JP3433230B2 - Reactor core and nuclear fuel material replacement method in the core - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core of a nuclear reactor and a method of replacing a nuclear fuel material in the core, and more specifically, it does not require a combustion reactivity compensating means such as a control rod and adds a burnable poison. The present invention relates to a core of a nuclear reactor excellent in operability, safety, and economic efficiency, which can be continuously operated for a long period using only the enriched uranium and the combustion start part, and a method of replacing nuclear fuel material in the core. is there.

[0002]

2. Description of the Related Art As a conventional combustion method in a nuclear reactor, a method adopted in a light water reactor, a block fuel type high temperature gas reactor or a fast reactor is the most general. That is, in this method, the criticality of the core is maintained by inserting a control rod for combustion control into the core or pulling out the control rod inserted into the core for about one year. That is, control rods are inserted into the core at the early stage of combustion when a large amount of fissionable nuclides are present in the core, and fission products (Fission Product
s, hereinafter referred to as “FP”) and the like, the reactivity decreases and the inserted control rod is pulled out from the core. Then, at the end of the operation cycle, these control rods are removed.

When the operation cycle ends, the reactor is shut down, and first, the most burned fuel is taken out from the core and new fuel is newly loaded, and also the fuel, which is not replaced, is appropriately rearranged. As a result, the criticality can be maintained even in the next operation cycle. This is because, for example, fuel with relatively advanced combustion is moved to the outer periphery of the core, or conversely, it is put inside to achieve flat output,
It is a large-scale work to move almost all the fuel, such as not placing new fuels close to each other so that extremely high power parts are not generated in the core.

[0004]

However, the core of such a nuclear reactor and the method for replacing the nuclear fuel material in the core have the following problems.

That is, as described above, the cores of the light water reactor, the block fuel type high temperature gas reactor, and the fast reactor are preferable in terms of neutron economy because they compensate the combustion reactivity by absorbing neutrons by the control rods. Not a thing.

Further, in the core in which the combustion reactivity is compensated by the control rods in this way, if the inserted control rods are pulled out from the core due to a failure of the control rod drive system, an accident, etc. There is a problem that the reactivity may be suddenly injected into the fuel cell, and an abnormal reactivity injection accident may occur in which the output sharply rises and some fuel may be melted.

In place of the control rods, there has been proposed a method of providing a reflector on the outer periphery of the core for reflecting neutrons and returning it to the center side of the core, and controlling the combustion reactivity of the core by this reflector. However, this limits the controllable region to the outer periphery of the core.

A coolant for removing heat energy generated by nuclear fission is provided in the core throughout the core.
It flows at a substantially constant velocity along the axial direction of the core. Therefore, it is conceivable to control the flow rate of this coolant to control the output in the core instead of the control rods, but in these cores, in the core radial direction which is the direction perpendicular to the core axial direction. Since it has an output distribution and that distribution changes with combustion, in order to control both core power and cooling simultaneously by adjusting the flow rate of the coolant, the coolant flow rate distribution for each channel It has to be changed, which is difficult. In the pebble bed fuel type high temperature gas reactor, which will be described later, the coolant having a high temperature flows toward the new fuel side, but this does not occur in the block fuel type high temperature gas reactor, and the coolant outlet temperature becomes low.

On the other hand, the replacement of nuclear fuel material in these cores requires a great deal of time and labor as described above, and the time until the reactor is restarted becomes long.
There is a problem of reducing the operating rate of nuclear power plants.

In the pebble bed type high temperature gas reactor, the ball-shaped fuel is continuously inserted into the upper part of the core, while the burned ball-shaped fuel is taken out from the lower part of the core.
Not only the control rods but also fuel relocation work is unnecessary. However, such continuous insertion and removal of fuel is accompanied by various difficulties, and there were problems in this part in the development in Germany. Further, in such a core, the combustion state of fuel and the flow of coolant cannot be controlled.

The present invention has been made in view of such circumstances, and a first object thereof is to make a fission chain reaction continue at a constant output without adjusting the reactivity by a control rod. , To provide a nuclear core excellent in both operability and safety.

[0012] The second purpose is to use the unburned nuclear fuel material on the downstream side of the combustion after the operation cycle as the combustion start portion of the next operation cycle. It is an object of the present invention to provide a method of replacing a nuclear fuel material in a core of a nuclear reactor, which makes it possible to efficiently burn the nuclear fuel material without requiring a complicated arrangement.

