JP4023758B2 - Reactor fuel assemblies and cores - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly and a core of a nuclear reactor, and more particularly to a fuel assembly and a core made of fuel rods enriched with plutonium.
[0002]
[Prior art]
In recent years, in the nuclear fuel cycle of nuclear reactors, the plutonium used to reprocess the spent fuel, collect the plutonium contained in the spent fuel, and load and use it on the BWR has been promoted. It has been. Hereinafter, a conventional fuel assembly composed of uranium fuel of BWR uses a so-called MOX fuel in which pellets composed of mixed oxide (MOX) in which plutonium oxide is mixed with uranium oxide as a base material are used. A case will be described.
[0003]
FIG. 4 is a longitudinal sectional view of a conventional BWR fuel assembly. The fuel assembly includes a plurality of fuel rods 1 in which a large number of fuel pellets (not shown) are enclosed, a rectangular tubular channel box 2 in which the fuel rods 1 are arranged in a grid, and an upper portion of the channel box 2. An upper tie plate 3 having a handle and a lower tie plate 4 below the channel box 2 and serving as a base for the entire fuel. Spacers 5 for maintaining gaps between the fuel rods 1 and the channel box 2 are provided at several locations in the axial direction of the fuel rod bundle.
[0004]
A conventional fuel rod arrangement for a MOX fuel assembly using MOX as a part of the fuel rod 1 will be described below. FIG. 5 is a cross-sectional view showing an example of a fuel rod arrangement of a conventional MOX fuel assembly, which is cited from FIG. 6 of Japanese Patent Laid-Open No. 3-128482. In the figure, reference symbols P1 to P6 denote fuel rods containing plutonium. The larger the subscript number, the lower the plutonium enrichment. The symbol G indicates a fuel rod which does not contain plutonium and is a uranium fuel rod having a concentration of 4.9% mixed with gadolinia as a flammable poison.
[0005]
As represented by the case shown in this figure, the MOX fuel assemblies that have been used in the mainstream in recent years include two types of MOX fuel rods that do not contain flammable poisons and uranium fuel rods that contain flammable poisons. It consists only of fuel rods. The reasons are mainly the following three points.
[0006]
(1) There are few irradiation data of the mixture of MOX and combustible poisons represented by gadolinia, and the characteristic is not fully grasped.
(2) Both plutonium oxide and gadolinia have lower thermal conductivity than uranium oxide, so we want to avoid combinations of these substances as much as possible.
(3) Since plutonium is extremely toxic to the human body and the nuclear material protection measures are severe, the manufacturing cost and transportation cost per unit of fuel assembly are very high.
[0007]
Therefore, as shown in FIG. 5, it is necessary to make all the fuel rods other than the fuel rod mixed with gadolinia MOX fuel rods and reduce the cost per unit plutonium amount as much as possible by increasing the amount of plutonium loaded in the fuel assembly integrally. .
[0008]
FIG. 6 is a cross-sectional view showing another example of the fuel rod arrangement of the conventional MOX fuel assembly, which is cited from FIG. 1 of the aforementioned Japanese Patent Laid-Open No. 3-128482. As in FIG. 5, reference symbols P1 to P3 denote fuel rods containing plutonium, and reference symbol G denotes a fuel rod which does not contain plutonium and which contains gadolinia as a flammable poison in a 4.5% enriched uranium fuel rod. ing. Reference numerals U1 to U3 denote uranium fuel rods having a concentration of 2.0 to 3.5%, which are arranged at the corners of the fuel assembly and the periphery thereof.
[0009]
In this case, the reason why the uranium fuel rod not containing the flammable poison is used in addition to the MOX fuel rod not containing the flammable poison and the uranium fuel rod added with the flammable poison is as follows.
[0010]
(4) Since the output of the fuel rod generally increases at the four corners of the fuel assembly and the periphery thereof, the plutonium enrichment is generally set low for the fuel rod at this portion. However, since MOX pellets have a higher molding cost than uranium pellets, it is economically inefficient to produce pellets with low plutonium enrichment.
