JP3651522B2

JP3651522B2 - Nuclear reactor core

Info

Publication number: JP3651522B2
Application number: JP23548896A
Authority: JP
Inventors: 紀之吉田; 直紀杉田; 毅中嶋; 政彦黒木
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1996-09-05
Filing date: 1996-09-05
Publication date: 2005-05-25
Anticipated expiration: 2016-09-05
Also published as: JPH1082879A

Description

【０００１】
【発明の属する技術分野】
本発明は沸騰水型原子炉の炉心における燃料の装荷技術に係り、特に炉心特性の改善および燃料経済性の向上を図った原子炉の炉心に関するものである。
【０００２】
【従来の技術】
図２５は、沸騰水型原子炉の炉心に装荷される燃料単位の一般的な構成例を示している。すなわち、図２５に示すように、断面十字型の制御棒Ａの周囲に４体の燃料集合体Ｂが装荷され、これによって燃料単位Ｃが構成される。
【０００３】
図２６は、多数の燃料集合体が配置された炉心Ｄの１／４（炉心中心Ｏから区分した四半分）の構成を示している。この図２６に示すように、炉心Ｄは一般に円柱に近い形であり、燃料単位Ｃが炉心全体に亘って多数配列されるとともに、反射体Ｅによって囲まれる炉心最外周の一部には燃料単位Ｃを構成しない燃料集合体Ｆが配列されている。なお、原子炉出力調整用の制御棒Ａ廻りに配置する燃料集合体Ｂで構成される燃料単位Ｃは、コントロールセルと呼ばれている。
【０００４】
このような原子炉では約１年強、すなわち１サイクル運転毎に燃料集合体の交換が行われ、全燃料集合体のうち１／３〜１／５の燃料が取り出され、代わりに新しい燃料集合体が装荷される。燃料交換の際に取り出される燃料集合体は、経済性の観点から最も残留濃縮度の低い燃料集合体である。例えば燃料集合体を１／４ずつ交換して運転した場合、４サイクル以後は燃焼が最も進んだ燃料集合体、燃焼が中程度進んだ燃料集合体、燃焼が進んでいない燃料集合体、すなわち残留濃縮度が最低、中程度、最高、の各燃料集合体が装荷された炉心となり、以後は燃料交換毎に残留濃縮度が最低の燃料集合体から取り出し、代わりに新燃料集合体を装荷すれば以降は同じ状態の炉心が維持される。
【０００５】
このように、最も残留濃縮度の低い燃料集合体を取り出し、新しい燃料集合体を装荷することを繰り返していくと、炉心は残留濃縮度の異なる数種類の燃料集合体が混在した定常的な状態になり、各サイクル毎の炉心特性の差がほとんどなくなる。このような炉心を平衡炉心と称している。平衡炉心では、燃料集合体が３〜５サイクルの間炉心の中にあり、残留濃縮度が低くなって出力があまり出なくなった時に取り出される。なお、装荷される燃料集合体は、後述する初装荷炉心に使用する初装荷燃料集合体と対比して、取替燃料集合体と呼ばれる。
【０００６】
初装荷炉心、すなわち原子炉が建設されて初めて燃料集合体が装荷された炉心では、すべての燃料集合体が新燃料である。しかしながら、第１サイクル終了後には燃料交換の必要が生じる。これは炉心全体の出力を維持するために新燃料を装荷する必要があるからである。同様に、第２サイクル終了後にも２サイクル使用されただけで取り出される燃料集合体が生じる。このように、原子炉が運転を開始して３サイクル程度の間は、短期間使用されただけで取り出される燃料集合体が発生する。
【０００７】
かつては、初装荷炉心をすべて同一の集合体平均燃焼度の初装荷燃料集合体で構成していたため、最初の数サイクルは、平衡炉心で取り出される燃料集合体に比べて残留濃縮度の高い燃料集合体が取り出されていた。このことは非常に不経済なことであった。
【０００８】
このような問題を解決するため、原子炉の初装荷炉心については、これまで様々な改良が加えられてきた。たとえば初装荷炉心に装荷する燃料集合体、すなわち初装荷燃料集合体を濃縮度の異なる数種類のものとし、濃縮度の高い順に１サイクル目燃料（新燃料）、２サイクル目燃料、３サイクル目燃料というようにみたて、平衡炉心を模擬した初装荷炉心を構成する。これは、多種類濃縮度炉心と呼ばれている炉心装荷方法である。
【０００９】
図２７はこの例として、集合体平均濃縮度の異なる４種類の初装荷燃料集合体を使用した炉心Ｄの１／４の燃料集合体配置構成を示している。最高濃縮度の燃料集合体７が平衡炉心の１サイクル目燃料を模擬し、２番目に濃縮度の高い燃料集合体８が２サイクル目燃料を模擬し、３番目に濃縮度の高い燃料集合体９が３サイクル目燃料を模擬し、最も濃縮度の低い燃料集合体１０が４、５サイクル目燃料を模擬している。
【００１０】
平衡炉心においては、中性子の漏れが大きく熱中性子束が低い炉心最外周部に、残留濃縮度の低い燃料から装荷する。多種類濃縮度炉心においても、これと同様の考えにより、炉心最外周部に濃縮度の最も低い燃料を装荷する。第１サイクル終了時には、濃縮度の最も低い燃料を取り出し、取替燃料集合体を新燃料として装荷する。第１サイクル終了時に取り出すのは、もともと低濃縮度の燃料であるため、残留濃縮度は低く、燃料経済性は向上する。
【００１１】
このように、平衡炉心を模擬した炉心に対し、初装荷燃料集合体の配置をより工夫することにより、一層燃料の経済性を向上させる発明が特開昭６０−７１９８７号公報「沸騰水型原子炉の運転方法」に開示されている。この開示された発明は、第１サイクルでは最高濃縮度の燃料集合体を炉心の最外周部に、最低濃縮度の燃料集合体をコントローセルに配置し、１サイクル運転した後には無限増倍率の最も低い燃料集合体を炉心の最外周およびコントローセルに装荷するというものである。
【００１２】
これにより、第１回の燃料交換で取り出される最低濃縮度の燃料集合体は、熱中性子束が低く燃焼があまり進まない炉心最外周ではなく、熱中性子束が高く燃焼が進む炉心中央部に装荷されるため、残留濃縮度はより低くなる。しかも、最外周に装荷されていた最高濃縮度の燃料集合体は燃焼があまり進んでおらず、第２サイクルにおいて高い出力を維持できるため、必要とされる新燃料の体数が減り、これによっても燃料の経済性が向上する。
【００１３】
このように、初装荷炉心の設計は当初の平衡炉心模擬から、より高い経済性を狙って改良が続けられている。経済性向上のためには、炉心平均濃縮度を上げることが必要であるが、最高濃縮度の初装荷燃料集合体の濃縮度を取替燃料集合体より高くすることは、燃料機械設計上の燃焼度の制約により困難である。また、第１、第２サイクル終了時に取り出される燃料集合体の初期濃縮度を上げることは、取り出し時の残留濃縮度を上げてしまうことになる。よって、初装荷炉心の改良は、初装荷燃料集合体中の最高濃縮度の燃料集合体の割合を増やす方向で進んでいる。
【００１４】
図２８は、このような初装荷炉心を例示した燃料集合体配置図であり、集合体平均濃縮度が異なる２種類の初装荷燃料集合体を使用した炉心の１／４の構成を示している。この炉心Ｄでは、コントロールセル１５を構成する燃料単位Ｃが高濃縮度燃料１６および低濃縮燃料１７とされているが、コントロールセル１５を構成する燃料集合体以外はほとんど高濃縮度燃料１６であり、全燃料集合体８７２体中、高濃縮度燃料１６が６７２体、低濃縮度燃料１７が２００体となっている。
【００１５】
【発明が解決しようとする課題】
ところで、炉心の設計においては最大線出力密度、最小限界出力比等の運転制限値を満足することが第一である。経済性向上をめざして初装荷燃料集合体中の最高濃縮度の燃料集合体の割合を増やした最近の初装荷炉心は、一般に平衡炉心より特性が厳しくなる傾向にあり、炉心特性の改善の手段なくしては初装荷炉心の経済性の向上は図れなくなってきている。
【００１６】
図２９は、最高濃縮度の初装荷燃料集合体と取替燃料集合体とにおける、無限増倍率の燃焼変化の一例を示している。最高濃縮度の初装荷燃料集合体の濃縮度は一般に使用されているように、取替燃料集合体と同一としている。濃縮度が同一であるから、無限増倍率の大きさの比較をそのまま出力の比較に置き換えることができる。
【００１７】
取替燃料集合体の無限増倍率は、燃焼初期には可燃性毒物であるガドリニア
（Ｇｄ₂Ｏ₃）によって抑えられており、１サイクルの燃焼が終るころにガドリニアがほぼ燃え尽き、無限増倍率が最大になる。これは炉心の余剰反応度をサイクルを通じてほぼ一定に保つためである。これにより、サイクル初期には２サイクル目の燃料の出力が高く、サイクル末期には１サイクル目の燃料の出力が高くなる。最高濃縮度の初装荷燃料集合体は、完全に平衡サイクル模擬ができている初装荷炉心では１サイクル目燃料を模擬するはずであるが、実際には経済性の向上のため燃焼初期から出力を担うことになるため、燃焼初期においては無限増倍率が取替燃料集合体の無限増倍率よりも高くなっている。
【００１８】
これは、ガドリニア入り燃料棒の本数を取替燃料集合体より少なくすることにより実現されている。ただし、第１サイクルの長さは試運転期間を含むことにより平衡サイクルより長くなること、第２サイクルの余剰反応度は高くなりやすいことにより、長期にわたって余剰反応度を適切に抑えるためガドリニアの濃度は最高濃縮度の初装荷燃料集合体のほうが取替燃料集合体よりも高い。
【００１９】
図３０は、最高濃縮度の初装荷燃料集合体および取替燃料集合体における局所ピーキング係数の燃焼変化の一例を示している。
【００２０】
沸騰水型原子炉では燃料集合体問に水ギャップがあるため、燃料集合体の外周の燃料棒の出力が相対的に高くなる。燃料集合体の外周の燃料棒の濃縮度を燃料集合体の内部の燃料棒の濃縮度より高くすることにより、ある程度出力を平坦化することができるが、核燃料物質の最高濃縮度が制限されている状況で、燃料集合体平均の濃縮度を確保するためには、燃料集合体の外周の燃料棒の濃縮度を充分に下げることはできない。
【００２１】
また、ガドリニア入り燃料棒は出力が低く、その分他の燃料棒の出力が相対的に高くなる。出力の高い燃料棒ほど核分裂性物質の消費が早いので出力が下がるのも早く、またガドリニア入り燃料棒はガドリニアが減少するにつれて出力が上がるため、燃料集合体は燃焼が進むと局所ピーキング係数が低下する。燃焼初期での取替燃料集合体を比較すると、ガドリニア入り燃料棒の本数の違いにより最高濃縮度の初装荷燃料集合体のほうが取替燃料集合体よりも若干小さくなるが、大きな差ではない。また、最小限界出力比の算出にはＲ因子という指標を用い、これが大きいほど厳しいが、やはり燃料集合体の燃焼初期が最も大きい。
【００２２】
最大線出力密度が問題となるのは、燃料集合体の出力が高く、局所ピーキング係数も高いところである。取替燃料集合体では局所ピーキング係数の最も高い燃料初期では燃料集合体の出力が低いため、熱的にあまり問題にならない。これに対し、初装荷炉心では最高濃縮度の初装荷燃料集合体の出力が第１サイクルの初期から高いため、この燃料の最大線出力密度が問題となってくる。また、最小限界出力比についても、最大線出力密度と同様の問題が生じる。よって、第１サイクル初期の最大線出力密度および最小限界出力比は、平衡サイクルより厳しくなる傾向にある。
【００２３】
そこで、本発明の第１の目的は、炉心平均濃縮度を高くして燃料経済性の向上を図りつつ、第１サイクル初期の炉心特性を良好にできる原子炉の初装荷炉心を提供することにある。
【００２４】
一方、前述したように、燃料集合体には可燃性毒物が含まれている。可燃性毒物とは、ガドリニウム（Ｇｄ）またはその酸化物であるガドリニア（Ｇｄ₂Ｏ₃）のように中性子を吸収する性質が強く、しかも中性子を吸収すると中性子を吸収する性質の弱い物質に変換するものである。これらの可燃性毒物は、Ｕ−２３５（ウラン２３５）などの核分裂性物質が過剰にあるサイクル初期には、中性子を吸収して核分裂性物質と中性子の核反応を抑制し、燃焼に伴って可燃性毒物も消費され、その中性子吸収能力を低下させる。燃料集合体には核燃料物質のみからなる燃料ぺレットを装荷した燃料棒の他に、核燃料物質のみならず可燃性毒物も含んだ燃料ぺレットを装荷した燃料捧が数本〜１０数本入っており、初期余剰反応度の抑制が行われている。
【００２５】
ところで一般に、原子炉の炉心においては、炉心中心付近が熱中性子束も高く燃焼が良く進む。一方、炉心の外周方向に行くに従って熱中性子の炉心外への漏れの効果が大きくなる。即ち熱中性子束も低くなり、燃焼の進み具合は悪くなる。このように、炉心外周領域に装荷した燃料と、炉心中心付近に装荷した燃料とでは、燃焼の進み具合に差が生じる。即ち、炉心中心付近に装荷した燃料は、可燃性毒物の燃焼も早い。
【００２６】
逆に炉心外周領域に装荷した燃料は、可燃性毒物の燃焼が遅い。仮に炉心中心付近に装荷する高濃縮燃料と、炉心最外周および最外周から２層目に装荷する高濃縮燃料とが全く同一の燃料であるとすると、まず炉心中心付近の高濃縮燃料が良く燃焼するため、可燃性毒物も効率よく減衰してゆき、それにつれてその領域の相対出力も高くなる。さらに燃焼が進み、可燃性毒物がある程度減少してしまうと、その後その領域の相対出力は低くなる一方である。
【００２７】
これに対し、炉心最外周および最外周から２層目の高濃縮燃料は燃焼が進みにくいため、炉心中心付近の可燃性毒物がある程度減少した時点においても可燃性毒物はかなり残っており、炉心外周領域の相対出力は未だ増加する方向にある。つまり、サイクル初期からある燃焼度までは、炉心断面における径方向の出力分布は炉心中心領域で高く、それ以降は炉心外周領域で高くなるというようにバランスが悪くなり、これが出力ピーキングの増加の原因となる。
【００２８】
炉心断面における径方向の出力ピーキングが増加すると、燃料集合体の線出力密度や最小限界出力比といった原子炉の運転に関する熱的パラメータが悪化し、運転裕度の減少につながる。従って、炉心断面における径方向の出力分布は平坦であるのが望ましい。
【００２９】
このように、濃縮度の異なる３種類の燃料、即ち高濃縮、中濃縮、および低濃縮燃料から構成され、コントロールセルには低濃縮燃料を装荷し、かつ炉心外周領域の燃料装荷パターンに関しては高い燃料経済性を実現するために炉心最外周のみならず最外周から２層目にも高濃縮燃料を装荷した炉心においては、前述したような炉心断面における径方向の出力ピーキングの増加をもたらすために、炉心の運転裕度が少なくなるという問題が生じる。
【００３０】
そこで、本発明の第２の目的は、第１サイクル終了後に取り出す燃料の残留濃縮度を低め、燃料経済性の向上をめざした炉心、即ち濃縮度の異なる３種類の燃料集合体から構成される炉心において、最外周と最外周から２層目にすべて高濃縮燃料が配置し、かつコントロールセルには全て低濃縮燃料を配置したものとし、サイクルを通じて炉心特性が悪化することのない原子炉の炉心を提供することにある。
【００３１】
【課題を解決するための手段】
前記の目的を達成するために、請求項１の発明では、１本の制御棒の周りに４体１組の燃料集合体を配置して燃料単位を構成し、この燃料単位を炉心全体に亘って多数配列する一方で、炉心外周側の一部には前記燃料単位を構成しない燃料集合体を配列させてなる原子炉の炉心において、前記燃料集合体を集合体平均濃縮度の違いにより２種類以上に種分けし、さらに集合体平均濃縮度の最も高い燃料集合体を当該燃料集合体に含まれる可燃性毒物入り燃料棒の本数の違いにより２種類に種分けし、燃料単位を構成しない炉心外周側の燃料集合体のすべてと、炉心外周を覆う反射体に面する燃料単位を構成する燃料集合体の４体中３体以上とを、最高濃縮度でかつ可燃性毒物入り燃料棒本数が少ない燃料集合体とし、前記反射体に面する燃料単位を構成する燃料集合体のうち、当該燃料単位のなかで最も炉心中心に近い配置のものを、最高濃縮度ではなく、かつ可燃性毒物入り燃料棒本数が少ないものでもない燃料集合体としたことを特徴とする原子炉の初装荷炉心を提供する。
【００３２】
本発明によれば、炉心の外周部に最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体が配置されることにより、炉心の外周部の出力が高くなり、炉心中央部の出力は相対的に低くなる。即ち径方向出力分布の平坦化が図れる。これにより最大線出力密度、最小限界出力比のような炉心の熱的特性の改善が可能となる。なお、一つの燃料単位に出力の高い燃料集合体が集中した場合には炉停止余裕に問題が起こる可能性があるが、炉心外周の反射体に面する燃料単位においては、最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体を３または４体装荷することが可能である。
【００３４】
また、本発明によれば、炉心外周の反射体に面する燃料単位に、最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体以外の燃料集合体が１体装荷される場合は、出力の高い最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体をできるだけ炉心の外部に配置するほうが、径方向出力分布の平坦化、および炉停止余裕の確保の点で有利である。
【００３５】
請求項２の発明では、請求項１記載の炉心において、反射体に面する燃料単位を構成する燃料集合体を、すべて最高濃縮度の燃料集合体としたことを特徴とする原子炉の初装荷炉心を提供する。
【００３６】
本発明によれば、炉停止余裕の点から、反射体に面する燃料単位を構成する燃料集合体４体中１体を最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体以外にする場合、その１体を最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の多い燃料集合体とすることが経済性の向上の点から有利である。また、炉停止余裕の確保も可能である。
【００３７】
請求項３の発明では、請求項１または２記載の炉心において、反射体に面する燃料単位以外の燃料単位であって、かつ反射体に面する燃料単位に隣接するものの燃料集合体の４体中２体以下を、最高濃縮度でかつ可燃性毒物入り燃料棒本数が少ない燃料集合体としたことを特徴とする原子炉の初装荷炉心を提供する。
【００３８】
本発明によれば、反射体に面してはいないが、反射体に面する燃料単位に隣接している燃料単位は、炉心の最外周部からやや離れて中性子の漏洩の効果も小さくなっている。出力の高くなり易い燃料集合体を過度に集中すると、この部分が炉心で最も出力が高くなってしまう。このため最高濃縮度でかつ可燃性毒物入り燃料棒の本数の少ない方の燃料集合体の体数を制限するが、炉停止余裕の点から各燃料単位の出力を均等にほぼ均等にしておくことが望ましい。そこで、最高濃縮度でかつ可燃性毒物入り燃料棒の本数の少ない燃料集合体の体数を各燃料単位毎に２体以下とする。
【００３９】
請求項４の発明では、請求項１から３までのいずれかに記載の炉心において、反射体に面するものではなく、また反射体に面する燃料単位に隣接するものでもない燃料単位については、最高濃縮度のものではなく、かつ可燃性毒物入り燃料棒本数が少ないものでもない燃料集合体を適用したことを特徴とする原子炉の初装荷炉心を提供する。
【００４０】
本発明によれば、炉心内部に最高濃縮度でかつ可燃性毒物入り燃料棒の本数の少ない燃料集合体を装荷すると、その燃料集合体の出力が高くなり過ぎ、熱的特性が厳しくなる。炉心外周部の出力を高くするために適正化されている燃料集合体は、炉心中央部に装荷するべきではない。なお、請求項１から請求項４までの炉心でも、炉心の最外周部の出力は未だかなり低い。炉心最外周部に装荷する燃料集合体は、可燃性毒物入り燃料棒の本数をより減らすことが可能である。
【００４１】
請求項５の発明では、１本の制御棒の周りに４体１組の燃料集合体を配置して燃料単位を構成し、この燃料単位を炉心全体に亘って多数配列する一方で、炉心外周側の一部には前記燃料単位を構成しない燃料集合体を配列させてなる原子炉の炉心において、前記燃料集合体を集合体平均濃縮度の違いにより２種類以上に種分けし、さらに集合体平均濃縮度の最も高い燃料集合体を当該燃料集合体に含まれる可燃性毒物入り燃料棒の本数の違いにより３種類に種分けし、炉心外周を覆う反射体に面する燃料集合体をすべて最高濃縮度で、かつ可燃性毒物入り燃料棒の本数が３種類のうち最少の本数のものとしたことを特徴とする原子炉の初装荷炉心を提供する。
【００４２】
本発明によれば、燃料集合体の種類が増えるのことにより、製造コストの点で不利ではあるが、径方向出力分布の平坦化に有効である。
【００４３】
請求項６の発明では、１本の制御棒の周りに４体１組の燃料集合体を配置して燃料単位を構成し、この燃料単位を炉心全体に亘って多数配列する一方で、炉心外周側の一部には前記燃料単位を構成しない燃料集合体を配列させてなる原子炉の炉心において、前記燃料集合体を集合体平均濃縮度の違いにより２種類以上に種分けし、さらに集合体平均濃縮度の最も高い燃料集合体を当該燃料集合体に含まれる可燃性毒物入り燃料棒の本数の違いにより２種類に種分けし、炉心外周を覆う反射体に面する燃料集合体をすべて最高濃縮度のものとし、また炉心最外周から２層目の燃料集合体をすべて最高濃縮度でかつ可燃性毒物入り燃料棒本数が少ないものとしないものとし、かつ最高濃縮度でかつ可燃性毒物入り燃料棒の本数が少ない燃料集合体は炉心の最外周から２層目以内に限って装荷したことを特徴とする原子炉の初装荷炉心を提供する。
【００４４】
炉心の最外周に最低濃縮度の燃料集合体を配置した炉心は、最外周に最高濃縮度の燃料集合体を配置した炉心に比べて、径方向出力分布の平坦化には不利である。しかしながら、本発明によれば、炉心の最外周から２層目の燃料集合体を、すべて最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体とすることにより、ある程度は径方向出力分布の平坦化が可能である。また、最高濃縮度でかつ可燃性毒物入り燃料棒の本数の少ない燃料集合体を炉心の最外周から２層目に限ることで、最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体の燃焼初期の無限増倍率をかなり大きくすることができる。
【００４５】
請求項７の発明では、１本の制御棒の周りに４体１組の燃料集合体を配置して燃料単位を構成し、この燃料単位を炉心全体に亘って多数配列する一方で、炉心外周側の一部には前記燃料単位を構成しない燃料集合体を配列させてなる原子炉の炉心において、前記燃料集合体を集合体平均濃縮度の違いにより３種類以上に種分けし、さらに集合体平均濃縮度の最も高い燃料集合体を当該燃料集合体に含まれる可燃性毒物入り燃料棒の本数の違いにより２種類に種分けし、炉心最外周の燃料集合体をすべて集合体平均濃縮度が中位の燃料集合体とし、また炉心最外周から２層目の燃料集合体をすべて最高濃縮度でかつ可燃性毒物入り燃料棒の本数が少ないものとし、かつ最高濃縮度でかつ可燃性毒物入り燃料棒の本数が少ない燃料集合体は炉心の最外周から２層目以内に限って装荷したことを特徴とする原子炉の初装荷炉心を提供する。
【００４６】
本発明によれば、炉心の最外周に中位の濃縮度の燃料集合体を配置した炉心においても、炉心の最外周から２層目の燃料集合体を、すべて最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体とすることで、径方向ピーキングの低減が有効的に図れるようになる。また、最高濃縮度でかつ可燃性毒物入り燃料棒の本数の少ない燃料集合体を炉心の最外周から２層目に限ることで、最高濃縮度で、かつ可燃性毒物入り燃料棒の本数の少ない燃料集合体の燃焼初期の無限増倍率をかなり大きくすることができる。
【００７９】
【発明の実施の形態】
以下、本発明に係る原子炉の炉心の実施形態について、図面を参照して説明する。
【００８０】
（第１の実施形態）（請求項１〜４対応）
図１は、本発明の第１の実施形態を示すもので、初装荷炉心Ｄの１／４部の燃料集合体を示す配置図である。
【００８１】