[0013]

In order to achieve the above object, the present invention takes the following means.

That is, in the core of a nuclear reactor according to the first aspect of the present invention, in order to solve the first object, the nuclear fuel consisting of enriched uranium of uniform enrichment to which neutron burnable poison is added at a uniform concentration is a rule. The nuclear fuel region that is arranged in a
A combustion initiation part provided at the lower end or the upper end of the nuclear fuel region for starting the combustion of the nuclear fuel by the fission chain reaction by supplying neutrons to the nuclear fuel, and fission generated by the fission chain reaction initiated by the combustion initiation part. The nuclear fuel region that has received energy and a coolant that cools the combustion start portion are provided, and the fission chain reaction is continued.

Therefore, the core of the nuclear reactor according to the invention of claim 1 has a simple nuclear fuel structure consisting of enriched uranium of uniform enrichment to which a neutron burnable poison is added at a uniform concentration, and the neutrons supplied by the combustion initiation part. Once the fission chain reaction is triggered in the nuclear fuel region, the criticality can be maintained autonomously thereafter.

In the core of a nuclear reactor according to a second aspect of the present invention, in order to solve the first object, the neutron burnable poison is added at a uniform concentration along the combustion axis direction to a uniform concentration along the combustion axis direction. A nuclear fuel region, in which nuclear fuel composed of enriched uranium with a high degree of enrichment is regularly arranged, is provided at the lower end or the upper end of the nuclear fuel region, and the combustion of the nuclear fuel by the fission chain reaction is started by supplying neutrons to the nuclear fuel. And a coolant for cooling the nuclear fuel region that has received the nuclear fission energy generated by the fission chain reaction initiated by the combustion initiation part and the combustion initiation part,
Continue the fission chain reaction.

Therefore, the core of the nuclear reactor of the invention of claim 2 is a combustion axis (an axis connecting the upper end and the lower end of the nuclear fuel region).
A simple nuclear fuel composition consisting of enriched uranium of uniform enrichment along the combustion axis direction with a uniform concentration of neutron burnable poisons added along the direction, and the fission chain once in the nuclear fuel region by the neutrons supplied by the combustion initiation part. When the reaction is triggered, the criticality can be maintained autonomously thereafter.

In order to solve the first object, in the invention of claim 3, in the core of the nuclear reactor according to claim 1 or 2, combustion by a fission chain reaction is performed with the duration of this combustion. Over a certain reactor operation period, the fuel is moved from the end side of the nuclear fuel region provided with the combustion start portion along the axis connecting the upper end portion and the lower end portion of the nuclear fuel region with substantially the same heat output distribution.

Therefore, in the core of the nuclear reactor of the third aspect of the present invention, the heat output distribution in the axial direction (vertical direction) of the combustion section, that is, the neutron flux distribution, can be kept constant over the reactor operation period. Therefore, it is possible to maintain a constant output distribution over the reactor operation period without adjusting the output distribution with a control rod or the like.

According to the invention of claim 4, in order to solve the first object, in the core of the nuclear reactor according to claim 3, the height of the nuclear fuel region and the heat output are generated in the nuclear fuel region. The reactor operating period is determined by dividing by the moving speed at which the heat output moves along the axis.

Therefore, in the reactor core of the invention of claim 4, by adjusting the height of the nuclear fuel region,
The reactor operating period can be adjusted. That is, when it is desired to realize a core having a long reactor operation period, the height of the nuclear fuel region may be increased, and conversely, when the height of the nuclear fuel region is decreased, the reactor operation period may be shortened.

According to a fifth aspect of the present invention, in order to solve the second object, there is provided a method for replacing nuclear fuel material in a core of a nuclear reactor according to the fourth aspect, wherein the heat output part is a combustion start part. When it moves from the end of the nuclear fuel region provided with to the vicinity of the other end of the nuclear fuel region along the axis, the nuclear fission chain reaction is stopped, and the nuclear fuel in the heat output part at this stop is removed. The core of the next reactor operation period is configured as a combustion start portion of the core of the next reactor operation period.