(5) When changing the plutonium enrichment of pellets in the production line, it is necessary to clean the line, but the cost is very high, so the cost required for production by minimizing the number of enrichment types There is a desire to save time and time as much as possible.
[0011]
As described above, in designing a fuel assembly using MOX fuel, in addition to the fuel characteristics, requirements relating to cost and the like are taken into consideration.
[0012]
[Problems to be solved by the invention]
It can be said that the design concept of the MOX fuel assembly as described above is an extension of the design concept of the conventional uranium fuel assembly. That is, among the fuel rods in uranium fuel, those containing Gd or those with a particularly low enrichment are unsuitable or uneconomical as MOX fuel rods. The uranium fuel rod is replaced with a corresponding enriched MOX fuel rod.
[0013]
Such an idea is based on a proven uranium fuel design and is moderate in design, but is not necessarily an optimal choice in light of the special circumstances of MOX fuel described above.
[0014]
That is, it can be said that it is a desirable design from the viewpoint of cost effectiveness to load a large amount of Pu as the whole fuel assembly and to reduce the number of MOX fuel rods. For example, as described in the above prior arts (3) and (4), considering that the MOX fuel assembly is high in transportation / manufacturing costs, the enrichment of the MOX pellets is increased as much as possible, and 1 It is desirable to increase the amount of plutonium loaded into the body fuel as much as possible. From a cost standpoint, it is ideal that most of the fissile material in the fuel is plutonium.
[0015]
However, looking at the design examples described with reference to the drawings in the above prior art, in any case, enriched uranium having a concentration of 4% or more is used for the fuel rod containing the combustible poison, and the combustible poison is also used. Enriched uranium with a degree of enrichment of 2.0 to 3.5% is used for the uranium-only fuel rod that does not contain uranium. In such a configuration, it is clear that the uranium fuel rods bear a significant portion of the reactivity of these fuels, and the corresponding amount of plutonium loading is reduced compared to the ideal design.
[0016]
In other words, it can be seen that the uranium fuel rods are subjected to high production costs and transportation costs similar to those of the MOX fuel assembly, and from this point of view, the design is greatly impaired in economic efficiency.
[0017]
From this point of view, an example of a design that is more desirable than the cases shown in FIGS. 5 and 6 will be introduced. FIG. 7 is a cross-sectional view showing another example of the arrangement of fuel rods in a conventional MOX fuel assembly, which is taken from FIG. 7 of Japanese Patent Publication No. 5-8398. In FIG. 7, reference symbols P <b> 1 to P <b> 3 are fuel rods containing plutonium. The greater the subscript number, the lower the plutonium enrichment. Reference symbol G indicates a fuel rod that does not contain plutonium and is mixed with gadolinia as a flammable poison in a uranium fuel rod made of natural uranium.
[0018]
In this configuration, the number of uranium fuel rods in the fuel assembly composed of 62 fuel rods is only four, which is indicated by the symbol G including a flammable poison. These fuel rods G are arranged at the corners of a fuel assembly having a large thermal neutron flux and have a large amount of neutron absorption. Therefore, the number of uranium fuel rods is smaller than the design examples shown in FIGS. It became possible to do. Furthermore, since natural uranium is used for these fuel rods G, the amount of uranium 235 contained in the uranium fuel rods in the fuel assembly is smaller than in the case of FIGS. 5 and 6 using enriched uranium. It is very low.
Therefore, it can be said that the loading amount of plutonium is almost maximized under the design conditions of the fuel by the design shown in FIG.
[0019]
However, in this case, since the gadolinia-containing fuel rods are arranged at the four corners of the fuel assembly, the thermal neutrons to the plutonium-enriched fuel rods arranged at the other corners at the initial stage of combustion. Supply is low. Then, since the rate of neutron moderation by the moderator in the fuel assembly becomes relatively high, the influence of the change in the boiling state on the reactivity increases. The ratio of the change in reactivity to the change in void fraction is called the void coefficient. When the absolute value increases, the change in the core output when a transient change occurs in the reactor increases. It will have a negative impact on health. At this time, it is also known that the stability deteriorates.