本実施形態においては、燃料集合体が集合体濃縮度の違いによって３種類、即ち高濃縮度燃料１１，１２、中濃縮度燃料１３、および低濃縮度燃料１４に分けられている。さらに、高濃縮度燃料１１，１２は、ガドリニア入り燃料棒本数の違いにより２種類に分けられ、一方は高濃縮度低Ｇｄ燃料１１、他方は高濃縮度高Ｇｄ燃料１２とされている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。
【００８２】
高濃縮度低Ｇｄ燃料１１は２００体、高濃縮度高Ｇｄ燃料１２は２２８体、中濃縮度燃料１３は１８４体、低濃縮度燃料１４は２６０体で、合計８７２体である。炉心Ｄの外周の燃料単位を構成しない燃料集合体は、すべて高濃縮度低Ｇｄ燃料１１とされている。炉心外周の反射体Ｅに面する燃料単位Ｃは、４体の高濃縮度低Ｇｄ燃料１１で構成され、または３体の高濃縮度低Ｇｄ燃料１１と１体の高濃縮度高Ｇｄ燃料１２とで構成されている。
【００８３】
３体の高濃縮度低Ｇｄ燃料１１と１体の高濃縮度高Ｇｄ燃料１２とで構成される燃料単位Ｃの中では、高濃縮度高Ｇｄ燃料１２が炉心Ｄの中心Ｏに最も近い位置に配置してある。反射体Ｅに面してはいないが、反射体Ｅに面する燃料単位Ｃに隣接する燃料単位Ｃには、高濃縮度低Ｇｄ燃料１１が２体または１体含まれるか、全く含まれていない。反射体Ｅに面しないで、かつ反射体Ｅに面する燃料単位に隣接もしない燃料単位Ｃには、高濃縮度低Ｇｄ燃料１１は一切含まれていない。
【００８４】
このような炉心Ｄの構成において、炉心外周部に位置する燃料集合体では中性子の漏れがあるため、出力はあまり高くならない。このため、炉心Ｄの中央部Ｏに位置する燃料集合体の出力が相対的に高くなり、熱的に厳しくなる。平衡炉心では、炉心最外周に燃焼の最も進んだ燃料集合体を配置し、中性子の漏れを少なくすることが経済的に有利であるが、初装荷炉心では、例えば特開昭６０−７１９８７号公報「沸騰水型原子炉の運転方法」のように、第１サイクルにおいて、低中性子漏洩型の炉心でなくても経済性上必ずしも不利にはならない。
【００８５】
炉心外周部の出力を高くすることができれば、相対的に炉心中央部Ｏの燃料集合体の出力は低くすることができるので、熱的特性の改善が可能となる。炉心Ｄの最外周部に最高濃縮度の燃料集合体１１，１２を配置するだけでも、最外周部に最低濃縮度の燃料集合体１４を配置した場合よりも径方向出力分布が平坦になる。ただし、炉心Ｄの内部にも最高濃縮度の燃料集合体１１，１２が多数配置されるため、外周部に配置される最高濃縮度の燃料集合体１１，１２の出力は高くするが、炉心内部の最高濃縮度の燃料集合体１１，１２の出力は上げない方法を考える必要がある。
【００８６】
最高濃縮度の燃料集合体１１，１２はガドリニア入り燃料棒により初期の出力を抑えているが、このガドリニア入り燃料棒の本数を変えることにより、同じ濃縮度の燃料集合体１１，１２でも燃焼初期の無限増倍率を異ならせることが可能である。
【００８７】
図２は、ガドリニア入り燃料棒の本数を異ならせた２種類の最高濃縮度の初装荷燃料集合体１１，１２の無限増倍率の燃焼度変化を示している。この図２に示すように、両者の無限増倍率の差は、燃焼初期において最も大きく、ガドリニアの燃焼とともに差が縮まり、ガドリニアが燃え尽きた後はほとんど差がなくなる。
【００８８】
図３は、本実施形態の効果を示したものであり、第１サイクル初期の運転状態の炉心半径方向の位置と相対出力との関係を示す特性図で、本実施形態の炉心Ｄの出力分布と従来の炉心の径方向の出力分布とを比較して示したものである。従来の炉心としては、平衡サイクル模擬の初装荷炉心を取り上げている。
【００８９】
本実施形態の炉心Ｄでは、炉心外周部に最高濃縮度でガドリニア入り燃料棒本数の少ない燃料集合体１１が配置されているため、従来の炉心に比べて出力が高くなり、相対的に炉心中央部の出力が低くなる。このようにして燃料集合体の出力の均一化を図ることにより、最大線出力密度や最小限界出力比のような熱的特性を改善することができる。
【００９０】
ただし、炉停止余裕の観点からは、出力の高くなりやすい燃料が集中することは不利である。炉停止余裕とは、冷温状態で最も反応度制御効果の大きい制御捧が引き抜かれ、他の制御棒はすべて挿入された状態での未臨界度をいう。一つの燃料単位に出力の高くなり易い燃料を集中すると、最も反応度制御効果の大きい制御棒が引き抜かれることによる反応度変化が大きくなる。冷温で殆どの制御棒が挿入された状態では、中性子の移動距離が運転状態に比べて小さくなるので炉心外周からの中性子の漏れの影響は炉心最外周の１〜２層程度に限定される。よって、炉心外周部で反射体Ｅに面する燃料単位Ｃには高濃縮度低Ｇｄ燃料１１を３体以上装荷できるが、それより内側の燃料単位Ｃでは高濃縮度低Ｇｄ燃料１１を２体以下にすることが炉停止余裕の点から必要である。
【００９１】
本実施形態では、径方向出力分布の平坦化により熱的特性を改善しつつ、炉停止余裕も従来の初装荷炉心に遜色ない初装荷炉心を実現することができる。
【００９２】
（第２の実施形態）（請求項５対応）
図４は本発明の第２の実施形態を示すもので、初装荷炉心の１／４部の燃料集合体配置図である。
【００９３】
本実施形態では、燃料集合体が集合体濃縮度の違いにより２種類、即ち高濃縮度燃料４１，４２，４３と、低濃縮度燃料４４とに種分けされ、さらに高濃縮度燃料４１，４２，４３はガドリニア入り燃料棒本数の違いにより３種類、即ち高濃縮度低Ｇｄ燃料４１、高濃縮度中Ｇｄ燃料４２、および高濃縮度高Ｇｄ燃料４３に分けられている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。高濃縮度低Ｇｄ燃料４１は１３２体、高濃縮度中Ｇｄ燃料４２は１６４体、高濃縮度高Ｇｄ燃料４３は３８０体、低濃縮度燃料４４は１９６体で、合計８７２体である。
【００９４】
この実施形態による炉心Ｄは極めて高い経済性を狙ったもので、コントロールセル１５以外は、ほとんどすべてが高濃縮度燃料４１，４２，４３である。この高濃縮度燃料４１，４２，４３が前記のように、ガドリニア入り燃料棒本数の違いにより分けられている。炉心最外周の燃料集合体９２体のほか、反射体Ｅにコーナ部で接する燃料集合体４０体が高濃縮度低Ｇｄ燃料４１である。中性子の漏れが大きい位置であるので、高濃縮度低Ｇｄ燃料４１の燃焼初期の無限増倍率をかなり大きくすることができる。
【００９５】
炉心内部には、高濃縮度中Ｇｄ燃料４２と高濃縮度高Ｇｄ燃料４３とが配置されているが、炉心外周から２，３層目には、高濃縮度中Ｇｄ燃料４２が比較的多くなるようにしている。最外周部に燃焼初期の無限増倍率のかなり大きな燃料集合体を配置できるので、高濃縮度燃料をガドリニア入り燃料棒本数の違いで２種類に分けている場合よりも、さらに径方向出力分布の平坦化が図れる。
【００９６】
（第３の実施形態）（請求項６対応）
図５は本発明の第３の実施形態を示すもので、初装荷炉心の１／４部の燃料集合体配置図である。
【００９７】
燃料集合体は集合体濃縮度の違いにより３種類、即ち高濃縮度燃料５１，５２、中濃縮度燃料５３、および低濃縮度燃料５４に種分けされ、さらに高濃縮度燃料５１，５２はガドリニア入り燃料棒本数の違いにより２種類、即ち高濃縮度低Ｇｄ燃料５１、高濃縮度高Ｇｄ燃料５２に分けられている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。高濃縮度低Ｇｄ燃料５１は８８体、高濃縮度高Ｇｄ燃料５２は３４４体、中濃縮度燃料５３は２０４体、低濃縮度燃料５４は２３６体で、合計８７２体である。
【００９８】
前述した第１，２の実施形態の初装荷炉心は、いずれも炉心Ｄの最外周に高濃縮度燃料を配置したものであるが、この第３の実施形態の初装荷炉心は、炉心Ｄの最外周に低濃縮度燃料５４を配置したものである。炉心Ｄの最外周に低濃縮度燃料５４を配置した場合には、高濃縮度燃料５１，５２を配置した場合よりも径方向出力分布の平坦化には不利であるが、ある程度の平坦化は可能である。このため、高濃縮度低Ｇｄ燃料５１を炉心の最外周から２層目に配置している。高濃縮度低Ｇｄ燃料５１は、炉心の最外周から２層目以外には装荷しないことで、かなり燃焼初期の無限増倍率を大きくすることができる。
【００９９】
（第４の実施形態）（請求項７対応）
図６は本発明の第４の実施形態を示しており、初装荷炉心の１／４部の燃料集合体配置図である。
【０１００】
燃料集合体は集合体濃縮度の違いにより３種類、即ち高濃縮度燃料６１，６２、中濃縮度燃料６３、および低濃縮度燃料６４に種分けされ、さらに高濃縮度燃料６１，６２はガドリニア入り燃料棒本数の違いにより２種類、即ち高濃縮度低Ｇｄ燃料６１と高濃縮度高Ｇｄ燃料６２とに分けられている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。高濃縮度低Ｇｄ燃料６１は８８体、高濃縮度高Ｇｄ燃料６２は３４８体、中濃縮度燃料６３は２３６体、低濃縮度燃料６４は２００体で、合計８７２体である。
【０１０１】
この第４の実施形態によれば、炉心Ｄの最外周部に中濃縮度燃料６３を配置した場合でも、第３の実施形態の初装荷炉心と同様に、炉心Ｄの最外周から２層目に高濃縮度低Ｇｄ燃料６１を配置することで、径方向出力分布を平坦化することができる。
【０１０２】
図７は本発明の関連技術を示すもので、初装荷炉心の１／４部の燃料集合体配置図である。
【０１０３】
燃料集合体は集合体濃縮度の違いにより３種類、即ち高濃縮度燃料７１、中濃縮度燃料７２，７３、および低濃縮度燃料７４に種分けされ、さらに中濃縮度燃料７２，７３はガドリニア入り燃料棒本数の違いにより２種類、即ち中濃縮度低Ｇｄ燃料７２と、中濃縮度高Ｇｄ燃料７３とに分けられている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。
【０１０４】
高濃縮度燃料７１は４３６体、中濃縮度低Ｇｄ燃料７２は９２体、中濃縮度高Ｇｄ燃料７３は１４４体、低濃縮度燃料７４は２００体で、合計８７２体である。
【０１０５】
本実施形態の初装荷炉心は、第４の実施形態の初装荷炉心と同様に、炉心Ｄの最外周部に中濃縮度燃料７２を配置しているが、中濃縮度燃料自身をガドリニア入り燃料棒本数の違いで２種類に分け、そのうちの中濃縮度低Ｇｄ燃料７２を炉心の最外周に配置することにより、径方向出力分布の平坦化を図っている。
【０１０６】
図８は本発明の関連技術を示すもので、初装荷炉心の１／４部の燃料集合体配置図である。
【０１０７】
本実施形態では、燃料集合体が集合体濃縮度の違いにより４種類、即ち最高濃縮度の燃料集合体８１と、２番目に濃縮度の高い燃料集合体８２と、３番目に濃縮度の高い燃料集合体８３と、最低濃縮度の燃料集合体８４とに種分けされている。よって、合計４種類の燃料集合体で炉心Ｄが構成されている。
【０１０８】
このうち、３番目に濃縮度の高い燃料集合体８３と、最低濃縮度の燃料集合体８４とは、ガドリニア入り燃料棒を有しない。最高濃縮度の燃料集合体８１は４３６体、２番目に濃縮度の高い燃料集合体８２は１４４体、３番目に濃縮度の高い燃料集合体８３は９２体、最低濃縮度の燃料集合体８４は２００体で、合計８７２体である。
【０１０９】
本実施形態において、ガドリニア入り燃料棒を有しない燃料集合体８３，８４については、燃焼初期から出力が急速に低下していく。この場合、ある程度濃縮度が高くガドリニア入り燃料棒を有しない燃料集合体８３を炉心最外周に用いれば、第１サイクルの初期だけは径方向出力分布を平坦化できる。炉心最外周の燃料集合体は出力が高くなりにくいため、かなり濃縮度の高い燃料集合体でも、ガドリニア入り燃料棒を入れる必要がない。
【０１１０】
図９〜図１６は、本発明の関連技術を示している。
【０１１１】
図９は、炉心Ｄの１／４部の燃料集合体配置図である。
【０１１２】
この炉心Ｄは濃縮度の異なる３種類の燃料集合体である低濃縮燃料集合体１、中濃縮燃料集合体２、および高濃縮燃料集合体３から構成され、さらに高濃縮燃料集合体３は可燃性毒物入り燃料棒本数が異なる高濃縮タイプ３Ａと高濃縮タイプ３Ｂからなる。
【０１１３】
高濃縮タイプ３Ａは、高濃縮タイプ３Ｂに比べて可燃性毒物を含んだ燃料捧の本数が１本以上多く、かつ炉心装荷体系無限配列における中性子増倍率が最大となる燃焼度がタイプ３Ｂのそれよりも高い。装荷パターンについては、炉心最外周と最外周から２層目（斜線で示す）にはタイプ３Ｂを配置し、制御棒Ａと４つの燃料集合体Ｂからなるコントロールセル１５には低濃縮燃料集合体１を配置し、その他の部分には低濃縮燃料集合体１、中濃縮燃料集合体２、高濃縮燃料タイプ３Ａ、および高濃縮燃料タイプ３Ｂを適切に配置することによって、複数の単位格子を構成している。
【０１１４】
なお、この装荷例においては、コントロールセル１５に面隣接する箇所に、低濃縮燃料集合体１を配置していない。その理由は以下の通りである。即ち、多種類濃縮度の燃料集合体で構成する初装荷炉心では、炉心内における燃料毎の中性子スペクトルの違いが顕著となる。低濃縮燃料集合体１が４体で構成されるコントロールセル１５は、中性子スペクトルが軟らかい、即ち炉心内において局所的に熱中性子束が高い領域となる。従って、コントロールセル１５に面隣接する箇所には、低濃縮燃料集合体１以外の燃料集合体を配置することにより、炉心内において局所的に熱中性子束が高い領域を拡大することがないように配慮したものである。最外周および最外周から２層目は中性子の漏れが大きく、熱中性子束が小さいため燃焼が進みにくい。それに対し、炉心中心付近は中性子の漏れが少なく、燃焼がよく進む。
【０１１５】
図１０は、燃料集合体の炉心装荷体系無限配列における中性子増倍率（無限増倍率）の燃焼度に対する変化を示すものである。
【０１１６】
燃料集合体の無限増倍率は、Ｕ−２３５などの核分裂性物質の濃縮度にほぼ比例するので、図１０の特性線ａに示すように、燃焼が進んで核分裂性物質が消費するとともに小さくなる。しかし、燃料に可燃性毒物を添加すると、特性線ｂに示すように、燃焼初期の無限増倍率が抑えられる。可燃性毒物は中性子を吸収すると自分自身の中性子吸収効果を失ってしまうので、燃焼が進むとともに可燃性毒物は減衰していく。それに伴い、無限増倍率の値は上昇していく。そして、ある燃焼度ｃ以降は無限増倍率が減少していくという傾向を示す。
【０１１７】
図１１は、異なる３種類の濃縮度の燃料集合体から構成される初装荷炉心の炉心装荷体系無限配列における中性子増倍率（無限増倍率）の燃焼度に対する変化を示した特性図である。この図１１において、実線ｄは低濃縮燃料集合体の特性を示し、破線ｂは中濃縮燃料集合体の特性を示し、一点鎖線ｆは高濃縮燃料集合体の特性を示している。
【０１１８】
低濃縮燃料集合体は濃縮度が〜１．５ｗｔ％程度と、もともと濃縮度が他の燃料集合体に比べて低く、無限増倍率の値が小さいうえに、大部分のものは第１サイクル終了後に取り出される。燃料経済性の観点からは第１サイクルにおいて、できる限り核分裂性物質を燃焼させ、残留濃縮度を低減する必要がある。従って、燃料捧が吸収した熱中性子をできるだけ核分裂に寄与させるために可燃性毒物を含む燃料棒はなく、実線ｄで示す低濃縮燃料集合体の無限増倍率は燃焼度とともに値が小さくなる。
【０１１９】
一方、中濃縮燃料集合体は濃縮度が２．数ｗｔ％であり、可燃性毒物を添加しないと燃焼初期の無限増倍率は１．２程度とかなり大きくなることから、それを抑制するために、数本の可燃性毒物を含む燃料捧が必要となる。また、中濃縮燃料集合体は大部分のものが第２サイクル終了時に取り出されるため、無限増倍率が最大となる燃焼度は破線ｅで示すように、第１サイクル終了時よりも若干低くなるように調整する。そうすることによって、第２サイクルにおける中濃縮燃料集合体は、あたかも第１サイクルにおける低濃縮燃料集合体の如くなるので、燃料棒が吸収した熱中性子を効率よく核分裂に寄与することができ、その結果取出時における残留濃縮度を低減する事ができる。
【０１２０】
さらに高濃縮燃料集合体は、中濃縮燃料集合体よりも濃縮度が高く、４．０ｗｔ％程度にまでなることもあり、燃焼初期の無限増倍率を抑制するための可燃性毒物入り燃料捧の本数は、中濃縮燃料集合体の場合よりも多く必要となる。また、高濃縮燃料集合体は第３サイクル以降に取り出されるため、一点鎖線ｆで示すように、無限増倍率が最大となる燃焼度は第２サイクル終了時の燃焼度よりも若干低くなるようにする必要がある。こうすることによって、第３サイクル以降における高濃縮燃料集合体は、あたかも第１サイクルにおける低濃縮燃料集合体や第２サイクルにおける中濃縮燃料集合体の如くなるので、取出時における残留濃縮度を低減することができる。
【０１２１】
しかしながら、現状の設計においては後述するように、可燃性毒物の濃度に関してその上限値が定められているため、可燃性毒物の濃度のみで所要の無限増倍率が最大となる燃焼度を得られない場合もある。そのようなときには、可燃性毒物を含む燃料棒の本数を増やすことによって、無限増倍率が最大となる燃焼度を延ばす必要がある。前述したように、可燃性毒物には、Ｇｄ（ガドリニウム）が酸化物の形態であるＧｄ₂Ｏ₃（ガドリニア）として多く用いられており、燃焼初期の無限増倍率を抑制するには、可燃性毒物を添加した燃料棒の本数を増大し、また無限増倍率が最大となる燃焼度を延ばすためには、可燃性毒物の濃度を高めればよい。しかしながら、Ｇｄ₂Ｏ₃濃度を上げると、ぺレットの熱伝導率が悪くなり融点が下がることが知られており、燃料棒の出力の制約が課せられる。
【０１２２】
このため現状では、Ｇｄ₂Ｏ₃の上限濃度を７．５ｗｔ％として制限している。前述したように、高濃縮燃料集合体の無限増倍率が最大となる燃焼度を得るためには、かなりの長さが要求されるため、最近の設計例では高濃縮燃料集合体のＧｄ₂Ｏ₃濃度として上限濃度である７．５ｗｔ％を用いている。高濃縮燃料集合体においてさらにタイプＡとタイプＢの無限増倍率が最大となる燃焼度の差をつけるには、大きく分けて３通りの考えがある。
【０１２３】
その第１案は可燃性毒物を含む燃料捧の本数を等しくして可燃性毒物の濃度を変える方法である。第２案は可燃性毒物の濃度を等しくして可燃性毒物を含む燃料棒の本数を変える方法である。第３案はそれらを組み合わせた発想であり、可燃性毒物の濃度と可燃性毒物を含む燃料捧の本数の両方とも変える方法である。
【０１２４】
第１案では図１２に示すように、無限増倍率が最大となる燃焼度は変わるものの未燃焼時の無限増倍率抑制効果はほとんど変わらない。一方、第２案では図１３に示すように、無限増倍率が最大となる燃焼度のみならず、未燃焼時の無限増倍率も異なった値が得られる。第３案では、これらを組み合わせた効果が得られる。結果的に、第２案および第３案のように可燃性毒物を含む燃料捧の本数を変えた方が、無限増倍率が最大となる燃焼度が異なる燃料が実現できることのみならず、未燃焼時における無限増倍率の異なる燃料集合体を装荷できるようになり、炉心設計上の融通性も増すので利点は大きい。
【０１２５】
第２案の場合は、炉心設計上の融通性は第３案より悪くなるが、第３案に比べて可燃性毒物の濃度種類が少ないので、製造上有利になるという特徴がある。ちなみに、図１２および図１３は、燃料集合体の平均濃縮度が同一である燃料集合体において、可燃性毒物の濃度が同じであるが可燃性毒物を含んだ燃料棒の本数が異なる場合（図１３）と、可燃性毒物を含んだ燃料棒の本数が同一であるが可燃性毒物の濃度が異なる場合（図１２）における炉心装荷体系無限配列における中性子増倍率（無限増倍率）を比較した例である。
【０１２６】
図１３に示す特性において、特性線ｇの燃料集合体に比べて、特性線ｈの燃料集合体は、可燃性毒物の本数が多い。また、図１２に示す特性において、特性線ｉの燃料集合体に比べて、特性線ｊの燃料集合体は可燃性毒物の濃度が高い。
【０１２７】
本実施形態のように、最外周および最外周から２層目に高濃縮燃料集合体を装荷する炉心の場合、燃焼の進みにくい最外周および最外周から２層目に装荷する高濃縮燃料集合体Ｂタイプは、無限増倍率が最大となる燃焼度ｃの短いものとし、それ以外の領域の一部には、無限増倍率が最大となる燃焼度ｃが高濃縮燃料Ｂタイプより高い高濃縮燃料Ａタイプを装荷することにより、図１４に例示すように、サイクルを通じた炉心断面径方向の出力バランスを保つことが可能となる。
【０１２８】
しかしながら、図１５の炉心装荷例に示すように、高濃縮燃料集合体３が唯１種類のみの場合、以下に記するような問題が生じる。即ち、図１６のＩに例示するように、サイクル初期における炉心断面における径方向出力分布をバランス良く維持していても、燃焼が進むにつれて炉心中心付近の可燃性毒物の燃焼が効率よく進むのに伴い、図１６のＩＩのように、炉心中心付近の相対出力が高くなる。しかしながら、さらに燃焼が進むと炉心中心付近の燃料の可燃性毒物は減少してしまい、その後は中性子増倍率が単調減少し、それに伴い相対出力も小さくなっていく。一方、炉心外周領域の燃料は炉心中心付近の燃料の可燃性毒物が減少した状態になっても、未だ可燃性毒物が十分残っている。つまり、炉心中心付近の相対出力は減少していく傾向にあるのに対し、炉心外周領域の燃料集合体は中性子増倍率がまだ増加しており、それに伴って相対出力も増加してゆく傾向にある。即ち、サイクル中期から末期になると、図１６のＩＩＩに示すように、炉心外周領域の相対出力が高くなる。
【０１２９】
このように、炉心断面の径方向における出力分布のバランスが悪くなる。このように、炉心径方向の出力ピーキングが高くなるということは、燃料の線出力密度や、最小限界出力比といった原子炉の運転において制約が課せられている熱的条件が厳しくなるということであり、それだけ原子炉運転上の裕度が減少するということである。このようなことにならないように、本実施形態の炉心においては、炉心断面の径方向における相対出力分布が図１４の如く、サイクルを通じてほぼ平坦になるようにＢタイプの無限増倍率が最大となる燃焼度ｃよりもＡタイプの無限増倍率が最大となる燃焼度ｃを高くするのである。無限増倍率が最大となる燃焼度を長くするには、前述した第１案のように、可燃性毒物の濃度を上げるか、あるいは第２案のように可燃性毒物を含む燃料棒の本数を増やすか、あるいは第３案のようにそれらを適切に組み合わせる方法があるが、本実施形態では炉心設計上の融通性の高い第２案あるいは第３案の考えを採用するものとし、高濃縮燃料ＢタイプよりもＡタイプの可燃性毒物を含む燃料棒本数を１本以上多くしている。
【０１３０】
第２案と第３案とを比較すると、第２案は可燃性毒物の濃度が固定しているのに対し、第３案は可燃性毒物の濃度および本数が制約を受けないという点で燃料設計上の自由度が大きい。即ち、第３案に基づく燃料集合体を用いると燃料設計および炉心設計はより容易になる。しかしながら、第２案は可燃性毒物の濃度が固定しているので、ぺレット製造上の加工経済性の観点から有利であるという特徴がある。従って、燃料および炉心設計の融通性を優先するのであれば、第３案を採用し、経済性を優先するのであれば第２案を採用するという様に使い分ければ良い。
【０１３１】
（第８の実施形態）（請求項１３〜１５対応）
本実施形態は各燃料の濃縮度、可燃性毒物の分布についてのものであり、図１７〜図２０にその分布例を示している。なお、これらの図においては、左上に制御捧Ａがある場合の各燃料棒の濃縮度および可燃性毒物分布を示しており、図１７は低濃縮燃料の濃縮度分布例、図１８は中濃縮燃料の濃縮度およびＧｄ分布例、図１９は高濃縮燃料Ａタイプの濃縮度およびＧｄ分布例、図２０は高濃縮燃料Ｂタイプの濃縮度およびＧｄ分布例をそれぞれ示している。
【０１３２】
これらの例では、燃料捧を９行９列に正方配列した燃料集合体が示してあり、○は燃料棒を、また○内の数字は燃料棒の濃縮度の種類を示しており、数字の若い順に濃縮度が高い。Ｗはウオータロッドであり、燃料集合体の中心付近における減速材を確保するために水を通す中空管である。このタイプの燃料では、太径のウオータロッドを２本有している。Ｇは可燃性毒物であるＧｄ₂Ｏ₃を含んだ燃料捧である。
【０１３３】
この実施形態は既に述ベた第２案に基づく一例であり、中濃縮燃料集合体、高濃縮燃料集合体（タイプＡ、タイプＢ）間で同一のＧｄ₂Ｏ₃濃度を用いてぺレットの共通化を図ることで燃料製造時における経済性向上を図っており、Ｇｄ₂Ｏ₃を含んだ燃料捧の本数によって無限増倍率が最大となる燃焼度を調整している。
【０１３４】
ここで装荷する低濃縮燃料集合体１、中濃縮燃料集合体２、高濃縮燃料集合体３（３Ａ，３Ｂ）の各燃料における集合体平均濃縮度、可燃性毒物棒本数、可燃性毒物濃度の一例を下記の表１に示す。
【０１３５】
【表１】