Therefore, in the method of replacing nuclear fuel material in the core of a nuclear reactor according to the fifth aspect of the present invention, the remaining nuclear fuel can be effectively used as the combustion starting portion of the next operation cycle. In addition, this replacement method does not require radial (planar) rearrangement of the nuclear fuel region, and can be realized by linearly moving the remaining nuclear fuel in the axial direction (vertical direction). Can be done at.
As a result, it is not necessary to manufacture the combustion start portion after the second operation cycle. Such a nuclear fuel material replacement method can be applied to a block fuel type high temperature gas reactor with almost no design change.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

An embodiment of the present invention will be described with reference to FIGS. 1 to 11.

The core of the nuclear reactor according to the embodiment of the present invention will be explained as an example when it is applied to a block fuel type high temperature gas reactor. FIG. 1 shows the atom according to the embodiment of the present invention. Fig. 2 is a plan sectional view showing a structural example of the core of the reactor.
FIG. 4 is a vertical cross-sectional view showing a structural example of a reactor pressure vessel that houses the core therein, FIG. 3 is a perspective view showing a structural example of a fuel body loaded in the core, and FIG. FIG. 5 is a perspective view showing a configuration example of a fuel rod, and FIG. 5 is a perspective view showing a configuration example of a fuel compact loaded in the fuel rod.

As shown in FIG. 1, the core 1 not only loads a plurality of fuel bodies 2 having a regular hexagonal cross section in the radial direction, but also stacks a plurality of fuel bodies 2 in the axial direction as shown in FIG. ing. Further, reflectors 3 made of, for example, graphite are provided on the radially outer side of the core 1 as shown in FIG. 1 and on the upper and lower sides of the core 1 as shown in FIG. 1 The effect of leaking out is reduced.

As shown in FIG. 2, the reactor 4 has a reactor pressure vessel 5 in which a reactor core 1, a reflector 3 and other internal structures are housed. This reactor pressure vessel 5 is equipped with a double pipe 7 consisting of an inner pipe and an outer pipe in its lower part, and a coolant 8 driven by a blower (not shown) is passed through a gap between the inner pipe and the outer pipe. It is supplied into the reactor pressure vessel 5. This coolant 8 is introduced into the reactor pressure vessel 5 through the gap between the inner pipe and the outer pipe of the double pipe 7 as shown by the arrow in FIG.
Ascend along the inner wall of the vessel 5 and lead to the upper part of the core 1,
After that, it descends, passes through the core 1, is introduced into the inner pipe of the double pipe 7, and goes out of the reactor pressure vessel 5.
By circulating the coolant 8 in this manner, the core 1 is cooled. Helium gas or carbon dioxide gas is used as the coolant 8.

As shown in FIG. 3, the fuel body 2 is a regular hexagonal columnar graphite block 11 holding a plurality of fuel rods 10.
It consists of. The graphite block 11 has storage openings for storing the fuel rods 10 for the number of the fuel rods 10,
The fuel rod 10 is stored and held in this storage port.

As shown in FIG. 4, the fuel rod 10 is sealed by loading a plurality of fuel compacts 14 in a long cylindrical cylinder called a graphite sleeve 13 and plugging both ends with end plugs 15. ing.

As shown in FIG. 5, the fuel compact 14 has a spherical fuel nucleus 18, a low density pyrolytic carbon 19 covering the outside thereof, and a high density pyrolytic carbon 20 covering the outside thereof.
The coated fuel particles 17 composed of silicon carbide 21 that covers the outside thereof and high-density pyrolytic carbon 22 that further covers the outside thereof are solidified and formed into an annular column shape. Fuel core 18
Is uranium dioxide using concentrated uranium with a constant concentration,
A predetermined ratio of neutron burnable poison (hereinafter, simply referred to as "burnable poison") is mixed and sintered. For example, gadolinia (Gd ₂ O ₃ ) is used as the burnable poison. The fuel core 18 made of only uranium dioxide and the fuel core 18 made of only burnable poisons may be manufactured, and these may be uniformly mixed at a predetermined ratio before the fuel compact 14 is formed. . In this way, the fuel compact 14 contains a predetermined ratio of burnable poison with respect to the enriched uranium having a constant enrichment.

FIG. 6 shows a fuel body 2 containing no burnable poison.
When an infinite number of cores are arranged in the core system as described above
5 is a diagram showing the relationship between the burnup and the infinite multiplication factor in FIG.
In FIG. 6, the burnup curve a₀Enriched uranium enriched
If the degree is low, below b ₀, C₀, D₀According to
Therefore, the degree of enrichment increases.