[0020]
Further, in the design of FIG. 7, the enrichment in the MOX fuel rod and the uranium fuel rod is all uniform in the axial direction, and excessive output peaking occurs at the lower part of the fuel assembly having a low void ratio depending on the operating conditions. there is a possibility. From that point of view, it may be better to use enriched uranium with as low enrichment as possible within the range in which the enrichment distribution in the axial direction can be set instead of natural uranium.
[0021]
Furthermore, when designing such MOX fuel, there are issues to be considered from the viewpoint of core operation. In other words, the conventional nuclear power plant is often used for base load as a power supply source, and the operation period is relatively constant. However, in recent years, the fluctuation of the operation period is increasing for each cycle due to various reasons. . Therefore, in a cycle with a long operation period, it is necessary to load a large amount of new fuel at the time of regular inspection and increase the amount of combustible poisons brought into the core. Generally, the core reactivity tends to be insufficient at the beginning of the cycle. In the cycle with a short operation period, the opposite situation occurs, and the core reactivity at the beginning of the cycle is too high. Therefore, in any case, it is required to reduce the fluctuation of the core reactivity at the beginning of the cycle.
[0022]
As one of the countermeasures, in the case of uranium fuel, the reactivity is adjusted by changing the loading ratio using two kinds of fuels with different amounts of flammable poisons and enrichment. It is desirable to use the same method for the MOX fuel. However, when manufacturing MOX fuel rods, it is necessary to make a long-term plan including a reprocessing process, and the time required for manufacturing is so long that the design change is not in time for the change in the operation plan immediately before the reactor operation. There are special circumstances. Therefore, when the above-described reactivity adjustment is performed on the MOX fuel, the uranium fuel rod in the fuel assembly must be changed in design.
[0023]
Examining the three design examples from FIG. 5 to FIG. 7 from this point of view, it can be seen that the number of gadolinia cannot be increased without changing the MOX fuel rods in FIG. 5 and FIG. In addition, in the example of FIG. 6, regardless of the presence or absence of gadolinia, a high enrichment is already used for the uranium fuel rod, so when the uranium enrichment is further increased, the output of the uranium fuel rod becomes excessively high, There is a possibility of damaging the margin.
[0024]
The present invention has been made to solve the above-mentioned problems. In the MOX fuel assembly, the manufacturing cost per unit amount of plutonium is reduced, and a fuel assembly with high fuel economy is provided. The purpose is to ensure the soundness of the fuel and improve the drivability.
[0025]
[Means for Solving the Problems]
In order to solve the above problems, the invention according to claim 1 of the present invention is:A plutonium-free first fuel rod consisting of natural uranium, depleted uranium by-produced in the uranium enrichment process, recovered uranium recovered by reprocessing spent fuel, or a mixture thereof, and plutonium-rich The fuel assembly of the nuclear reactor is formed by bundling a second fuel rod formed into a gas and a water rod in which a coolant circulates in a lattice shape and surrounded by a channel box, and is located at the corner of the fuel assembly. At least a part of the fuel rod is the first fuel rod to which a flammable poison is added, and the fuel rod disposed at a position adjacent to the fuel rod is the first fuel rod to which the flammable poison is not added. A fuel assembly for a nuclear reactor is provided.
[0028]
With this configuration, the number of uranium fuel rods containing combustible poisons in the fuel assembly can be minimized and the plutonium loading can be maximized under given design conditions without deteriorating the void coefficient.