【０１３６】
図２１は、これら燃料集合体の炉心装荷体系無限配列における中性子増倍率
（無限増倍率）の燃焼度に対する変化を示している。即ち図２１に示すように、可燃性毒物を含んだ燃料棒本数が増えると可燃性毒物の寿命、つまり無限増倍率が最大となるまでの燃焼度も高くなり、例えば低濃縮燃料集合体１が０本（ａ_１）では０GWd/t 、中濃縮燃料集合体２が４本（ａ_２）では９GWd/t 、高濃縮燃料タイプ３Ｂが１０本（ａ_３ｂ）では１７GWd/t 、高濃縮燃料タイプ３Ａが１４本（第２案におけるａ_３Ａ）では１９GWd/t となる。
【０１３７】
ここで示すタイプ３Ａとタイプ３Ｂは、それぞれ可燃性毒物であるＧｄ₂Ｏ₃を含んだ燃料棒の本数がそれぞれ１４本、１０本であり、その差は４本である。そして、無限増倍率が最大となるまでの燃焼度の差は約２GWd/t である。これらは請求項１３、１４、１５の好適な１例にもなっている。
【０１３８】
第２案のように、可燃性毒物の濃度がタイプ３Ａとタイプ３Ｂで同一であるという制約を受ける場合、最低でも４本程度は可燃性毒物を含む燃料棒数の差をつけないと、タイプ３Ａおよびタイプ３Ｂの燃料集合体の無限増倍率が最大となる燃焼度の有為な差が得られない。
【０１３９】
一方、第３案の場合、可燃性毒物の濃度をタイプ３Ａとタイプ３Ｂで異なる値を用いることができるので、可燃性毒物を含む燃料棒本数の差が４本未満であっても、タイプ３Ａおよびタイプ３Ｂ燃料集合体の無限増倍率が最大となる燃焼度の有為な差をつけることができる。
【０１４０】
第３案の低濃縮燃料集合体１、中濃縮燃料集合体２、高濃縮燃料集合体３Ａ、高濃縮燃料集合体３Ｂ各燃料集合体の集合体平均濃縮度、可燃性毒物棒本数、可燃性毒物濃度の一例を下記の表２に示す。
【０１４１】
【表２】