That is, when the burnable poison is not contained, the infinite multiplication factor of the fuel body 2 increases as the enrichment of enriched uranium increases, as shown in FIG. That is, the higher the enrichment of enriched uranium, the easier it is to make the core 1 critical, and the longer the operating cycle length of the nuclear reactor 4 can be made. However, there is a surplus in the initial stage of combustion (stage with low burnup). Will have reactivity. The infinite multiplication factor of the fuel body 2 decreases almost linearly as the burnup increases.

On the other hand, FIG. 7 shows the burnup and the infinite multiplication factor in the case where an infinite number of fuel bodies 2 uniformly containing a combustible poison in a predetermined ratio with respect to uranium dioxide are arranged in the core system as described above. It is a figure which shows the relationship of. In FIG. 7, the burnup curve a ₁ indicates the enrichment of uranium enriched in FIG.
It is the same as the fuel enrichment of the burnup curve a ₀ shown in FIG. Similarly, b ₁ , c ₁ , and d ₁ also show the enrichment of uranium enriched in FIG.
Is the same as the fuel enrichment of the burnup curves b ₀ , c ₀ , d ₀ shown in FIG.

As shown in FIG.
When the burnable poison is added, the added burnable poison is neutral.
Since it absorbs the child, the initial stage of combustion (burnup
Lower stage) lowers the infinite multiplication factor. In Figure 7
Each burnup curve a shown by the dotted line in ₀~ D₀Are respectively shown in FIG.
Each burnup curve a₀~ D₀The same as flammable
It shows the infinite multiplication factor of the fuel body 2 with no poison added.
Of. And burnable poisons absorb neutrons
And converts it to a substance that does not absorb neutrons, so the burnup is
Burnable poisons decrease as the temperature increases, and
The multiplication factor is increasing, and all the burnable poisons are converted to other substances.
After conversion, do not include combustible poisons as shown in Figure 6.
Similar to the fuel unit 2, it is almost linear with increasing burnup.
Will be reduced to.

That is, the addition of the burnable poison to the fuel body 2 has the effect of reducing the infinite multiplication factor in the initial stage of combustion (the stage where the burnup is low) corresponding to the area A shown by hatching in FIG. . Therefore, the reactor 4
By increasing the enrichment of enriched uranium in order to prolong the operating cycle length of the fuel cell, and even when it has an excessive reactivity in the initial stage of combustion, by adding a burnable poison, Initial stage (stage with low burnup)
It is possible to adjust the reactivity of the core 1 by reducing the excess reactivity in the core.

Further, as shown in FIG. 9, the core 1 is provided with a combustion starting portion 24 at the lower end portion in addition to the nuclear fuel material region 23 composed of enriched uranium and combustible poisons. The combustion starting portion 24 may be provided at the upper end portion of the core 1 using, for example, the fuel body 2 containing a large amount of plutonium,
By generating neutrons, combustion by the fission chain reaction in the nuclear fuel material region 23 is started.

FIG. 10 is a conceptual diagram showing an image of output and mass balance in the core 1 of the nuclear reactor according to the embodiment of the present invention.

That is, in the reactor core 1 according to the embodiment of the present invention, the fission chain reaction is started by the neutrons generated from the combustion start portion 24 at the lower end of the core,
The neutron flux φ rises as shown in FIG. Hereinafter, the part where this neutron flux φ is rising is the combustion part B
Called. Further, since FP is generated from the nuclear fuel material M by this nuclear fission, the nuclear fuel material M decreases from the core lower end side and the FP accumulates from the core lower end side as shown in FIG.

On the other hand, nuclear fission reduces the amount of nuclear fuel material, but some of the neutrons generated by this fission are absorbed by uranium 238 (U-238), and this uranium 238 is converted to plutonium (Pu).

Since the amount of the nuclear fuel material M is larger in the upper part of the combustion part B than in the lower part thereof, the infinite multiplication factor increases in the upper part of the combustion part B. For this reason, the region where the neutron flux φ rises and the fission chain reaction is occurring moves upward along the axis. The heat energy generated by this nuclear fission is removed by the coolant 8. When the region where the fission chain reaction occurs moves from the lower side to the upper side along the axis, it is preferable that the coolant 8 also flows from the lower side to the upper side of the core 1, but FIG. It is also possible to cool the core 1 by flowing the coolant 8 from the upper side to the lower side of the core 1 as shown in FIG.