[0029]
Also,Claim 2According to the present invention, at least a part of the first fuel rod includes a plurality of regions in which at least one of the combustible poison concentration and the uranium enrichment is different in the axial direction. With this configuration, the assembly axial direction output distribution can be adjusted so as not to produce excessive output locally, and the soundness of the fuel can be improved.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 shows a fuel assembly according to a first embodiment of the present invention. FIG. 1 (a) is a cross-sectional view showing a fuel arrangement in the fuel assembly, and FIG. 1 (b) is a shaft of each fuel rod. It is a figure which shows direction enrichment and gadolinia distribution. In the figure, the same parts as those in the prior art are denoted by the same reference numerals, and redundant description is omitted.
[0036]
In FIG. 1A, symbols P1, P2, and P3 are MOX fuel rods containing plutonium (full length fuel rods), symbol PV is an MOX fuel rod containing plutonium (partial length fuel rods), and symbol U1 is uranium made of natural uranium. The fuel rod, symbol G1, indicates a uranium fuel rod obtained by mixing gadolinia as a flammable poison in natural uranium. In addition, a water rod represented by the symbol W and through which the coolant flows is disposed at the center of the fuel assembly.
[0037]
In FIG. 1B, symbols p1, p2, and p3 indicate the plutonium enrichment, and the greater the subscript number, the smaller the plutonium enrichment. Further, e1 indicates the uranium enrichment, g1 and g2 indicate gadolinia concentrations, and g2 is set to be smaller than g1.
[0038]
As shown in FIG. 1, gadolinia-containing uranium fuel rods G1 are arranged at the corners of the fuel assembly. Two natural uranium fuel rods U1 not containing plutonium are disposed adjacent to each gadolinia-containing uranium fuel rod G1, and fuel rods P3 or PV having a low plutonium enrichment are disposed adjacent to U1. Further, this gadolinia-containing uranium fuel rod G1 changes the gadolinia concentration in the axial direction and uses natural uranium at the lower end and the upper end, but the upper axial region (e1 + g1) excluding the upper end is The gadolinia density is set higher than the axially lower region (e1 + g2) excluding the lower end.
[0039]
Further, a natural uranium fuel rod U1 is disposed at a position in two directions in contact with a water rod W provided in the center of the fuel assembly, and a fuel rod P1 having a high plutonium enrichment is disposed at a position adjacent to U1 or W. To do.
[0040]
That is, in FIG. 1 illustrating the present embodiment, the fuel rod U1 or G1, which does not contain plutonium and has a low enrichment of uranium 235, is positioned adjacent to the water rod on two sides as a position not adjacent to the channel box. Two and twelve are arranged at positions adjacent to the channel box. Further, four of the twelve rods adjacent to the channel box are composed of fuel rods G1 containing gadolinia as a flammable poison.
[0041]
As an example of the uranium enrichment e1 of the natural uranium fuel rod U1 or gadolinia-containing uranium fuel rod G1 used here, an average value of natural uranium is assumed to be about 0.7%. However, since these fuel rods U1 and G1 are arranged at the corners of the fuel assembly, in the production of U1 and G1, a low enrichment having a enrichment lower than the enrichment of uranium fuel rods that have undergone a normal enrichment process. Uranium must be used, but this is not limited to natural uranium.
[0042]
That is, instead of natural uranium, the fuel rods U1 and G1 may use deteriorated uranium produced as a by-product in the uranium enrichment process or recovered uranium recovered by reprocessing spent fuel. The fuel rods U1 and G1 may be a mixture of combinations selected from natural uranium, deteriorated uranium, and recovered uranium. In this case, it shall be manufactured without purifying the mixture after purification.
[0043]
Even when deteriorated uranium or recovered uranium is used, the uranium enrichment of the fuel rods U1 and G1 is set to be low as in the case of natural uranium. The enrichment of natural uranium is about 0.7% and the average enrichment of depleted uranium is 0.2-0.3%. On the other hand, the enrichment of recovered uranium varies depending on the enrichment before combustion of the spent fuel before reprocessing and the combustion conditions, and is generally in the range up to about 1.2%. Therefore, in order to obtain a high effect of reducing the enrichment and reducing the cost for fuel production, the uranium enrichment of the fuel rods U1, G1 other than natural uranium is set to a value not exceeding about 1.2%. Is preferred.