上記の低濃縮燃料集合体１、中濃縮燃料集合体２、高濃縮燃料集合体タイプ３Ｂの濃縮度および可燃性毒物濃度分布の一例は図１７、図１８、図２０と同じである。
【０１４２】
一方、タイプ３Ａの濃縮度および可燃性毒物濃度分布の一例を図２２に示す。
【０１４３】
この典型例に示すタイプ３Ａでは、図２１の３Ａ（第３案）に示すように、無限増倍率が最大となる燃焼度は第２案のそれと同じ１９GWd/t となるように可燃性毒物を含む燃料棒数および可燃性毒物濃度を調整したものである。仮にタイプ３Ａの可燃性毒物本数をタイプ３Ｂのそれと同じ１０本にして、可燃性毒物濃度を上記値よりもさらに上げることによっても、図２１の３Ａ（第１案）に示すように、無限増倍率が最大となる燃焼度として前記と同じ程度の値を得る事は可能である。これが第１案の考え方であるが、第２案および第３案のように、タイプ３Ａとタイプ３Ｂの可燃性毒物を含む燃料棒の本数差を１本以上つけることによって、未燃焼時における無限増倍率の値の差もつけることができ、炉心設計上における融通性は高い。
【０１４４】
第１案、第２案、第３案いずれの方法であっても、炉心断面における径方向相対出力分布は局所的に多少の違いはあっても、サイクルを通じてほぼ図２３に示すような傾向を取る。即ち、サイクルを通じて径方向における相対出力の最大値である出力ピーキングがあまり高くなることがなく、良好な運転特性が得られる。ただし、第１案の場合は、タイプ３Ａとタイプ３Ｂの未燃焼時における無限増倍率がほぼ等しいので、炉心設計上の融通性が少ないのに対し、第２案、第３案の場合は炉心設計上の融通性が高く、図９の燃料装荷パターン例以外にも良好な炉心特性を有する設計が何通りか得られる。
【０１４５】
図２４は、タイプ３Ａとタイプ３Ｂの無限増倍率が最大となる燃焼度の差が７GWd/t のときの、炉心断面における径方向相対出力分布の例である。このとき図１６の傾向とは逆に、まず炉心外周領域における相対出力が高くなり、次に炉心中心付近の相対出力が高くなることがわかる。即ち、タイプ３Ａとタイプ３Ｂの無限増倍率が最大となる燃焼度の差は大きすぎてもよくない。従って、本発明の請求項１３に示すように、タイプＡとタイプＢの無限増倍率が最大となる燃焼度の差は５GWd/t 以下がよい。
【０１４６】
【発明の効果】
以上のように、本発明に係る原子炉の初装荷炉心によれば、第１サイクル、特にその初期における炉心の径方向出力分布を平坦化することにより、熱的特性を改善することができ、炉心平均濃縮度を高めて経済性の向上が図れる。
【０１４７】
また、本発明に係る原子炉の炉心によれば、濃縮度の異なる３種類の燃料集合体、低濃縮燃料、中濃縮燃料、および高濃縮燃料から構成され、炉心最外周と最外周から２層目に全て高濃縮燃料が配置し、コントロールセルは全て低濃縮燃料を配置する炉心において、高濃縮燃料を２つのタイプに分け、一方のタイプを他方のタイプに比べて、可燃性毒物入り燃料棒本数が１本以上多く、かつ炉心装荷体系無限配列における中性子増倍率が最大となる燃焼度が低くすることにより、炉心特性を悪化することなく、取出燃料の残留濃縮度を低めて燃料経済性をさらに向上することができるという効果が奏される。
【図面の簡単な説明】
【図１】本発明の第１の実施形態による初装荷炉心を示す燃料集合体の配置図。
【図２】本発明の第１の実施形態の初装荷炉心に使用する高濃縮度燃料の無限増倍率の燃料変化を示す図。
【図３】本発明の第１の実施形態の初装荷炉心の第１サイクル初期の運転状態の炉心半径方向の位置と相対出力との関係を示す特性図。
【図４】本発明の第２の実施形態による初装荷炉心を示す燃料集合体の配置図。
【図５】本発明の第３の実施形態による初装荷炉心を示す燃料集合体の配置図。
【図６】本発明の関連技術による初装荷炉心を示す燃料集合体の配置図。
【図７】本発明の関連技術による初装荷炉心を示す燃料集合体の配置図。
【図８】本発明の関連技術による初装荷炉心を示す燃料集合体の配置図。
【図９】本発明の関連技術による炉心を示す燃料集合体の配置図。
【図１０】本発明の関連技術における燃料集合体の炉心装荷体系無限配列における中性子増倍率の燃焼度に対する変化を示す図。
【図１１】本発明の関連技術における多種類濃縮度炉心に装荷される低濃縮燃料、中濃縮燃料、高濃縮燃料それぞれの炉心装荷体系無限配列における中性子増倍率の燃焼度に対する変化を示す図。
【図１２】本発明の関連技術における可燃性毒物燃料棒本数が同じで可燃性毒物濃度が異なる場合における中性子増倍率の燃焼度に対する変化を示す図。
【図１３】本発明の関連技術における可燃性毒物濃度が同じで可燃性毒物燃料棒本数が異なる場合における中性子増倍率の燃焼度に対する変化を示す図。
【図１４】本発明の関連技術における炉心断面の径方向における相対出力分布を示す概念図であり、サイクルを通じて相対出力の最大値、即ち出力ピーキングが小さい安定した状態を示す図。
【図１５】本発明の関連技術において、高濃縮燃料の仕様が唯１種類のときで、最外周および最外周から２層目に高濃縮燃料を装荷した状態を示す配置図。
【図１６】本発明の関連技術において、炉心断面の径方向における相対出力分布を示す概念図。
【図１７】本発明の関連技術において、低濃縮燃料、中濃縮燃料、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料のー実施例を示す濃縮度・可燃性毒物濃度分布図。
【図１８】本発明の関連技術において、低濃縮燃料、中濃縮燃料、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料のー実施例を示す濃縮度・可燃性毒物濃度分布図。
【図１９】本発明の関連技術において、低濃縮燃料、中濃縮燃料、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料のー実施例を示す濃縮度・可燃性毒物濃度分布図。
【図２０】本発明の関連技術において、低濃縮燃料、中濃縮燃料、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料のー実施例を示す濃縮度・可燃性毒物濃度分布図。
【図２１】関連技術として記載した低濃縮燃料、中濃縮燃料、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料の炉心装荷体系無限配列における中性子増倍率（無限増倍率）の燃焼度に対する変化を示す図。
【図２２】本発明の関連技術における高濃縮タイプ３Ａ燃料を示す図。
【図２３】本発明の関連技術において、炉心径方向の相対出力分布を示す図。
【図２４】本発明の関連技術において、高濃縮タイプＡ燃料、高濃縮タイプＢ燃料の無限増倍率が最大となる燃焼度の差が７GWd/tである時における炉心径方向の相対出力分布を示す概念図。
【図２５】本発明が適用される燃料単位を示す模式的な平面図。
【図２６】本発明が適用される炉心の燃料配置を示す模式的な平面図。
【図２７】従来の初装荷炉心で平衡炉心を模擬した燃料集合体の配置図。
【図２８】従来の初装荷炉心で高濃縮度燃料の割合を増やした燃料集合体の配置図。
【図２９】従来例を説明するもので、最高濃縮度の初装荷燃料集合体と取替燃料集合体の無限増倍率の燃焼変化を比較して示す図。
【図３０】従来例を説明するもので、最高濃縮度の初装荷燃料集合体と取替燃料集合体との局所ピーキング係数の燃焼変化を比較して示す図。
【符号の説明】
Ａ制御棒
Ｂ燃料集合体
Ｃ燃料単位
Ｄ燃料単位を構成しない燃料集合体
Ｅ炉心
Ｆ反射体
Ｏ炉心中心
１低濃縮燃料
２中濃縮燃料
３高濃縮燃料
３Ａ高濃縮タイプＡ燃料
３Ｂ高濃縮タイプＢ燃料
１１高濃縮度低Ｇｄ燃料
１２高濃縮度高Ｇｄ燃料
１３中濃縮度燃料
１４低濃縮度燃料
１５コントロールセル
４１高濃縮度低Ｇｄ燃料
４２高濃縮度中Ｇｄ燃料
４３高濃縮度高Ｇｄ燃料
４４低濃縮度燃料
５１高濃縮度低Ｇｄ燃料
５２高濃縮度高Ｇｄ燃料
５３中濃縮度燃料
５４低濃縮度燃料
６１，６２高濃縮度燃料
６３中濃縮度燃料
６４低濃縮度燃料
７１高濃縮度燃料
７２中濃縮度低Ｇｄ燃料
７３中濃縮度高Ｇｄ燃料
７４低濃縮度燃料
８１１番目に濃縮度の高い燃料集合体
８２２番目に濃縮度の高い燃料集合体
８３３番目に濃縮度の高い燃料集合体
８４最低濃縮度の燃料集合体[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel loading technique in a core of a boiling water reactor, and more particularly to a nuclear reactor core that is intended to improve core characteristics and fuel economy.
[0002]
[Prior art]
FIG. 25 shows a general configuration example of the fuel unit loaded in the core of the boiling water reactor. That is, as shown in FIG. 25, four fuel assemblies B are loaded around the control rod A having a cross-shaped cross section, and thereby a fuel unit C is configured.
[0003]
FIG. 26 shows a configuration of a quarter (a quarter divided from the core center O) of the core D in which a large number of fuel assemblies are arranged. As shown in FIG. 26, the core D is generally shaped like a cylinder, a large number of fuel units C are arranged over the entire core, and a fuel unit is provided at a part of the outermost periphery of the core surrounded by the reflector E. Fuel assemblies F that do not constitute C are arranged. The fuel unit C composed of the fuel assemblies B arranged around the control rod A for adjusting the reactor power is called a control cell.
[0004]
In such a nuclear reactor, the fuel assemblies are exchanged for about one year, that is, every cycle operation, and 1/3 to 1/5 of the total fuel assemblies are taken out. The body is loaded. The fuel assembly taken out at the time of refueling is the fuel assembly having the lowest residual enrichment from the viewpoint of economy. For example, when the fuel assemblies are operated by exchanging 1/4 each, the fuel assemblies that have undergone the most combustion after 4 cycles, the fuel assemblies that have undergone moderate combustion, the fuel assemblies that have not undergone the combustion, ie, residual If the core is loaded with the fuel assemblies with the lowest, medium, and highest enrichment, then the fuel assemblies with the lowest residual enrichment will be removed after each fuel change and loaded with new fuel assemblies instead. Thereafter, the core in the same state is maintained.
[0005]
In this way, when the fuel assembly with the lowest residual enrichment is taken out and a new fuel assembly is loaded repeatedly, the core will be in a steady state where several types of fuel assemblies with different residual enrichments are mixed. Thus, there is almost no difference in core characteristics for each cycle. Such a core is called an equilibrium core. In a balanced core, the fuel assemblies are in the core for 3-5 cycles and are removed when the residual enrichment is low and output is not very high. The fuel assembly to be loaded is referred to as a replacement fuel assembly, in contrast to the initially loaded fuel assembly used in the initial loading core described later.
[0006]
In the initial loading core, that is, the core in which the fuel assemblies are loaded for the first time after the reactor is constructed, all the fuel assemblies are new fuels. However, it is necessary to change the fuel after the first cycle. This is because it is necessary to load new fuel to maintain the power of the entire core. Similarly, after the second cycle is completed, a fuel assembly that is taken out only by using two cycles is generated. As described above, during the period of about 3 cycles after the operation of the nuclear reactor, a fuel assembly is generated that is extracted only after being used for a short period of time.
[0007]
In the past, all of the initial loaded cores consisted of initially loaded fuel assemblies with the same average burnup, so the first few cycles were fuels with a high residual enrichment compared to the fuel assemblies removed from the equilibrium core. The aggregate was taken out. This was very uneconomical.
[0008]
In order to solve such problems, various improvements have been made to the initial loading core of a nuclear reactor. For example, the fuel assemblies loaded in the first loaded core, that is, the first loaded fuel assemblies are of several types with different enrichments, and the first cycle fuel (new fuel), the second cycle fuel, the third cycle fuel in descending order of enrichment. In view of this, an initial loading core simulating an equilibrium core is constructed. This is a core loading method called a multi-concentration core.
[0009]
FIG. 27 shows, as an example, a fuel assembly arrangement configuration of a quarter of the core D using four types of initially loaded fuel assemblies with different assembly average enrichments. The highest enrichment fuel assembly 7 simulates the first cycle fuel of the equilibrium core, the second enrichment fuel assembly 8 simulates the second cycle fuel, and the third enrichment fuel assembly. 9 simulates the fuel in the third cycle, and the fuel assembly 10 having the lowest enrichment simulates the fuel in the fourth and fifth cycles.
[0010]
In an equilibrium core, fuel is loaded from a low residual enrichment fuel at the outermost periphery of the core where neutron leakage is large and thermal neutron flux is low. In a multi-type enrichment core, the fuel having the lowest enrichment is loaded on the outermost periphery of the core based on the same concept. At the end of the first cycle, the fuel with the lowest enrichment is taken out and the replacement fuel assembly is loaded as new fuel. Since the low enrichment fuel is originally taken out at the end of the first cycle, the residual enrichment is low and the fuel economy is improved.
[0011]
In this way, an invention for further improving the fuel economy by further devising the arrangement of the initially loaded fuel assembly with respect to the core simulating an equilibrium core is disclosed in Japanese Patent Application Laid-Open No. 60-71987, “Boiling Water Atoms”. Furnace operating method ". In the disclosed invention, in the first cycle, the fuel assembly with the highest enrichment is arranged on the outermost peripheral portion of the core, the fuel assembly with the lowest enrichment is placed on the control cell, and after one cycle operation, the infinite multiplication factor is obtained. The lowest fuel assembly is loaded on the outermost periphery of the core and the control cell.
[0012]
As a result, the lowest enriched fuel assembly taken out in the first refueling is loaded not at the outermost periphery of the core where the thermal neutron flux is low and the combustion does not progress so much, but at the center of the core where the thermal neutron flux is high and the combustion proceeds. Therefore, the residual concentration becomes lower. In addition, the highest-concentration fuel assembly loaded on the outermost periphery does not burn much and can maintain a high output in the second cycle, thereby reducing the number of new fuel bodies required. However, the fuel economy is improved.
[0013]
Thus, the design of the initial loading core has been improved from the initial equilibrium core simulation aiming at higher economic efficiency. In order to improve economic efficiency, it is necessary to increase the average enrichment of the core, but it is necessary to increase the enrichment of the first loaded fuel assembly at the highest enrichment to be higher than that of the replacement fuel assembly. Difficult due to burnup constraints. Further, increasing the initial enrichment of the fuel assembly taken out at the end of the first and second cycles will increase the residual enrichment at the time of removal. Therefore, the improvement of the initial loading core is proceeding in the direction of increasing the ratio of the highest enriched fuel assembly in the initial loading fuel assembly.
[0014]
FIG. 28 is a fuel assembly layout diagram exemplifying such an initially loaded core, and shows a quarter configuration of the core using two types of initially loaded fuel assemblies having different average enrichments of the assemblies. . In the core D, the fuel unit C constituting the control cell 15 is the high enrichment fuel 16 and the low enrichment fuel 17, but the fuel unit C except for the fuel assembly constituting the control cell 15 is almost the highly enriched fuel 16. Among the 872 fuel assemblies, 672 high-

