JP2004109085A - Fuel assembly for boiling water reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel assembly for a boiling water reactor for minimizing reactivity loss and effectively using the uranium resource while securing a sufficient thermal operation margin when reprocessed uranium is loaded in a fuel for the boiling water rector intended for high burn-up. <P>SOLUTION: A fuel pellet made of a nuclear fuel material is filled in cladding tubes to form a group of fuel rods. The group of fuel rods are regularly arranged in square lattice arrangement. A large-diameter water rod, a non-boiling region occupying a region corresponding to a plurality of the fuel rods, is provided for an approximately center location of the lattice arrangement. Part of the fuel pellet contains the reprocessed uranium acquired by reprocessing a spent fuel in the fuel assembly for the boiling water reactor. In an upper region and a lower region of the fuel effective region in an axial direction, the lower region contains a larger amount of the reprocessed uranium than that contained in the upper region, and a most circumferential region of the lower region contains a larger amount of the reprocessed uranium than that contained in other regions. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a fuel assembly using recovered uranium obtained by reprocessing spent fuel.
[0002]
[Prior art]
A fuel assembly loaded in a boiling water reactor usually stays in a core of four to five cycles and is taken out after achieving a predetermined burnup. The removed spent fuel still contains about 0.8 wt% of fissile uranium 235. Therefore, it is desirable to effectively use uranium resources by reloading uranium recovered by reprocessing of spent fuel into a fuel assembly.
[0003]
However, there are the following problems when reconcentrating recovered uranium and reusing it as fuel for a nuclear reactor. That is, while burning in the furnace, uranium 235 is depleted by fission reactions, while absorbing neutrons to produce uranium 236. Therefore, uranium 235 and uranium 236 exist in the spent fuel. However, since these two nuclides have almost the same mass, when the uranium 235 is concentrated using the centrifugation technique, the uranium 235 and the uranium 236 cannot be completely separated. Therefore, as the enrichment of uranium 235 increases, the concentration of uranium 236 also increases.
[0004]
Uranium 236 has a large neutron absorption cross section and acts as a neutron poison in the reactor core. In the recovered uranium, the above-mentioned uranium 236 is present in addition to the uranium 238 and the uranium 235, but there is also a uranium 234 slightly acting as a neutron poison similar to the uranium 236. For example, when the recovered uranium is reconcentrated to uranium 235 of 4 wt%, uranium 236 is present at about 1 to 2 wt%. On the other hand, the uranium 234 in this case is 0.1 wt% or less, and the effect of the presence of the uranium 234 can be ignored. Due to the influence of uranium 236, the fuel loaded with recovered uranium has a lower reactivity (reactivity loss) than the unloaded fuel, and it may be necessary to increase the enrichment of uranium 235 to compensate for the reduced reactivity. There is even. (For example, see Patent Document 1)
[0005]
This is not preferable from the viewpoint of effective utilization of uranium resources, and it is desirable that the reactivity of the recovered uranium-loaded fuel be reduced as much as possible and that the reactivity compensation be unnecessary. Also, the amount of spent fuel generated is increasing year by year, and it is desirable to load as much recovered uranium as possible into the fuel assembly and reuse it.However, the reduction in reactivity at this time should be minimized. is important.
[0006]
[Patent Document 1]
JP-A-62-90595
[Problems to be solved by the invention]
On the other hand, in order to increase the economic efficiency of the nuclear reactor, it has been promoted to increase the extraction energy per fuel assembly to improve the average extraction burnup. For this reason, it has become necessary to increase the average enrichment of the fuel. However, this causes the following problems. That is, generally, when the average enrichment of the fuel assembly is increased, the spectrum becomes harder, and as a result, the void reactivity increases to the negative side. As a result, during operation, the reactivity of the upper portion of the fuel having a high void ratio is lower than that of the lower portion, and the axial power distribution tends to be further distorted. Increasing the output below the fuel is accompanied by an increase in the maximum linear power density, which reduces the thermal operating margin.
[0008]
The present invention minimizes reactivity loss while loading recovered uranium into boiling water reactor fuel aiming at high burnup while minimizing reactivity loss while effectively utilizing uranium resources. It is an object of the present invention to obtain a boiling water reactor fuel assembly contributing to the following.
[0009]
[Means for Solving the Problems]
In the fuel assembly for a boiling water reactor according to the invention described in claim 1, the fuel rods in which the fuel pellets made of the nuclear fuel material are filled in the cladding tube are regularly arranged in a square lattice-like arrangement. A large-diameter water rod, which is a non-boiling region occupying a region corresponding to a plurality of fuel rods, is provided at a substantially central position of the lattice array, and a part of the fuel pellets contains recovered uranium obtained by reprocessing spent fuel. A fuel assembly for a boiling water reactor,
In the upper region and the lower region of the axial fuel effective region, the amount of recovered uranium contained in the lower region is larger than that in the upper region, and further,
In the lower region, the amount of recovered uranium contained in the outermost peripheral region is larger than the amount of recovered uranium contained in other regions.