FIG. 11 shows how the ratio of nuclides in the enriched uranium fuel changes with combustion. When the enrichment of enriched uranium is, for example, 5% by weight, the state of fresh fuel is as shown in FIG.
As shown in FIG. 1 (a), uranium 235 (U-235) and uranium 238 (U-238) had a weight ratio of 5:95, but when they began to absorb neutrons from below, FIG. First, as shown in b), burnable poisons are reduced.
Further, the burnable poison is reduced, and a part of uranium 235 is converted into FP by fission, and another part is converted into uranium 236 by neutron capture. A part of uranium 238 captures fast neutrons, and becomes plutonium 239 through two β-decays. The number of fissile nuclides decreases as a whole.

As the combustion progresses further, as shown in FIG. 11 (c), the burnable poison is further reduced, and the uranium 235 is further converted into FP and uranium 236. Also, uranium 238
, While plutonium 239 is generated as described above, from the already generated plutonium 239,
FP is generated by fission, and plutonium 240 is generated by neutron capture. The number of fissile nuclides decreases as a whole.

As the combustion progresses further, the burnable poisons disappear as shown in FIG. 11 (d). Uranium 235 further converts to FP and uranium 236. Plutonium 239 is generated from uranium 238 as described above, while FP is generated from fission 239 that has already been generated due to fission, and plutonium 240 is generated due to neutron capture. The number of fissile nuclides decreases as a whole.

When combustion further progresses, as shown in FIG. 11 (e), the uranium 235 is further converted into FP and uranium 236, and the amount thereof is reduced. Plutonium 239 is produced from uranium 238 as described above, but the number of fissile nuclides is reduced.

As the combustion progresses, the amount of fissile nuclides decreases, and uranium 235 and plutonium 239
FP is generated and accumulated by nuclear fission of or plutonium 241 or the like. Since FP also contains a substance that absorbs neutrons well, as shown in FIG.
When the amount of is higher than a certain level, it becomes difficult to continue the fission chain reaction in this part, the neutron flux φ level decreases, the nuclear reaction does not occur, and the nuclide ratio hardly changes.

In the core 1, FIG. 11 (b) to FIG.
Although the criticality is maintained in the region corresponding to (e), the portion of these regions corresponding to FIG.
The state becomes 1 (f), and the region where the criticality is maintained is exited.

On the other hand, the portion corresponding to FIG. 11 (a) eventually becomes the state of FIG. 11 (b), and enters the criticality maintaining region. Since the portion corresponding to FIG. 11A is the upper portion of the fuel body 2 and the portion corresponding to FIG. 11F is the lower portion of the fuel body 2, the combustion portion B in which the neutron flux φ rises is It moves upward as it burns.

As described above, in the core 1 of the nuclear reactor according to the embodiment of the present invention, the combustion part B in which the neutron flux φ rises is as shown in FIGS. 10 (a) and 10 (b). Combustion proceeds by moving upward along the axial direction of the core 1. Then, when the combustion part B in which the neutron flux φ rises moves to the upper end part of the core 1, the nuclear fuel material M is no longer present in the upper part, so that the nuclear fission chain reaction stops and the operation cycle ends. To do. That is, since the length of the operation cycle is determined by the height of the core 1, the higher the height of the core 1, the longer the operation cycle length of the core 1.

When the combustion section B reaches the upper end of the core 1,
The criticality cannot be maintained and the combustion peak where the neutron flux φ is rising does not reach the upper end of the core 1. Therefore, as shown in FIG. 10B, both the nuclear fuel material M and the burnable poison BP remain in the vicinity of the upper end of the core 1.

In the nuclear reactor according to the embodiment of the present invention, in order to shift to the next operation cycle, as shown in FIGS. 10 (b) and 10 (c), the uppermost stage of the core 1 is moved in the operation cycle. The fuel body 2 loaded in the core is taken out from the core 1. Then, after the other fuel bodies 2 are taken out from the core 1 as spent fuel, the fuel bodies 2 loaded on the uppermost stage in the operation cycle are loaded on the lowermost stage of the core 1,
It is used as the combustion starter 24 for the next operation cycle. That is, the fuel body 2 loaded in the uppermost stage in the operation cycle is in the combustion region, so that the fuel body 2 is effectively used as the combustion start portion 24 in the next operation cycle. The replacement fuel body 2 is stacked on the fuel body 2 used as the combustion starting portion 24 as described above to form the core 1 for the next operation cycle.