enrichment fuels

16 and 200 low-enrichment fuels 17 are provided.
[0015]
[Problems to be solved by the invention]
By the way, in the design of the core, it is the first to satisfy the operation limit values such as the maximum line power density and the minimum limit power ratio. Recent initial loaded cores, which have increased the proportion of the highest enriched fuel assemblies in the initially loaded fuel assemblies with the aim of improving economy, generally tend to be more stringent than equilibrium cores, and are a means of improving core characteristics. Without it, the economics of the first loaded core cannot be improved.
[0016]
FIG. 29 shows an example of a change in combustion at infinite multiplication factor in the initially loaded fuel assembly and the replacement fuel assembly with the highest enrichment. The enrichment of the initially loaded fuel assembly with the highest enrichment is the same as the replacement fuel assembly, as is commonly used. Since the degree of enrichment is the same, the comparison of the infinite multiplication factor can be directly replaced with the output comparison.
[0017]
The infinite multiplication factor of the replacement fuel assembly is gadolinia, which is a flammable poison at the beginning of combustion.
(Gd₂O_Three), The gadolinia is almost burned out at the end of one cycle of combustion, and the infinite multiplication factor is maximized. This is to keep the excess reactivity of the core almost constant throughout the cycle. Thereby, the fuel output of the second cycle is high at the beginning of the cycle, and the fuel output of the first cycle is high at the end of the cycle. The first-loaded fuel assembly with the highest enrichment should simulate the first-cycle fuel in the first-loaded core that has been fully simulated for the equilibrium cycle. Therefore, in the early stage of combustion, the infinite multiplication factor is higher than the infinite multiplication factor of the replacement fuel assembly.
[0018]
This is realized by making the number of gadolinia-containing fuel rods smaller than that of the replacement fuel assembly. However, the length of the first cycle is longer than the equilibrium cycle by including the trial run period, and the excess reactivity of the second cycle tends to be high, so the concentration of gadolinia is The highest enrichment initial loaded fuel assembly is higher than the replacement fuel assembly.
[0019]
FIG. 30 shows an example of the combustion change of the local peaking coefficient in the first enriched fuel assembly and the replacement fuel assembly with the highest enrichment.
[0020]
In a boiling water reactor, since there is a water gap in the fuel assembly, the output of the fuel rods on the outer periphery of the fuel assembly is relatively high. By making the enrichment of the fuel rods on the outer periphery of the fuel assembly higher than the enrichment of the fuel rods inside the fuel assembly, the output can be flattened to some extent, but the maximum enrichment of nuclear fuel material is limited. In this situation, in order to secure the average enrichment of the fuel assembly, the enrichment of the fuel rods on the outer periphery of the fuel assembly cannot be lowered sufficiently.
[0021]
Further, the output of the fuel rod with gadolinia is low, and the output of other fuel rods is relatively high accordingly. Fuel rods with higher power consumption consume more fissile material, so the power output decreases more quickly, and fuel rods with gadolinia increase in power as gadolinia decreases, so the fuel assembly decreases in local peaking coefficient as combustion progresses To do. Comparing the replacement fuel assemblies at the early stage of combustion, the initial loaded fuel assembly with the highest enrichment is slightly smaller than the replacement fuel assembly due to the difference in the number of fuel rods with gadolinia, but this is not a big difference. The calculation of the minimum limit output ratio uses an index called an R factor, and the larger the value, the more severe, but the initial combustion stage of the fuel assembly is the largest.
[0022]
The maximum linear power density becomes a problem when the output of the fuel assembly is high and the local peaking coefficient is also high. In the replacement fuel assembly, since the output of the fuel assembly is low at the initial stage of the fuel having the highest local peaking coefficient, it is not a thermal problem. On the other hand, since the output of the initially loaded fuel assembly with the highest concentration is high from the beginning of the first cycle in the initially loaded core, the maximum linear power density of this fuel becomes a problem. Further, the same problem as the maximum linear power density occurs with respect to the minimum limit power ratio. Therefore, the maximum linear power density and the minimum limit power ratio at the beginning of the first cycle tend to be stricter than the equilibrium cycle.
[0023]
Accordingly, a first object of the present invention is to provide an initial loading core of a nuclear reactor that can improve the core economy in the first cycle while improving the fuel economy by increasing the core average enrichment. is there.
[0024]
On the other hand, as described above, the fuel assembly contains a flammable poison. The flammable poison is gadolinium (Gd) or its oxide gadolinia (Gd).₂O_Three), Which has a strong property of absorbing neutrons, and when it absorbs neutrons, it is converted into a material with a weak property of absorbing neutrons. These flammable poisons absorb neutrons and suppress nuclear reactions between fissile materials and neutrons at the beginning of the cycle when there is an excessive amount of fissile materials such as U-235 (uranium 235). Sex poisons are also consumed, reducing their ability to absorb neutrons. In addition to the fuel rods loaded with fuel pellets made of only nuclear fuel material, the fuel assembly contains several to 10 or more fuels loaded with fuel pellets containing not only nuclear fuel materials but also flammable poisons. The initial excess reactivity is suppressed.
[0025]
In general, in the core of a nuclear reactor, the vicinity of the core center has a high thermal neutron flux and the combustion proceeds well. On the other hand, the effect of leakage of thermal neutrons to the outside of the core increases as it goes toward the outer periphery of the core. That is, the thermal neutron flux is also lowered, and the progress of combustion is worsened. In this way, there is a difference in the progress of combustion between the fuel loaded in the core peripheral region and the fuel loaded near the core center. That is, the fuel loaded near the core center burns flammable poisons quickly.
[0026]
On the other hand, the fuel loaded in the outer peripheral region of the core is slow in burning the combustible poison. If the highly enriched fuel loaded near the core center and the highly enriched fuel loaded in the second layer from the outermost periphery and the outermost periphery are exactly the same fuel, the highly enriched fuel near the core center will burn well first. Therefore, the flammable poison also attenuates efficiently, and the relative output of the region increases accordingly. As the combustion progresses further and the flammable poisons are reduced to some extent, the relative output in that region then decreases.
[0027]
On the other hand, since the highly concentrated fuel in the second layer from the outermost periphery and the outermost periphery of the core is difficult to burn, even when the combustible poison near the core center is reduced to some extent, the combustible poison remains considerably. The relative output of the area is still increasing. In other words, from the beginning of the cycle to a certain burnup, the power distribution in the radial direction in the core cross section is high in the core center region, and thereafter is high in the core outer peripheral region. It becomes.
[0028]
If the radial power peaking in the core cross section increases, the thermal parameters related to the operation of the reactor, such as the linear power density of the fuel assembly and the minimum critical power ratio, will deteriorate, leading to a reduction in operating margin. Accordingly, it is desirable that the radial power distribution in the core cross section is flat.
[0029]
In this way, it is composed of three types of fuels with different enrichments, ie, high enrichment, medium enrichment, and low enrichment fuels. In a core loaded with highly concentrated fuel not only in the outermost periphery of the core but also in the second layer from the outermost periphery in order to realize fuel economy, in order to bring about an increase in the radial power peaking in the core cross section as described above The problem arises that the operating margin of the core is reduced.
[0030]
Therefore, the second object of the present invention is comprised of a core for reducing the residual enrichment of the fuel taken out after the end of the first cycle and improving the fuel economy, that is, three kinds of fuel assemblies having different enrichments. In the reactor core, the highly enriched fuel is placed in the outermost layer and the second layer from the outermost periphery, and the low enriched fuel is placed in the control cell. The reactor core does not deteriorate the core characteristics throughout the cycle. Is to provide.
[0031]
[Means for Solving the Problems]
  In order to achieve the above object, according to the first aspect of the present invention, a fuel unit is constituted by arranging four fuel assemblies of four bodies around one control rod, and the fuel unit is spread over the entire core. In the reactor core in which the fuel assemblies that do not constitute the fuel unit are arranged on a part of the outer periphery of the core, the fuel assemblies are divided into two types depending on the difference in the average enrichment of the assemblies. A core that does not constitute a fuel unit by classifying as described above and further classifying the fuel assembly with the highest average enrichment of the assembly into two types according to the number of fuel rods containing combustible poisons contained in the fuel assembly. All of the fuel assemblies on the outer peripheral side and three or more of the four fuel assemblies constituting the fuel unit facing the reflector covering the outer periphery of the core have the highest enrichment and the number of fuel rods containing flammable poisons. Less fuel assembliesOf the fuel assemblies constituting the fuel unit facing the reflector, the fuel unit disposed closest to the center of the core is not the highest enrichment and the number of fuel rods containing flammable poisons is small. A fuel assembly that is not a thingAn initial loading core of a nuclear reactor is provided.
[0032]
According to the present invention, a fuel assembly having the highest enrichment and a small number of fuel rods containing flammable poisons is disposed on the outer peripheral portion of the core, so that the output of the outer peripheral portion of the core is increased and the core central portion is increased. Output is relatively low. That is, the radial output distribution can be flattened. This makes it possible to improve the thermal characteristics of the core such as maximum line power density and minimum limit power ratio. If a fuel assembly with high output is concentrated in one fuel unit, there is a possibility that a problem will occur in the reactor shutdown margin, but in the fuel unit facing the reflector on the outer periphery of the core, In addition, it is possible to load three or four fuel assemblies with a small number of fuel rods containing flammable poisons.
[0034]
  Also,According to the present invention, when one fuel assembly other than a fuel assembly having the highest enrichment and a small number of flammable poison-containing fuel rods is loaded on the fuel unit facing the reflector on the outer periphery of the core In order to flatten the radial power distribution and to secure a margin for shutting down the reactor, it is better to place fuel assemblies with the highest enrichment with high power and a small number of fuel rods containing flammable poisons outside the core as much as possible. It is advantageous.
[0035]
  Claim 2According to another aspect of the present invention, there is provided an initial loading core of a nuclear reactor according to claim 1, wherein all of the fuel assemblies constituting the fuel unit facing the reflector are the highest enrichment fuel assemblies. To do.
[0036]
According to the present invention, from the viewpoint of the furnace stop margin, one of the four fuel assemblies constituting the fuel unit facing the reflector has the highest enrichment and the number of fuel rods containing flammable poisons is small. In the case of other than the body, it is advantageous from the viewpoint of improving the economy that one body is a fuel assembly having the highest concentration and a large number of fuel rods containing flammable poisons. In addition, it is possible to secure a furnace shutdown margin.
[0037]
  Claim 3In the invention ofClaim 1 or 2In the reactor core, the fuel unit other than the fuel unit facing the reflector, and adjacent to the fuel unit facing the reflector, two or less of the four fuel assemblies have the highest enrichment and flammability. Provided is a nuclear reactor initial loading core characterized by a fuel assembly having a small number of poisonous fuel rods.
[0038]
According to the present invention, the fuel unit that does not face the reflector but is adjacent to the fuel unit that faces the reflector is slightly separated from the outermost periphery of the core, and the effect of neutron leakage is reduced. Yes. If the fuel assemblies that are likely to have high output are excessively concentrated, this portion will have the highest output in the core. For this reason, the number of fuel assemblies with the highest enrichment and the smaller number of fuel rods containing flammable poisons is limited, but the output of each fuel unit should be made to be approximately equal in terms of the furnace shutdown margin. Is desirable. Therefore, the number of fuel assemblies having the highest enrichment and a small number of fuel rods containing flammable poisons is set to two or less for each fuel unit.
[0039]
  Claim 4In the invention of claim 1,To 3In the reactor core according to any one of the above, the fuel unit that does not face the reflector and is not adjacent to the fuel unit that faces the reflector is not of the highest concentration and contains a flammable poison. An initial loading core of a nuclear reactor characterized by applying a fuel assembly that does not have a small number of fuel rods is provided.
[0040]
  According to the present invention, when a fuel assembly having the highest enrichment and a small number of fuel rods containing flammable poisons is loaded into the core, the output of the fuel assembly becomes too high and the thermal characteristics become severe. A fuel assembly that has been optimized to increase the power at the outer periphery of the core should not be loaded in the center of the core. From claim 1Claim 4Even in the cores up to, the output at the outermost periphery of the core is still quite low. The fuel assembly loaded on the outermost periphery of the core can further reduce the number of fuel rods containing flammable poisons.
[0041]
  Claim 5In this invention, a set of four fuel assemblies is arranged around one control rod to form a fuel unit, and a large number of fuel units are arranged over the entire core, while one on the outer peripheral side of the core. In the reactor core in which the fuel assemblies that do not constitute the fuel unit are arranged in the section, the fuel assemblies are classified into two or more types according to the difference in the average assembly enrichment, and further the average assembly enrichment The highest fuel assembly is classified into three types according to the number of fuel rods containing combustible poisons contained in the fuel assembly, and all fuel assemblies facing the reflector covering the outer periphery of the core are at the highest enrichment. In addition, there is provided an initial loading core of a nuclear reactor characterized in that the number of fuel rods containing flammable poisons is the smallest of the three types.
[0042]
According to the present invention, the increase in the number of types of fuel assemblies is disadvantageous in terms of manufacturing cost, but is effective in flattening the radial output distribution.
[0043]
  Claim 6In this invention, a set of four fuel assemblies is arranged around one control rod to form a fuel unit, and a large number of fuel units are arranged over the entire core, while one on the outer peripheral side of the core. In the reactor core in which the fuel assemblies that do not constitute the fuel unit are arranged in the section, the fuel assemblies are classified into two or more types according to the difference in the average assembly enrichment, and further the average assembly enrichment The highest fuel assembly is classified into two types according to the number of fuel rods containing combustible poisons contained in the fuel assembly, and all fuel assemblies facing the reflector covering the outer periphery of the core are of the highest enrichment. In addition, all fuel assemblies in the second layer from the outermost periphery of the core shall not have the highest enrichment and the number of fuel rods containing flammable poisons.In addition, fuel assemblies with the highest enrichment and a small number of fuel rods containing flammable poisons were loaded only within the second layer from the outermost periphery of the core.An initial loading core of a nuclear reactor is provided.
[0044]
  A core having the lowest enriched fuel assembly disposed on the outermost periphery of the core is disadvantageous for flattening the radial power distribution as compared with a core having the highest enriched fuel assembly disposed on the outermost periphery. However, according to the present invention, the fuel assemblies in the second layer from the outermost periphery of the core are all fuels with the highest enrichment and a small number of fuel rods containing flammable poisons. The directional output distribution can be flattened.Also, by limiting the fuel enrichment with the highest enrichment and the number of fuel rods with flammable poisons to the second layer from the outermost periphery of the core, the number of fuel rods with the highest enrichment and combustible poisons is small. The infinite multiplication factor at the initial stage of combustion of the fuel assembly can be considerably increased.
[0045]
  Claim 7In this invention, a set of four fuel assemblies is arranged around one control rod to form a fuel unit, and a large number of fuel units are arranged over the entire core, while one on the outer peripheral side of the core. In the reactor core in which the fuel assemblies that do not constitute the fuel unit are arranged in the section, the fuel assemblies are classified into three or more types according to the difference in the average assembly enrichment, and further the average assembly enrichment The highest fuel assembly is classified into two types according to the difference in the number of fuel rods containing combustible poisons contained in the fuel assembly, and all the fuel assemblies on the outermost periphery of the core have the medium average enrichment. It is assumed that the fuel assemblies and all the fuel assemblies in the second layer from the outermost periphery of the core have the highest enrichment and the number of fuel rods containing flammable poisons is small.In addition, fuel assemblies with the highest enrichment and a small number of fuel rods containing flammable poisons were loaded only within the second layer from the outermost periphery of the core.An initial loading core of a nuclear reactor is provided.
[0046]
  According to the present invention, even in a core in which a medium enriched fuel assembly is disposed on the outermost periphery of the core, the fuel assemblies in the second layer from the outermost periphery of the core are all at the highest enrichment and combustible. By using a fuel assembly with a small number of poisonous fuel rods, radial peaking can be effectively reduced.Also, by limiting the fuel enrichment with the highest enrichment and the number of fuel rods with flammable poisons to the second layer from the outermost periphery of the core, the number of fuel rods with the highest enrichment and combustible poisons is small. The infinite multiplication factor at the initial stage of combustion of the fuel assembly can be considerably increased.
[0079]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a reactor core according to the present invention will be described with reference to the drawings.
[0080]
  First Embodiment (Claim 1)~ 4Correspondence)
  FIG. 1 shows a first embodiment of the present invention and is a layout diagram showing a fuel assembly of a quarter part of the initial loading core D. FIG.
[0081]
In the present embodiment, the fuel assemblies are divided into three types, that is, high enrichment fuels 11 and 12, medium enrichment fuel 13, and low enrichment fuel 14 according to the difference in assembly enrichment. Further, the highly enriched