[0044]
Four points will be described as operational effects in the present embodiment.
(1) In this embodiment, the fuel rods other than the MOX constituting the fuel assembly are fuel rods U1 and G1, all using natural uranium, and the amount of U235 is very small. Therefore, the amount of plutonium in the MOX fuel rod can be increased as compared with the case where conventional enriched uranium is used.
[0045]
(2) The fuel rod including gadolinia is only the fuel rod G1 disposed at the corner of the fuel assembly. By disposing G1 only at a position where the value as a flammable poison is high, the number of fuel rods containing gadolinia in the fuel assembly can be reduced to four.
[0046]
(3) A natural uranium fuel rod U1 is disposed adjacent to the gadolinia-containing fuel rod G1. As a result, since the thermal neutron flux becomes higher around U1, the thermal neutron flux is sufficiently supplied to the MOX fuel rods P1, P2, P3, and PV arranged on the inner side of the outermost periphery of the fuel assembly. The coefficient can be lowered. In addition, the arrangement of U1 also increases the supply of thermal neutron flux to the fuel rod G1 with gadolinia, so that the gadolinia value of the fuel rod G1 can be further increased.
[0047]
(4) By arranging two natural uranium fuel rods U1 that do not contain enriched uranium inside the fuel assembly with a hard neutron spectrum and poor fuel combustion efficiency, the enrichment of MOX pellets in the MOX fuel rods is as much as possible It is increasing. This arrangement also has the effect of softening the neutron spectrum inside the fuel assembly.
[0048]
As described above, according to the present embodiment, an economical fuel in which the amount of MOX pellets produced is minimized by increasing the enrichment of MOX pellets while maximizing the amount of plutonium loaded in the fuel assembly. Can be realized. Furthermore, by reducing the void coefficient, it is possible to realize a highly stable fuel that does not impair fuel soundness even during a transient change.
[0049]
In the present embodiment, the arrangement configuration has been described by taking a typical 9 × 9 type fuel assembly in a boiling water reactor as an example, but this type of arrangement may be another type such as a 10 × 10 type. The present invention can also be applied to other fuel assemblies. Further, the same arrangement can be given to a pressurized water reactor fuel assembly such as a 17 × 17 type.
[0050]
(Second Embodiment)
FIG. 2 shows a fuel assembly according to a second embodiment of the present invention. FIG. 2 (a) is a cross-sectional view showing the fuel arrangement in the fuel assembly, and FIG. 2 (b) is the axis of each fuel rod. It is a figure which shows direction enrichment and gadolinia distribution.
[0051]
The fuel assembly shown in FIG. 2 uses two types of MOX fuel rods and arrangement positions in the case of FIG. 1 as they are, reduces the number of uranium fuel rods U1, and adds two kinds of flammable poisons, gadolinia. The fuel rod is loaded.
[0052]
In FIG. 2, symbols P1 to P3 and PV denote the same MOX fuel rods as those of the same symbols in FIG. 1, and symbol U1 also denotes a uranium fuel rod made of natural uranium as in FIG. Symbol G3 indicates a fuel rod mixed with gadolinia only in the lower region in the axial direction excluding the lower end portion of the uranium fuel rod U1. E1 and e2 indicate uranium enrichment, and g3, g4, and g5 indicate gadolinia concentrations.
[0053]
That is, in FIG. 2 illustrating this embodiment, the fuel rods U1, G2 or G3 not containing plutonium and having a low enrichment of uranium 235 are adjacent to the water rod on two sides as positions not adjacent to the channel box. Two at the position and twelve at the position adjacent to the channel box. Of the 12 rods adjacent to the channel box, seven are composed of fuel rods G2 or G3 containing gadolinia as a flammable poison.