The core 1 thus constructed also has
As shown in FIG. 10C and FIG.
As shown in FIGS. 10A and 10B, the combustion part B moves upward along the axial direction of the core 1.

Next, the operation of the core of the nuclear reactor according to the embodiment of the present invention configured as described above will be described.

When neutrons are emitted from the neutron source of the combustion start portion 24, the neutrons cause nuclear fission of the fuel contained in the fuel body 2 (combustion start portion 24), as shown in FIG. 10 (a). The neutron flux φ rises at.

Since the nuclear fuel material M is changed to FP by this nuclear fission, the nuclear fuel material M decreases from the core lower end side and the FP accumulates from the core lower end side as shown in FIG. 10 (a). The burnable poison BP absorbs neutrons generated by nuclear fission and converts them into other substances, and thus decreases from the core lower end side.

Since FP also contains a substance that absorbs neutrons well, as shown in FIG. 11 (f), when the amount of FP exceeds a certain level, the fission chain reaction is no longer continued in this part. It becomes difficult, the neutron flux φ level decreases, nuclear reaction does not occur, and the nuclide ratio hardly changes. 11 (b) to FIG. 11 in the core 1.
Although the criticality is maintained in the region corresponding to (e), the portion of these regions corresponding to FIG.
The state becomes 1 (f), and the region where the criticality is maintained is exited.

On the other hand, the portion corresponding to FIG. 11 (a) eventually becomes the state of FIG. 11 (b), and enters the region where the criticality is maintained. Since the portion corresponding to FIG. 11 (a) is the upper portion of the fuel body 2 and the portion corresponding to FIG. 11 (f) is the lower portion of the fuel body 2, the combustion portion B in which the neutron flux φ rises burns. With it moves upwards.

As described above, in the core 1 of the nuclear reactor according to the embodiment of the present invention, the combustion part B in which the neutron flux φ rises moves upward along the axial direction of the core 1, The fuel body 2 moves to the upper stage of the combustion section B. Also in the fuel body 2 arranged in the upper stage of the combustion section B, the combustible poison is reduced as described above, and the combustion section B moves to the fuel body 2 arranged in the upper stage.

In this way, in the reactor core 1 of the reactor according to the embodiment of the present invention, as shown in FIGS.
As shown in (b), the combustion part B moves along the axis connecting the upper and lower ends of the core 1 while maintaining the shape of the power distribution (distribution of neutron flux φ). Therefore, once the flow rate of the coolant 8 and its distribution for each cooling channel are optimized and the operation cycle is started, it is not necessary to change the flow rate until the operation cycle ends. Further, by increasing the height of the core, the operation cycle is lengthened.

It should be noted that an average of 200 per nuclear fission
MeV (1 eV to about 1.6 × ^{10 - 19} J) energy is released. This nuclear fission energy raises the temperature of the fuel body 2, but is removed by the coolant 8 using the helium gas or carbon dioxide gas flowing on the outer periphery of the fuel body 2.

The operation cycle is completed by moving the combustion section B to the upper end of the core 1. The fuel body 2 loaded on the uppermost stage of the core 1 in the operation cycle
Are taken out of the core 1. Since both the nuclear fuel material M and the burnable poison BP remain in this fuel body 2 and plutonium is accumulated, after the other fuel bodies 2 are taken out from the core 1 as spent fuel,
It is reloaded to the lowermost stage of the core 1 and used as the combustion start portion 24 in the next operation cycle. The replacement fuel bodies 2 are stacked on the fuel body 2 used as the combustion start portion 24 to form the core 1 for the next operation cycle.

Reactor core 1 according to an embodiment of the present invention
Since such a method of replacing the nuclear fuel material is adopted, the nuclear fuel material can be effectively used. Furthermore, this replacement method can be implemented in a short period of time because it does not require relocation of the fuel body 2 in the radial direction of the core 1 and can be realized by linearly moving only in the axial direction. Also,
When the core 1 of the next operation cycle configured in this way is restarted, the special combustion start part 24 becomes unnecessary.