fuels

11 and 12 are divided into two types according to the difference in the number of fuel rods containing gadolinia, one being a highly enriched low Gd fuel 11 and the other being a highly enriched high Gd fuel 12. Therefore, the core D is composed of a total of four types of fuel assemblies.
[0082]
The high enrichment low Gd fuel 11 is 200, the high enrichment high Gd fuel 12 is 228, the medium enrichment fuel 13 is 184, and the low enrichment fuel 14 is 260, for a total of 872. The fuel assemblies that do not constitute the fuel unit on the outer periphery of the core D are all highly enriched and low Gd fuel 11. The fuel unit C facing the reflector E on the outer periphery of the core is composed of four high enrichment low Gd fuels 11, or three high enrichment low Gd fuels 11 and one high enrichment low Gd fuel 12. It consists of and.
[0083]
Among the fuel units C composed of three high enrichment low Gd fuels 11 and one high enrichment high Gd fuel 12, the position where the high enrichment high Gd fuel 12 is closest to the center O of the core D It is arranged in. Although not facing the reflector E, the fuel unit C adjacent to the fuel unit C facing the reflector E contains two, one, or none of the high enrichment low Gd fuel 11. Absent. The fuel unit C that does not face the reflector E and is not adjacent to the fuel unit that faces the reflector E does not contain any high enrichment low Gd fuel 11.
[0084]
In such a configuration of the core D, the fuel assembly located at the outer peripheral portion of the core has a neutron leakage, so the output is not so high. For this reason, the output of the fuel assembly located in the central portion O of the core D becomes relatively high and becomes thermally severe. In an equilibrium core, it is economically advantageous to arrange a fuel assembly with the most advanced combustion on the outermost periphery of the core and reduce neutron leakage. Like the “operation method of the boiling water reactor”, even if it is not a low neutron leakage core in the first cycle, it is not necessarily disadvantageous in terms of economic efficiency.
[0085]
If the output of the outer periphery of the core can be increased, the output of the fuel assembly in the core central portion O can be relatively lowered, so that the thermal characteristics can be improved. Even if the highest enriched

fuel assemblies

11 and 12 are arranged on the outermost peripheral portion of the core D, the radial output distribution becomes flatter than the case where the lowest enriched fuel assemblies 14 are arranged on the outermost peripheral portion. However, since

many fuel assemblies

11 and 12 with the highest enrichment are arranged inside the core D, the output of the

fuel assemblies

11 and 12 with the highest enrichment disposed at the outer peripheral portion is increased, but the inside of the reactor core is increased. It is necessary to consider a method in which the output of the

fuel assemblies

11 and 12 having the highest enrichment is not increased.
[0086]
The

fuel assemblies

11 and 12 with the highest enrichment suppress the initial output by the fuel rods with gadolinia. By changing the number of the fuel rods with gadolinia, the

fuel assemblies

11 and 12 with the same enrichment are in the initial stage of combustion. It is possible to vary the infinite multiplication factor.
[0087]
FIG. 2 shows the change in the burnup at infinite multiplication factor of the two kinds of maximum enrichment initially loaded

fuel assemblies

11 and 12 with different numbers of fuel rods containing gadolinia. As shown in FIG. 2, the difference between the infinite multiplication factors is the largest in the early stage of combustion, the difference is reduced with the combustion of gadolinia, and the difference is almost eliminated after gadolinia is burned out.
[0088]
FIG. 3 shows the effect of this embodiment, and is a characteristic diagram showing the relationship between the position in the core radial direction and the relative output in the operating state in the initial stage of the first cycle, and the power distribution of the core D of this embodiment. And the power distribution in the radial direction of the conventional core are compared. As a conventional core, an initial loaded core that simulates an equilibrium cycle is taken up.
[0089]
In the core D of the present embodiment, since the fuel assembly 11 having the highest enrichment and a small number of gadolinia-containing fuel rods is arranged on the outer periphery of the core, the output is higher than that of the conventional core, and the core is relatively The output of the part becomes low. In this way, by making the output of the fuel assembly uniform, the thermal characteristics such as the maximum linear power density and the minimum critical power ratio can be improved.
[0090]
However, from the viewpoint of the furnace shutdown margin, it is disadvantageous to concentrate the fuel that tends to increase the output. Furnace shutdown margin refers to the subcriticality when the control element having the highest reactivity control effect is pulled out in the cold state and all other control rods are inserted. When the fuel whose output tends to be high is concentrated in one fuel unit, the change in the reactivity due to the control rod having the highest reactivity control effect being pulled out increases. In a state where most control rods are inserted at a low temperature, the neutron travel distance is smaller than in the operating state, so the influence of neutron leakage from the outer periphery of the core is limited to about one or two layers on the outermost periphery of the core. Therefore, three or more highly enriched low Gd fuels 11 can be loaded on the fuel unit C facing the reflector E on the outer periphery of the core, but two highly enriched low Gd fuels 11 are loaded on the fuel unit C inside. The following is necessary from the viewpoint of furnace shutdown margin.
[0091]
In the present embodiment, it is possible to realize an initial loading core that improves thermal characteristics by flattening the radial power distribution and has a furnace shutdown margin that is comparable to the conventional initial loading core.
[0092]
  (Second Embodiment) (BillingItem 5 pairsResponse)
  FIG. 4 shows a second embodiment of the present invention and is a fuel assembly layout diagram of a quarter part of the initial loading core.
[0093]
In the present embodiment, the fuel assemblies are classified into two types, that is, high enrichment fuels 41, 42, 43 and low enrichment fuels 44, depending on the difference in the enrichment of the assemblies, and further, the highly enriched fuels 41, 42. , 43 are divided into three types according to the difference in the number of fuel rods containing gadolinia, that is, a highly enriched low Gd fuel 41, a highly enriched medium Gd fuel 42, and a highly enriched high Gd fuel 43. Therefore, the core D is composed of a total of four types of fuel assemblies. The high enrichment low Gd fuel 41 is 132 bodies, the high enrichment medium Gd fuel 42 is 164 bodies, the high enrichment high Gd fuel 43 is 380 bodies, the low enrichment fuel 44 is 196 bodies, a total of 872.
[0094]
The core D according to this embodiment is aimed at extremely high economic efficiency, and almost all except the control cell 15 are highly enriched fuels 41, 42 and 43. As described above, the highly enriched fuels 41, 42, and 43 are divided according to the difference in the number of fuel rods containing gadolinia. In addition to the 92 fuel assemblies at the outermost periphery of the core, the 40 fuel assemblies that come into contact with the reflector E at the corners are the high enrichment low Gd fuel 41. Since the leakage of neutrons is large, the infinite multiplication factor at the initial stage of combustion of the high enrichment low Gd fuel 41 can be considerably increased.
[0095]
Inside the core, the highly enriched medium Gd fuel 42 and the highly enriched high Gd fuel 43 are arranged. In the second and third layers from the outer periphery of the core, the relatively high medium enriched Gd fuel 42 is present. It is trying to become. Since a fuel assembly with an infinite multiplication factor at the initial stage of combustion can be arranged on the outermost peripheral part, the output distribution in the radial direction is further increased compared to the case where high enriched fuel is divided into two types according to the number of fuel rods containing gadolinia. Flattening can be achieved.
[0096]
  (Third embodiment)Item 6 pairsResponse)
  FIG. 5 shows the third embodiment of the present invention and is a fuel assembly layout diagram of a quarter of the initial loading core.
[0097]
The fuel assemblies are classified into three types, that is, high enrichment fuels 51 and 52, medium enrichment fuel 53, and low enrichment fuel 54 depending on the assembly enrichment, and the high enrichment fuels 51 and 52 are gadolinia. Depending on the number of fuel rods entered, there are two types: high enrichment low Gd fuel 51 and high enrichment high Gd fuel 52. Therefore, the core D is composed of a total of four types of fuel assemblies. There are 88 high enrichment low Gd fuels 51, 344 high enrichment high Gd fuels 52, 204 medium enrichment fuels 53 and 236 low enrichment fuels 54, a total of 872.
[0098]
The first-loaded cores of the first and second embodiments described above are those in which highly enriched fuel is arranged on the outermost periphery of the core D, but the first-loaded core of the third embodiment is the core of the core D. The low enrichment fuel 54 is arranged on the outermost periphery. When the low enrichment fuel 54 is arranged on the outermost periphery of the core D, it is disadvantageous for the flattening of the radial power distribution as compared with the case where the high enrichment fuels 51 and 52 are arranged. Is possible. For this reason, the highly enriched low Gd fuel 51 is disposed in the second layer from the outermost periphery of the core. The high enrichment low Gd fuel 51 can be considerably increased in infinite multiplication factor at the initial stage of combustion by not loading the fuel other than the second layer from the outermost periphery of the core.
[0099]
  (Fourth embodiment)Item 7 pairsResponse)
  FIG. 6 shows a fourth embodiment of the present invention and is a fuel assembly layout diagram of a quarter part of the initial loading core.
[0100]
The fuel assemblies are classified into three types, that is, high enrichment fuels 61 and 62, medium enrichment fuel 63, and low enrichment fuel 64 depending on the assembly enrichment, and the high enrichment fuels 61 and 62 are gadolinia. There are two types of fuel rods depending on the number of fuel rods, ie, high enrichment low Gd fuel 61 and high enrichment high Gd fuel 62. Therefore, the core D is composed of a total of four types of fuel assemblies. There are 88 high enrichment low Gd fuels 61, 348 high enrichment high Gd fuels 62, 236 medium enrichment fuels 63, 200 low enrichment fuels 64, a total of 872.
[0101]
According to the fourth embodiment, even when the intermediate enrichment fuel 63 is disposed on the outermost peripheral portion of the core D, the second layer from the outermost periphery of the core D is the same as in the initially loaded core of the third embodiment. By arranging the highly enriched low Gd fuel 61 in the radial direction, the radial output distribution can be flattened.
[0102]
  FIG. 7 shows the related technology of the present invention.FIG. 4 is a fuel assembly layout diagram of a quarter of the initial loading core.
[0103]
The fuel assemblies are classified into three types, that is, high enrichment fuel 71, medium enrichment fuels 72 and 73, and low enrichment fuel 74, depending on the assembly enrichment, and the intermediate enrichment fuels 72 and 73 are gadolinia. There are two types according to the difference in the number of fuel rods, that is, medium enrichment low Gd fuel 72 and medium enrichment high Gd fuel 73. Therefore, the core D is composed of a total of four types of fuel assemblies.
[0104]
There are 436 high enrichment fuels 71, 92 medium enrichment low Gd fuels 72, 144 medium enrichment high Gd fuels 73, and 200 low enrichment fuels 74, a total of 872.
[0105]
In the initial loading core of the present embodiment, the intermediate enrichment fuel 72 is arranged on the outermost peripheral portion of the core D as in the initial loading core of the fourth embodiment. By dividing the number of rods into two types, the medium enriched low Gd fuel 72 is arranged on the outermost periphery of the reactor core, thereby flattening the radial power distribution.
[0106]
  FIG. 8 shows related technology of the present invention.FIG. 4 is a fuel assembly layout diagram of a quarter of the initial loading core.
[0107]
In this embodiment, there are four types of fuel assemblies depending on the difference in the enrichment of the assemblies, that is, the fuel assembly 81 with the highest enrichment, the fuel assembly 82 with the second highest enrichment, and the third with the highest enrichment. The fuel assemblies 83 are classified into the fuel assemblies 84 having the lowest enrichment. Therefore, the core D is composed of a total of four types of fuel assemblies.
[0108]
Among these, the fuel assembly 83 with the third highest enrichment and the fuel assembly 84 with the lowest enrichment do not have gadolinia-containing fuel rods. 436 fuel assemblies 81 with the highest enrichment, 144 fuel assemblies 82 with the second highest enrichment, 92 fuel assemblies 83 with the third most enrichment, and fuel assemblies 84 with the lowest enrichment. There are 200 bodies, a total of 872.
[0109]
In the present embodiment, the output of the