[0054]
In FIG. 2B, symbol e2 indicates the uranium enrichment, and its value is set higher than e1. That is, the gadolinia-filled uranium fuel rod G2 changes both the gadolinia concentration and the uranium enrichment in the axial direction, and uses natural uranium at the lower end and the upper end, but the upper axial region excluding the upper end (E2 + g3) sets the uranium enrichment higher than the axial lower region (e1 + g4) excluding the lower end. In addition, the gadolinia density is set to be different in these two regions.
[0055]
Therefore, the amount of uranium 235 loaded in the fuel assembly shown in FIG. 2 is larger than that in the fuel assembly in FIG. 1 by the amount of enrichment set higher in the upper region of the gadolinia-filled uranium fuel rod G2. High degree. Therefore, when the operation period of the nuclear reactor is increased, it can be coped with by mixing the fuel assemblies in FIG. 2 in a necessary ratio and using them by the same handling as the new fuel.
[0056]
Also, in order to prevent deterioration of core characteristics due to increased reactivity, the number of fuel rods containing gadolinia is increased to suppress reactivity from the beginning to the middle of the combustion, and at the same time prevent peaking in the lower axial direction due to the difference in the upper and lower enrichment be able to.
[0057]
The enrichment e2 in the upper region in the axial direction of the fuel rod G2 is set higher than the enrichment e1 in the lower region. On the other hand, it is desirable that the enrichment e2 is as low as possible for the purpose of improving the void coefficient. When natural uranium is used in the lower region, the enrichment e1 is about 0.7%.
[0058]
Usually, when there is an axial enrichment distribution in one fuel rod, it is necessary to provide a significant enrichment difference so that the distribution can be detected by the apparatus when performing nondestructive inspection after manufacturing. Considering that the distribution detection limit in the current non-destructive fuel rod non-destructive inspection system is 0.5%, the enrichment e1 and enrichment distribution of natural uranium are attached. .2% is preferable, and it is desirable to make it less than that in order to improve the void coefficient.
[0059]
In the present embodiment, the same operational effects as those of the first embodiment are obtained, and further, the fuel economy is excellent, and the MOX fuel having excellent fuel soundness is used, and the operation plan can be flexibly dealt with. A core that can be realized can be realized.
[0060]
In the present embodiment, the arrangement configuration has been described by taking a 9 × 9 type fuel assembly as an example, but the present invention can also be applied to other types of fuel assemblies such as a 10 × 10 type.
(Third embodiment)
FIG. 3 is a ¼ cross-sectional view showing an example of the arrangement of fuel rods in the core of a nuclear reactor according to the third embodiment of the present invention. The reactor core of the present embodiment is configured using the fuel assemblies according to the first and second embodiments. FIG. 3 is an overhead view of a quarter of the core in which fuel assemblies 8 shown in a square are arranged in a substantially cylindrical shape, and the cross shown by reference numeral 9 indicates the reactivity by inserting control rods during normal operation. The position to adjust is shown. Further, the square in the figure represents one fuel assembly, the symbol U is a uranium fuel assembly made up of uranium fuel rods not containing plutonium, and the symbols L and H are the first and second symbols, respectively. The MOX fuel assemblies according to the embodiment are respectively shown.
[0061]
As shown in FIG. 3, MOX fuel assemblies L and H are loaded with uranium fuel assemblies U in four adjacent directions. Although FIG. 3 is a quarter cross-sectional view of the core, uranium fuel assemblies U not containing plutonium are arranged in four adjacent directions of the MOX fuel assemblies even at a core position (not shown).
[0062]
The MOX fuel assemblies L and H have a plurality of MOX fuels not only having the same number of each type of MOX fuel rods indicated by reference numerals P1, P2, P3 and PV in FIGS. 1 and 2, but also having different enrichments. The arrangement position of the bars is common. That is, the MOX fuel assemblies L and H have exactly the same design with respect to the MOX fuel rods, the only difference being the configuration of the uranium fuel rods including low enrichment uranium fuel rods and gadolinia. That is, the difference between the fuel assemblies L and H is that the enrichment of uranium fuel rods, the axial distribution of enrichment, the number of fuel rods containing flammable poisons in the uranium fuel rods, or the combustible poisons. It can be said that this is a point relating to the axial distribution of the concentration of flammable poisons. (Note that any one of these elements can be the same design.)