As described above, in the reactor core 1 according to the embodiment of the present invention, all the fuel bodies 2 other than the fuel body 2 used as the combustion start portion 24 are enriched uranium having a single enrichment. Not only can it be constructed by mixing combustible poisons in a single ratio, but since it does not require a control rod, its configuration can be extremely simplified, and abnormal reactivity is injected. You can eliminate accidents.

Further, when the reactor is in operation, FIG.
And as shown in FIG. 10 (b), the power distribution only moves along the axial direction of the core 1 without changing its shape. Therefore, once the coolant flow rate is optimized at the start of the operation cycle, You can easily continue driving without changing it until. Moreover, during the operation cycle, it is not necessary to take in and out the fuel unlike the pebble bed type high temperature gas furnace, so that the operation can be extremely simplified in combination with not requiring the control rod.

Further, the core characteristics such as reactivity are not changed even if combustion progresses. With this feature, the driving method and the response at the time of the accident will not change due to combustion,
It will be easy. Especially in plutonium combustion furnaces, uranium 2
Since the parent substances such as 38 cannot be put into the core, we expect a negative reactivity coefficient for burnable poisons. In this case, as the combustion progresses, the absolute value of the negative reactivity becomes smaller and the reactor shifts to the dangerous side. Therefore, in a plutonium combustion reactor or the like, measures must be taken against this, but in the reactor core of the reactor according to the embodiment of the present invention, such measures are unnecessary.

Further, by increasing the height of the core 1, the length of the operation cycle can be easily lengthened.
That is, since the length of the operation cycle can be made substantially equal to the life of the plant of the nuclear power plant, it becomes possible to eliminate the need for refueling. As a result, the lid of the reactor pressure vessel 5 will not be opened, and construction in a developing country can be expected.

The reactor core 1 of the reactor according to the above-described embodiment of the present invention can be applied to most reactors of the type in which solid fuel is loaded in the core 1. In particular, the design change is hardly required to apply it to the block fuel type high temperature gas reactor, which is in the stage of practical use in the United States and has been put into operation in Japan. Further, even in a light water reactor, the neutron utilization efficiency is lower than that of the block fuel type high temperature gas reactor due to the difference in the size of the core 1, the specifications of the fuel body 2, the reflector 3, or the coolant 8, The core 1 can be established by increasing the enrichment of uranium enrichment, appropriately adding a combustible poison, and reducing the excess reactivity at the beginning of the operation cycle.

Further, in the reactor core 1 according to the embodiment of the present invention, the fuel body 2 containing the enriched uranium remaining in the previous operation cycle and the accumulated plutonium is used for the combustion start portion of the next operation cycle. Since it is used as 24, the nuclear fuel material can be effectively used. Further, the fuel replacement performed before the start of the operation cycle is a movement only in the axial direction that moves the fuel body 2 loaded in the previous operation cycle to the combustion start position, and does not move in the radial direction. Since it is not accompanied, it can be performed in a short time.

The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such configurations. In the scope of the technical idea described in the claims, those skilled in the art can come up with various modifications and modifications, and those modifications and modifications are also technical aspects of the present invention. It is understood that it belongs to the range.

[0070]

As described above, according to the present invention,
The fission chain reaction can be continued at a constant output without adjusting the reactivity by the control rod. As described above, it is possible to realize a nuclear core excellent in both operability and safety.

Further, according to the present invention, after the end of the operation cycle, the unburned nuclear fuel substance on the downstream side of the combustion can be used as the combustion start portion of the next operation cycle. As described above, it is possible to realize a method of replacing a nuclear fuel material in a core of a nuclear reactor, which does not require complicated arrangement of the nuclear fuel material and can efficiently burn the nuclear fuel material.

[Brief description of drawings]

FIG. 1 is a plan sectional view showing a configuration example of a core of a nuclear reactor according to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing a configuration example of a reactor pressure vessel that internally houses the reactor core of the same embodiment.

FIG. 3 is a perspective view showing a configuration example of a fuel body loaded in a core of a nuclear reactor according to the same embodiment.

FIG. 4 is a perspective view showing a configuration example of a fuel rod which constitutes a fuel body loaded in a core of a nuclear reactor according to the same embodiment.

FIG. 5 is a perspective view showing a configuration example of a fuel compact loaded in a fuel rod which constitutes a fuel body loaded in a core of a nuclear reactor according to the same embodiment.

FIG. 6 is a diagram showing the relationship between infinite multiplication factor and burnup in a core system of a fuel body containing no combustible poison.