fuel assemblies

83 and 84 that do not have gadolinia-containing fuel rods rapidly decreases from the beginning of combustion. In this case, if a fuel assembly 83 that has a certain degree of enrichment and does not have gadolinia-containing fuel rods is used on the outermost periphery of the core, the radial output distribution can be flattened only at the initial stage of the first cycle. Since the fuel assembly at the outermost periphery of the core is unlikely to have high output, it is not necessary to insert gadolinia-containing fuel rods even in a highly concentrated fuel assembly.
[0110]
9 to 16 show related techniques of the present invention.Is shown.
[0111]
FIG. 9 shows the core DFIG. 4 is a layout diagram of a fuel assembly of a quarter part.
[0112]
The core D is composed of a low-enriched fuel assembly 1, a medium-enriched fuel assembly 2, and a highly-enriched fuel assembly 3, which are three types of fuel assemblies having different enrichments, and the highly-enriched fuel assembly 3 is combustible. It consists of a highly enriched type 3A and a highly enriched type 3B with different numbers of fuel rods containing sex poisons.
[0113]
The highly enriched type 3A has one or more fuels containing combustible poisons compared to the highly enriched type 3B, and the burnup that maximizes the neutron multiplication factor in the infinite array of core loading systems is that of the type 3B. Higher than. As for the loading pattern, the type 3B is disposed in the outermost periphery of the core and the second layer (indicated by hatching) from the outermost periphery, and the control cell 15 comprising the control rod A and the four fuel assemblies B has a low enriched fuel assembly. 1 is disposed, and a low-enriched fuel assembly 1, a middle-enriched fuel assembly 2, a highly enriched fuel type 3A, and a highly enriched fuel type 3B are appropriately disposed in the other portions to form a plurality of unit cells. doing.
[0114]
In this loading example, the low-enriched fuel assembly 1 is not disposed at a location adjacent to the control cell 15. The reason is as follows. That is, in the initial loading core composed of fuel assemblies of various enrichments, the difference in the neutron spectrum for each fuel in the core becomes remarkable. The control cell 15 composed of four low-enriched fuel assemblies 1 has a soft neutron spectrum, that is, a region where the thermal neutron flux is locally high in the core. Therefore, by disposing a fuel assembly other than the low enriched fuel assembly 1 at a location adjacent to the control cell 15, a region having a high thermal neutron flux is not locally expanded in the core. It is a consideration. In the outermost circumference and the second layer from the outermost circumference, neutron leakage is large and the thermal neutron flux is small, so that combustion does not proceed easily. On the other hand, there is little neutron leakage near the center of the core, and combustion proceeds well.
[0115]
FIG. 10 shows the change of the neutron multiplication factor (infinite multiplication factor) with respect to the burnup in the infinite array of fuel assemblies in the core loading system.
[0116]
Since the infinite multiplication factor of the fuel assembly is substantially proportional to the enrichment of the fissile material such as U-235, as the characteristic line a in FIG. 10 shows, the combustion progresses and the fissile material is consumed and becomes smaller. . However, when a flammable poison is added to the fuel, as indicated by the characteristic line b, the infinite multiplication factor at the initial stage of combustion is suppressed. When a flammable poison absorbs neutrons, it loses its own neutron absorption effect. As the combustion progresses, the flammable poison attenuates. Along with this, the value of the infinite multiplication factor increases. And after a certain burnup c, the infinite multiplication factor tends to decrease.
[0117]
FIG. 11 is a characteristic diagram showing a change in neutron multiplication factor (infinite multiplication factor) with respect to burnup in an infinite array of core loading systems of an initial loading core composed of three different enrichment fuel assemblies. In FIG. 11, the solid line d indicates the characteristics of the low enriched fuel assembly, the broken line b indicates the characteristics of the medium enriched fuel assembly, and the alternate long and short dash line f indicates the characteristics of the highly enriched fuel assembly.
[0118]
Low enriched fuel assemblies have an enrichment level of ~ 1.5wt%, which is originally lower than other fuel assemblies, the infinite multiplication factor is small, and most of them end the first cycle It will be taken out later. From the viewpoint of fuel economy, in the first cycle, it is necessary to burn the fissile material as much as possible to reduce the residual enrichment. Therefore, there is no fuel rod containing a flammable poison in order to make the thermal neutrons absorbed by the fuel dedicated as much as possible to fission, and the infinite multiplication factor of the low-enriched fuel assembly indicated by the solid line d decreases with the burnup.
[0119]
On the other hand, the enrichment of the intermediate enriched fuel assembly is 2. If the flammable poison is not added, the infinite multiplication factor in the early stage of combustion becomes as large as about 1.2, so a fuel devotion containing several flammable poisons is necessary to suppress it. It becomes. In addition, since most of the medium enriched fuel assembly is taken out at the end of the second cycle, the burnup at which the infinite multiplication factor is maximized is slightly lower than at the end of the first cycle, as shown by the broken line e. Adjust to. By doing so, the intermediate enriched fuel assembly in the second cycle looks like the low enriched fuel assembly in the first cycle, so that the thermal neutrons absorbed by the fuel rods can contribute to fission efficiently. It is possible to reduce the residual concentration at the time of taking out the result.
[0120]
Furthermore, the highly enriched fuel assembly has a higher enrichment than the intermediate enriched fuel assembly, and may be as high as 4.0 wt%. A larger number is required than in the case of a medium enriched fuel assembly. In addition, since the highly concentrated fuel assembly is taken out after the third cycle, the burnup at which the infinite multiplication factor is maximized is slightly lower than the burnup at the end of the second cycle, as indicated by the alternate long and short dash line f. There is a need to. By doing so, the highly enriched fuel assemblies in the third cycle and after become like low enriched fuel assemblies in the first cycle and medium enriched fuel assemblies in the second cycle, so that the residual enrichment at the time of extraction is reduced. can do.
[0121]
However, in the current design, as will be described later, since the upper limit value is determined with respect to the concentration of the flammable poison, it is not possible to obtain a burnup at which the required infinite multiplication factor is maximized only by the concentration of the flammable poison. In some cases. In such a case, it is necessary to increase the burnup at which the infinite multiplication factor is maximized by increasing the number of fuel rods containing flammable poisons. As described above, Gd (gadolinium) is an oxide form of Gd.₂O_ThreeIn order to suppress the infinite multiplication factor at the beginning of combustion, to increase the number of fuel rods added with a flammable poison, and to increase the burnup at which the infinite multiplication factor is maximized Should just increase the concentration of flammable poisons. However, Gd₂O_ThreeIncreasing the concentration is known to lower the thermal conductivity of the pellet and lower the melting point, which imposes restrictions on the output of the fuel rods.
[0122]
Therefore, at present, Gd₂O_ThreeThe upper limit concentration is limited to 7.5 wt%. As described above, in order to obtain a burnup at which the infinite multiplication factor of the highly enriched fuel assembly is maximized, a considerable length is required. Therefore, in recent design examples, the Gd of the highly enriched fuel assembly is required.₂O_ThreeThe upper limit concentration of 7.5 wt% is used as the concentration. In order to obtain a difference in burnup at which the infinite multiplication factor between Type A and Type B is maximized in a highly concentrated fuel assembly, there are roughly three ways of thinking.
[0123]
The first proposal is a method of changing the concentration of the flammable poison by equalizing the number of fuels containing the flammable poison. The second plan is to change the number of fuel rods containing flammable poisons by making the concentration of the flammable poisons equal. The third idea is an idea that combines them, and is a method of changing both the concentration of the flammable poison and the number of fuels containing the flammable poison.
[0124]
In the first proposal, as shown in FIG. 12, although the degree of combustion at which the infinite multiplication factor is maximized is changed, the effect of suppressing the infinite multiplication factor when unburned is hardly changed. On the other hand, in the second plan, as shown in FIG. 13, not only the burnup at which the infinite multiplication factor is maximized, but also different values are obtained for the infinite multiplication factor when unburned. In the 3rd plan, the effect which combined these is acquired. As a result, changing the number of fuels containing flammable poisons as in the second and third plans not only realizes a fuel with a different burnup that gives the maximum infinite multiplication factor, but also unburned fuel. It is possible to load fuel assemblies with different infinite multiplication factors at the time, and the flexibility in the core design is increased, which is a great advantage.
[0125]
In the case of the second plan, the flexibility in the core design is worse than that in the third plan. However, since the concentration type of the flammable poison is less than that in the third plan, it is advantageous in manufacturing. Incidentally, FIG. 12 and FIG. 13 show the case where the number of fuel rods containing the combustible poison is different in the fuel assembly having the same average enrichment of the fuel assembly but having the same concentration of the combustible poison (see FIG. 12). 13) and an example comparing the neutron multiplication factor (infinite multiplication factor) in the core loading system infinite array when the number of fuel rods containing the flammable poison is the same but the concentration of the flammable poison is different (Fig. 12) It is.
[0126]
In the characteristics shown in FIG. 13, the number of flammable poisons is larger in the fuel assembly in the characteristic line h than in the fuel assembly in the characteristic line g. Further, in the characteristics shown in FIG. 12, the concentration of the flammable poison is higher in the fuel assembly of the characteristic line j than in the fuel assembly of the characteristic line i.
[0127]
In the case of a core in which the highly concentrated fuel assembly is loaded in the second layer from the outermost periphery and the outermost periphery as in the present embodiment, the highly concentrated fuel assembly loaded in the second layer from the outermost periphery and the outermost periphery where combustion is difficult to proceed. The B type is assumed to have a short burnup c at which the infinite multiplication factor is maximized, and in some other regions, the highly enriched fuel having a burnup c at which the infinite multiplication factor is maximized is higher than that of the highly enriched fuel B type. By loading the A type, as shown in FIG. 14, it becomes possible to maintain the power balance in the core cross-sectional radial direction throughout the cycle.
[0128]
However, as shown in the core loading example of FIG. 15, when there is only one type of highly enriched fuel assembly 3, the following problems arise. That is, as illustrated in I of FIG. 16, even if the radial power distribution in the core cross section in the initial stage of the cycle is maintained in a well-balanced manner, the combustion of the combustible poison near the core center proceeds efficiently as the combustion proceeds. Along with this, as shown by II in FIG. 16, the relative output near the core center increases. However, as the combustion proceeds further, the flammable poison in the fuel near the center of the core decreases, and thereafter, the neutron multiplication factor decreases monotonically and the relative output decreases accordingly. On the other hand, even if the fuel in the outer periphery region of the core is in a state where the flammable poison in the fuel near the center of the core is reduced, the burnable poison still remains. In other words, while the relative power near the core center tends to decrease, the neutron multiplication factor of the fuel assemblies in the outer periphery of the core still increases, and the relative power tends to increase accordingly. is there. That is, from the middle of the cycle to the end of the cycle, as shown in III of FIG.
[0129]
Thus, the balance of the power distribution in the radial direction of the core cross section becomes worse. In this way, the power peaking in the core radial direction is high, which means that the thermal conditions imposed in the operation of the reactor such as the linear power density of the fuel and the minimum critical power ratio become severe. That is, the margin for reactor operation is reduced accordingly. In order to avoid this, in the core of the present embodiment, the B type infinite multiplication factor is maximized so that the relative power distribution in the radial direction of the core cross section becomes substantially flat throughout the cycle as shown in FIG. The burn-up c at which the A type infinite multiplication factor is maximized is made higher than the burn-up c. To increase the burnup at which the infinite multiplication factor is maximized, increase the concentration of the flammable poison as in the first proposal, or increase the number of fuel rods containing the flammable poison as in the second proposal. There is a method of increasing or combining them appropriately as in the third plan, but in this embodiment, the idea of the second or third plan having high flexibility in the core design is adopted, and the highly concentrated fuel The number of fuel rods containing a flammable poison of type A is increased by one or more than type B.
[0130]
Comparing the second and third proposals, the second proposal has a fixed concentration of flammable poisons, whereas the third proposal does not restrict the concentration and number of flammable poisons. Great design freedom. That is, when the fuel assembly based on the third plan is used, the fuel design and the core design become easier. However, since the 2nd plan has the fixed density | concentration of a combustible poison, it has the characteristics that it is advantageous from a viewpoint of processing economy on pellet manufacture. Therefore, if priority is given to the flexibility of fuel and core design, the third plan may be adopted, and if priority is given to economy, the second plan may be adopted.
[0131]
(Eighth embodiment) (Corresponding to claims 13 to 15)
The present embodiment relates to the concentration of each fuel and the distribution of combustible poisons, and examples of such distribution are shown in FIGS. In these figures, the enrichment of each fuel rod and the distribution of flammable poisons when control dedication A is in the upper left are shown. FIG. 17 shows an example of the enrichment distribution of the low enriched fuel, and FIG. 18 shows the intermediate enrichment. Fuel enrichment and Gd distribution example, FIG. 19 shows a highly enriched fuel A type enrichment and Gd distribution example, and FIG. 20 shows a highly enriched fuel B type enrichment and Gd distribution example, respectively.
[0132]
In these examples, a fuel assembly is shown in which fuel dedications are arranged in a square array of 9 rows and 9 columns, ○ indicates a fuel rod, and the number in ○ indicates the type of enrichment of the fuel rod. Concentration is higher in younger order. W is a water rod, which is a hollow tube through which water passes to secure a moderator in the vicinity of the center of the fuel assembly. This type of fuel has two thick water rods. G is a flammable poison Gd₂O_ThreeIt is a fuel dedication that includes.
[0133]
This embodiment is an example based on the second proposal already described, and the same Gd is used between the medium enriched fuel assembly and the highly enriched fuel assembly (type A, type B).₂O_ThreeConcentration of pellets using concentration makes it possible to improve economy at the time of fuel production.₂O_ThreeThe burnup at which the infinite multiplication factor is maximized is adjusted according to the number of fuels that contain.
[0134]
The aggregate enrichment, the number of flammable poison rods, and the concentration of the flammable poison in each fuel of the low enriched fuel assembly 1, medium enriched fuel assembly 2, and high enriched fuel assembly 3 (3A, 3B) loaded here. An example is shown in Table 1 below.
[0135]
[Table 1]