In this way, by changing the design of the uranium fuel rods that have the same specifications for the MOX fuel rods and a short manufacturing period, the ratio of the numbers of the MOX fuel assemblies L and H can be easily changed. It is possible to easily cope with design changes.
[0063]
Further, as described above, the MOX fuel assembly H according to the second embodiment has a large gadolinia content and is designed to suppress the output peaking in the lower part of the fuel. In addition, since the average fissile material concentration is high, the contribution to the core reactivity is large at the end of the cycle. On the other hand, since the MOX fuel assembly L according to the first embodiment has a lower gadolinia content than H, even if it is placed in a place where the output is low and the burnup is difficult to proceed, the gadolinia burns by the end of the cycle. It does not cause a loss of core reactivity.
[0064]
Therefore, in this embodiment, as shown in FIG. 3 as an example of the core arrangement, L is arranged in the periphery of the core and around the control rod inserted during operation, and H is arranged in other portions with a small thermal margin. As a result, a thermal core with a large thermal margin and a high reactivity at the end of the cycle can be realized, and the power distribution and reactivity of the core can be adjusted flexibly in response to changes in the operation plan, etc. .
[0065]
【The invention's effect】
According to the present invention, the load of plutonium is increased while minimizing the manufacturing cost of MOX fuel, and a fuel assembly and a reactor core having excellent transient characteristics, stability, operational flexibility, and operational characteristics are obtained. Can do.
[Brief description of the drawings]
FIG. 1A is a diagram showing a fuel rod arrangement of a fuel assembly according to a first embodiment of the present invention, and FIG. 1B is a diagram showing an axial concentration distribution and a gadolinia distribution of each fuel rod.
2A is a diagram showing a fuel rod arrangement of a fuel assembly according to a second embodiment of the present invention, and FIG. 2B is a diagram showing an axial concentration distribution and a gadolinia distribution of each fuel rod.
FIG. 3 is a ¼ cross-sectional view showing an example of fuel rod arrangement in a reactor core according to a third embodiment of the present invention.
FIG. 4 is an elevational view showing a conventional fuel assembly in a partial cross section.
FIG. 5 is a cross-sectional view of an example of a conventional MOX fuel assembly.
FIG. 6 is a cross-sectional view of an example of a conventional MOX fuel assembly.
FIG. 7 is a cross-sectional view of an example of a conventional MOX fuel assembly.
[Explanation of symbols]
1 ... Fuel rod, 2 ... Channel box, 3 ... Upper tie plate,
4 ... Lower tie plate, 5 ... Spacer, 6 ... Water rod,
7 ... Control rod, 8 ... Fuel assembly,
9: Control rod position inserted to adjust reactivity during normal operation.

Claims (2)

A plutonium-free first fuel rod consisting of natural uranium, depleted uranium by-produced in the uranium enrichment process, recovered uranium recovered by reprocessing spent fuel, or a mixture thereof, and plutonium-rich The fuel assembly of the nuclear reactor is formed by bundling a second fuel rod formed into a gas and a water rod in which a coolant circulates in a lattice shape and surrounded by a channel box, and is located at the corner of the fuel assembly. At least a part of the fuel rod is the first fuel rod to which a flammable poison is added, and the fuel rod disposed at a position adjacent to the fuel rod is the first fuel rod to which the flammable poison is not added. A fuel assembly for a nuclear reactor, characterized by being a fuel rod. 2. The nuclear reactor according to claim 1, wherein at least a part of the first fuel rod includes a plurality of regions in which at least one of combustible poison concentration and uranium enrichment is different in an axial direction. Fuel assembly.

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