FIG. 7 is a diagram showing the relationship between the infinite multiplication factor and the burnup in a core system of a fuel body that uniformly contains a burnable poison in a predetermined ratio with respect to a nuclear fuel material.

FIG. 8 is a comparative diagram showing the relationship between infinite multiplication factor and burnup between a fuel body containing burnable poison and a fuel body not containing burnable poison.

FIG. 9 is a conceptual diagram showing the arrangement relationship between the nuclear fuel material and the neutron source in the core.

FIG. 10 is a conceptual diagram showing an image of output and mass balance in the core of the nuclear reactor according to the same embodiment.

FIG. 11 is a schematic diagram showing changes in the ratio of nuclides of uranium, plutonium, and FP.

[Explanation of symbols]

A ... area B ... Combustion section M ... Nuclear fuel material BP ... Combustible poison φ ... Neutron flux 1 ... core 2 ... Fuel 3 ... Reflector 4 ... Reactor 5 ... Reactor pressure vessel 7 ... Double piping 8 ... Coolant 10 ... Fuel rod 11 ... Graphite block 13 ... Graphite sleeve 14 ... Fuel compact 15 ... End plug 17 ... Coated fuel particles 18 ... Fuel core 19 ... Low density pyrolytic carbon 20 ... High-density pyrolytic carbon 21 ... Silicon Carbide 22 ... High-density pyrolytic carbon 23 ... Nuclear fuel material area 24 ... Combustion start part

Continuation of front page (51) Int.Cl. ⁷ Identification code FI G21C 3/32 G (56) References Hiroshi Sekimoto, Kohichi Ryuu, Demonstracing the Feasir reef feat. American Nuclear Society, December 12, 2000, 83, 45 (58) Fields investigated (Int.Cl. ⁷ , DB name) G21C 1/02 G21C 5/00 G21C 3/326 G21C 3/328 G21C 5/18

Claims (5)

(57) [Claims] 1. A nuclear fuel region in which a nuclear fuel composed of enriched uranium with a uniform enrichment to which a neutron burnable poison is added at a uniform concentration is regularly arranged, and a nuclear fuel region is provided at a lower end or an upper end of the nuclear fuel region, A combustion initiation part for initiating combustion of the nuclear fuel by a fission chain reaction by supplying neutrons to the nuclear fuel; the nuclear fuel region and the combustion receiving the fission energy generated by the fission chain reaction initiated by the combustion initiation part. A core of a nuclear reactor, comprising a cooling material for cooling a starting part, and adapted to continue the fission chain reaction. 2. A nuclear fuel region in which nuclear fuel composed of enriched uranium of uniform enrichment is regularly arranged along the combustion axis direction, in which neutron burnable poison is added at a uniform concentration along the combustion axis direction, A combustion start part provided at the lower end or the upper end of the nuclear fuel region to start the combustion of the nuclear fuel by a fission chain reaction by supplying neutrons to the nuclear fuel, and a fission chain reaction started by the combustion initiation part. A nuclear reactor core provided with the nuclear fuel region that has received the generated nuclear fission energy and a coolant that cools the combustion start portion, and that continues the nuclear fission chain reaction. 3. Combustion by the nuclear fission chain reaction is performed from the end side of the nuclear fuel region where the combustion initiation part is provided to the upper end of the nuclear fuel region over the reactor operation period which is the duration of the combustion. The core of the nuclear reactor according to claim 1 or 2, wherein the core is moved with substantially the same heat output distribution along an axis connecting the lower portion and the lower portion. 4. The nuclear reactor operating period is determined by dividing the height of the nuclear fuel region by the moving speed at which the heat output portion generating heat output in the nuclear fuel region moves along the axis. The core of the nuclear reactor according to claim 3, wherein 5. The method of replacing nuclear fuel material in a core of a nuclear reactor according to claim 4, wherein the heat output portion is provided from an end portion of the nuclear fuel region where the combustion initiation portion is provided to the shaft. When it moves to the vicinity of the other end of the nuclear fuel region along with, the nuclear fission chain reaction is stopped, and the nuclear fuel in the heat output part at the time of this shutdown is burned in the core during the next reactor operation period. As a starting point,
A method for replacing nuclear fuel material in a core of a nuclear reactor, which is configured to form a core during the next reactor operation period.