[0136]
FIG. 21 shows a neutron multiplication factor in an infinite array of core loading systems of these fuel assemblies.
The change with respect to the burnup of (infinite multiplication factor) is shown. That is, as shown in FIG. 21, when the number of fuel rods containing a flammable poison increases, the life of the flammable poison, that is, the degree of combustion until the infinite multiplication factor is maximized increases. 0 (a₁) 0GWd / t, 4 medium enriched fuel assemblies 2 (a₂) 9GWd / t, 10 highly concentrated fuel type 3B (a_3b) 17 GWd / t, 14 highly concentrated fuel type 3A (a in the second plan)_3A) Is 19GWd / t.
[0137]
Type 3A and type 3B shown here are Gd, which are flammable poisons, respectively.₂O_ThreeThere are 14 and 10 fuel rods each including, and the difference is four. The difference in burnup until the infinite multiplication factor is maximized is about 2 GWd / t. These are also preferred examples of

claims

13, 14, and 15.
[0138]
As in the second plan, when the concentration of the flammable poison is restricted to be the same between the type 3A and the type 3B, the type must be at least 4 so long as there is no difference in the number of fuel rods containing the flammable poison. A significant difference in the burnup at which the infinite multiplication factor of the 3A and type 3B fuel assemblies is maximized cannot be obtained.
[0139]
On the other hand, in the case of the third plan, since different values can be used for the type 3A and type 3B concentrations of the flammable poison, even if the difference in the number of fuel rods containing the flammable poison is less than four, the type 3A In addition, a significant difference in burnup can be made so that the infinite multiplication factor of the type 3B fuel assembly is maximized.
[0140]
The average enrichment of each fuel assembly, the number of flammable poison rods, the flammability of the low enriched fuel assembly 1, medium enriched fuel assembly 2, highly enriched fuel assembly 3A, and highly enriched fuel assembly 3B An example of the toxic concentration is shown in Table 2 below.
[0141]
[Table 2]

An example of the enrichment and flammable poison concentration distribution of the low enriched fuel assembly 1, medium enriched fuel assembly 2, and highly enriched fuel assembly type 3B is the same as that shown in FIGS.
[0142]
On the other hand, FIG. 22 shows an example of the concentration of type 3A and the flammable poison concentration distribution.
[0143]
In the type 3A shown in this typical example, as shown in 3A (3rd plan) of FIG. 21, the combustible poison is used so that the burnup at which the infinite multiplication factor becomes the maximum is 19 GWd / t as in the 2nd plan. The number of fuel rods included and the concentration of flammable poisons are adjusted. Even if the number of flammable poisons of type 3A is set to 10 that is the same as that of type 3B and the concentration of flammable poisons is further increased from the above value, an infinite increase is obtained as shown in 3A (first plan) in FIG. It is possible to obtain the same value as described above as the burnup with the maximum magnification. This is the idea of the first plan, but as in the second and third plans, by adding one or more difference in the number of fuel rods containing type 3A and type 3B flammable poisons, an infinite number of unburned fuel can be obtained. Differences in gain values can also be given, and flexibility in core design is high.
[0144]
In any of the first, second, and third methods, the radial relative power distribution in the core cross-section has a tendency as shown in FIG. take. That is, the output peaking, which is the maximum value of the relative output in the radial direction throughout the cycle, does not become so high, and good operating characteristics can be obtained. However, in the case of the first plan, the infinite multiplication factor when the type 3A and the type 3B are not burned is substantially equal, so the flexibility in the core design is low, whereas in the case of the second and third plans, the core The design flexibility is high, and several designs having good core characteristics can be obtained in addition to the fuel loading pattern example of FIG.
[0145]
FIG. 24 is an example of the radial relative power distribution in the core cross section when the difference in burnup at which the infinite multiplication factor between Type 3A and Type 3B is maximum is 7 GWd / t. At this time, contrary to the tendency of FIG. 16, it can be seen that the relative output in the core outer peripheral region first increases, and then the relative output near the core center increases. That is, the difference in burnup at which the infinite multiplication factor between Type 3A and Type 3B is maximized may not be too large. Therefore, as shown in claim 13 of the present invention, the difference in burnup at which the infinite multiplication factor between Type A and Type B is maximized is preferably 5 GWd / t or less.
[0146]
【The invention's effect】
As described above, according to the initial loading core of the nuclear reactor according to the present invention, the thermal characteristics can be improved by flattening the radial power distribution of the core in the first cycle, particularly in the initial stage, Economical improvement can be achieved by increasing the average concentration of the core.
[0147]
Further, according to the core of the nuclear reactor according to the present invention, it is composed of three types of fuel assemblies having different enrichments, a low enriched fuel, a medium enriched fuel, and a highly enriched fuel, and two layers from the outermost periphery and the outermost periphery of the core. In the core where all the highly concentrated fuel is placed in the eyes and all the control cells are placed in the low enriched fuel, the highly enriched fuel is divided into two types, and one type is compared to the other type, and a fuel rod containing a flammable poison. By reducing the burnup at which the number of neutrons is one or more and the neutron multiplication factor in the core loading system infinite arrangement is maximized, the residual concentration of the extracted fuel can be lowered and fuel economy can be improved without deteriorating the core characteristics. There is an effect that it can be further improved.
[Brief description of the drawings]
FIG. 1 is a layout diagram of a fuel assembly showing an initial loading core according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a fuel change at an infinite multiplication factor of a highly enriched fuel used in the initially loaded core according to the first embodiment of the present invention.
FIG. 3 is a characteristic diagram showing the relationship between the position in the core radial direction and the relative output in the initial operating state of the first cycle of the initially loaded core according to the first embodiment of the present invention.
FIG. 4 is a layout diagram of a fuel assembly showing an initial loading core according to a second embodiment of the present invention.
FIG. 5 is a layout diagram of a fuel assembly showing an initial loading core according to a third embodiment of the present invention.
FIG. 6 of the present inventionRelated technologyFIG. 3 is a layout diagram of a fuel assembly showing an initial loading core according to FIG.
[Fig. 7] of the present invention.Related technologyFIG. 3 is a layout diagram of a fuel assembly showing an initial loading core according to FIG.
[Fig. 8] of the present inventionRelated technologyFIG. 3 is a layout diagram of a fuel assembly showing an initial loading core according to FIG.
FIG. 9 shows the present invention.Related technologyFIG. 3 is a layout diagram of a fuel assembly showing a core by
FIG. 10 shows the present invention.Related technologyThe figure which shows the change with respect to the burnup of the neutron multiplication factor in the core loading system infinite arrangement of the fuel assembly in FIG.
FIG. 11 shows the present invention.Related technologyThe figure which shows the change with respect to the burnup of the neutron multiplication factor in the core loading system infinite arrangement of each of the low enrichment fuel, the medium enrichment fuel, and the highly enriched fuel loaded in the multi-enrichment core in FIG.
FIG. 12 shows the present invention.Related technologyThe figure which shows the change with respect to the burnup of the neutron multiplication factor in the case where the number of the burnable poison fuel rods is the same and the burnable poison concentration is different.
FIG. 13 shows the present invention.Related technologyThe figure which shows the change with respect to the burnup of the neutron multiplication factor in case the combustible poison density | concentration in the same and the number of combustible poison fuel rods differ.
FIG. 14 shows the present invention.Related technologyIt is a conceptual diagram which shows the relative output distribution in the radial direction of the core cross-section in FIG., And is a figure which shows the stable state with the minimum value of a relative output, ie, output peaking, small throughout a cycle.
FIG. 15 shows the present invention.Related technologyFIG. 5 is a layout view showing a state in which the highly enriched fuel is loaded on the outermost periphery and the second layer from the outermost periphery when the specification of the highly enriched fuel is only one type.
FIG. 16 shows the present invention.Related technologyIn FIG. 2, the conceptual diagram which shows the relative power distribution in the radial direction of a core cross section.
FIG. 17 shows the present invention.Related technologyFIG. 2 is a concentration / combustible poison concentration distribution diagram showing an embodiment of low enriched fuel, medium enriched fuel, highly enriched type A fuel, and highly enriched type B fuel.
FIG. 18 shows the present invention.Related technologyFIG. 2 is a concentration / combustible poison concentration distribution diagram showing an embodiment of low enriched fuel, medium enriched fuel, highly enriched type A fuel, and highly enriched type B fuel.
FIG. 19 shows the present invention.Related technologyFIG. 2 is a concentration / combustible poison concentration distribution diagram showing an embodiment of low enriched fuel, medium enriched fuel, highly enriched type A fuel, and highly enriched type B fuel.
FIG. 20 shows the present invention.Related technologyFIG. 2 is a concentration / combustible poison concentration distribution diagram showing an embodiment of low enriched fuel, medium enriched fuel, highly enriched type A fuel, and highly enriched type B fuel.
FIG. 21As related technologyThe figure which shows the change with respect to the burnup of the neutron multiplication factor (infinite multiplication factor) in the core loading system infinite arrangement of the described low enrichment fuel, medium enrichment fuel, highly enriched type A fuel, and highly enriched type B fuel.
FIG. 22 shows the present invention.Related technologyThe figure which shows the highly concentrated type 3A fuel in.
FIG. 23 shows the present invention.Related technologyFIG. 3 is a diagram showing a relative power distribution in the core radial direction.
FIG. 24 of the present inventionRelated technology2 is a conceptual diagram showing the relative power distribution in the core radial direction when the difference in burnup at which the infinite multiplication factor of the highly enriched type A fuel and the highly enriched type B fuel becomes maximum is 7 GWd / t.
FIG. 25 is a schematic plan view showing a fuel unit to which the present invention is applied.
FIG. 26 is a schematic plan view showing a fuel arrangement in a core to which the present invention is applied.
FIG. 27 is a layout diagram of a fuel assembly simulating an equilibrium core in a conventional initially loaded core.
FIG. 28 is a layout diagram of a fuel assembly in which the ratio of highly enriched fuel is increased in a conventional initial loading core.
FIG. 29 is a diagram illustrating a conventional example and comparing combustion changes at an infinite multiplication factor between a first-loaded fuel assembly having the highest enrichment and a replacement fuel assembly.
FIG. 30 is a diagram illustrating a conventional example and comparing combustion changes in local peaking coefficients between the first loaded fuel assembly and the replacement fuel assembly having the highest enrichment.
[Explanation of symbols]
A Control rod
B Fuel assembly
C Fuel unit
D Fuel assembly that does not constitute a fuel unit
E core
F reflector
O Core center
1 Low enriched fuel
2 Medium concentrated fuel
3 Highly enriched fuel
3A High enrichment type A fuel
3B highly enriched type B fuel
11 High enrichment low Gd fuel
12 High enrichment high Gd fuel
13 Medium enrichment fuel
14 Low enrichment fuel
15 Control cell
41 High enrichment low Gd fuel
42 Gd fuel with high enrichment
43 High enrichment high Gd fuel
44 Low enrichment fuel
51 High enrichment low Gd fuel
52 High enrichment high Gd fuel
53 Medium enrichment fuel
54 Low enrichment fuel
61,62 High enrichment fuel
63 Medium enrichment fuel
64 Low enrichment fuel
71 Highly enriched fuel
72 Medium enrichment low Gd fuel
73 Medium enrichment high Gd fuel
74 Low enrichment fuel
81 The first highly concentrated fuel assembly
82 Second most concentrated fuel assembly
83 The third most concentrated fuel assembly
84 Minimum enrichment fuel assembly

Claims

A fuel unit is configured by arranging a set of four fuel assemblies around one control rod, and a large number of fuel units are arranged over the entire core. In a nuclear reactor core in which fuel assemblies that do not constitute a fuel unit are arranged, the fuel assemblies are classified into two or more types according to the difference in the average enrichment of the assemblies, and the fuel having the highest average enrichment of the assemblies The assembly is classified into two types according to the number of fuel rods containing flammable poisons contained in the fuel assembly, and all of the fuel assemblies on the outer periphery side of the core that do not constitute a fuel unit and the reflector that covers the outer periphery of the core Three or more of the four fuel assemblies constituting the fuel unit facing the fuel assembly are the fuel assemblies having the highest enrichment and a small number of fuel rods containing flammable poisons, and constituting the fuel unit facing the reflector Of the fuel unit In most things placed near the core center, the maximum concentration not the degree, and initial core of a nuclear reactor, characterized in that the there is no fuel assemblies intended less burnable poison containing fuel rods number.

2. The reactor core according to claim 1, wherein all of the fuel assemblies constituting the fuel unit facing the reflector are the highest enriched fuel assemblies.

The reactor core according to claim 1 or 2 , wherein at least two of the four fuel assemblies other than the fuel unit facing the reflector and adjacent to the fuel unit facing the reflector are at most An initial loading core of a nuclear reactor characterized by a fuel assembly with a high degree of enrichment and a small number of fuel rods containing flammable poisons.

In the core according to any one of claims 1 to 3 , the fuel unit that does not face the reflector and is not adjacent to the fuel unit that faces the reflector is not of the highest enrichment. An initial loading core of a nuclear reactor characterized by applying a fuel assembly that does not have a small number of fuel rods containing flammable poisons.

A fuel unit is configured by arranging a set of four fuel assemblies around one control rod, and a large number of fuel units are arranged over the entire core. In a nuclear reactor core in which fuel assemblies that do not constitute a fuel unit are arranged, the fuel assemblies are classified into two or more types depending on the difference in the average enrichment of the assemblies, and the fuel having the highest average enrichment of the assemblies The assembly is classified into three types according to the number of fuel rods containing flammable poisons contained in the fuel assembly, and all the fuel assemblies facing the reflector covering the outer periphery of the core are fully enriched and combustible. An initial loading core of a nuclear reactor characterized in that the number of poisonous fuel rods is the smallest of the three types.

A fuel unit is configured by arranging a set of four fuel assemblies around one control rod, and a large number of fuel units are arranged over the entire core. In a nuclear reactor core in which fuel assemblies that do not constitute a fuel unit are arranged, the fuel assemblies are classified into two or more types according to the difference in the average enrichment of the assemblies, and the fuel having the highest average enrichment of the assemblies The assembly is classified into two types according to the difference in the number of fuel rods containing combustible poisons contained in the fuel assembly, and all the fuel assemblies facing the reflector covering the outer periphery of the core are of the highest concentration. The fuel assembly in the second layer from the outermost periphery of the core has the highest enrichment and the number of fuel rods with flammable poisons, and the fuel assembly with the highest enrichment and the number of fuel rods with flammable poisons Is limited to the second layer from the outermost circumference of the core Initial core of a nuclear reactor, characterized in that it was loaded Te.

A fuel unit is configured by arranging a set of four fuel assemblies around one control rod, and a large number of fuel units are arranged over the entire core. In a nuclear reactor core in which fuel assemblies that do not constitute a fuel unit are arranged, the fuel assemblies are classified into three or more types according to the difference in the average enrichment of the assemblies, and the fuel having the highest average enrichment of the assemblies The assemblies are classified into two types according to the difference in the number of fuel rods containing combustible poisons contained in the fuel assemblies, and all the fuel assemblies on the outermost periphery of the core are set as the fuel assemblies having the medium average enrichment. In addition, the fuel assemblies in the second layer from the outermost periphery of the core shall all have the highest enrichment and the number of fuel rods with combustible poisons, and the number of fuel rods with the highest enrichment and combustible poisons will be small. The fuel assembly is within the second layer from the outermost periphery of the core Initial core of a nuclear reactor, characterized in that the loading only.