[0010]
In the fuel assembly for a boiling water reactor according to the second aspect of the present invention, in the upper area described in the first aspect, the recovery is included in an area other than the outermost periphery and not adjacent to the large diameter water rod. It is characterized by the largest amount of uranium.
[0011]
In the fuel assembly for a boiling water reactor according to the invention described in claim 3, the upper region and the lower region described in claim 1 or 2 are arranged such that each of the upper region and the lower region is approximately one-third of the axial effective fuel region from above. And approximately 1/3 from the bottom.
[0012]
A fuel assembly for a boiling water reactor according to the invention described in claim 4 includes a fuel rod including a burnable poison-containing fuel pellet in the fuel assembly according to any one of claims 1 to 3. It is characterized in that the burnable poison-containing fuel pellet contains the recovered uranium.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the recovered uranium amount in the lower region is larger than that in the upper region, and the recovered uranium amount included in the fuel rods in the outermost peripheral region is larger than the recovered uranium amount in the fuel rods in regions other than the outermost periphery and not adjacent to the water rod. It is a fuel assembly for a boiling water reactor that has been increased. For this reason, when the recovered uranium is used for the fuel assembly, it is possible to provide a fuel that minimizes the decrease in reactivity from the case where the recovered uranium is not used and increases the operating margin with respect to the maximum linear power density.
[0014]
Hereinafter, advantages of using the recovered uranium in the lower part in the axial direction will be described. Since the neutron absorption of uranium 236 is centered on the resonance energy region, when the neutron spectrum is softer than the resonance energy region, the probability of escaping the neutron resonance absorption of uranium 236 increases, and the decrease in reactivity can be minimized. The region where the neutron spectrum is soft in the core is the axial lower part in the boiling water reactor. This is because in a boiling water reactor, the void fraction at the lower part in the axial direction is small, and neutron deceleration is promoted. Therefore, in order to minimize the decrease in the reactivity, it is preferable to arrange a large amount of recovered uranium in the lower part in the axial direction.
[0015]
FIG. 1 is a diagram showing a cross section of a fuel for a boiling water reactor aiming at a high burnup. In this example, a plurality of fuel rods 1 are bundled in a grid of 9 × 9 arrangement, and a square water It has a shape in which a rod (water channel) 2 is arranged. FIG. 2 shows a line connecting the midpoints of each side of the assembly, and an infinite multiplication factor where the enrichment of all the fuel rods is assumed to be a uniform distribution (4.4 wt%). FIG. 9 is an explanatory diagram showing a change in reactivity loss when four fuel rods 1b located in the layer are loaded with recovered uranium. That is, it is a diagram showing the burnup dependence of the reactivity loss due to the loading of recovered uranium. In this figure, the reactivity loss increases as the void fraction increases, and the reactivity difference between 70% void and 0% void is about 0.03% ３k at the beginning of combustion. This indicates that it is most advantageous to dispose the recovered uranium at the lower portion in the axial direction where the void ratio is small for the purpose of minimizing the reactivity loss.
[0016]
Next, the radial arrangement of the recovered uranium will be described. FIG. 3 shows the infinite multiplication factor when the enrichment of all the fuel rods is assumed to be a uniform distribution (4.4 wt%). FIG. 7 is an explanatory diagram showing a comparison of reactivity loss when recovered uranium is loaded on each of four fuel rods 1a adjacent to the fuel rod 1a, a fuel rod 1b located on the second layer from the outermost circumference, and a fuel rod 1c on the outermost circumference. . Since the neutron deceleration is good around the water gap or the fuel rod adjacent to the water rod and the relative output of the fuel rod is large, the neutron absorption effect is large when the recovered uranium is placed at this position, and it is adjacent to both. In this example, the reactivity loss is increased by about 0.01% △ k as compared with the case where the recovered uranium is disposed on the inner peripheral portion which is not used.
[0017]
In general, when viewed from a cross section of a fuel assembly, the fuel rod located at the outermost periphery has a soft neutron spectrum because it is close to a water gap, and thus tends to have the highest output as compared with other fuel rods. In this case, if the output of the outermost fuel rod can be reduced, the maximum value (local peaking coefficient) of the relative output of the fuel rod can be reduced, so that the maximum linear output density during operation can be reduced. For this reason, the present invention proposes to arrange more recovered uranium in the fuel rod located at the outermost periphery with respect to the axial lower portion where the linear power density tends to increase during operation. It goes without saying that this utilizes the neutron absorption effect of uranium 236 to suppress the output of the outermost fuel rods.
[0018]
Summarizing the above, the following conclusions were obtained.
(1) A comparison of the amount of change in the reactivity loss between the analysis example in which the arrangement position of the recovered uranium is changed in the axial direction and the analysis example in which the arrangement position of the recovered uranium is changed in the radial direction shows that the arrangement of the recovered uranium in the axial direction. The amount of change is larger when the position is changed. That is, it is more effective to optimize the arrangement in the axial direction from the viewpoint of reducing the reactivity loss. It was also found that it is effective to arrange more recovered uranium on the outermost circumference to reduce the local output peaking coefficient for the purpose of reducing the maximum linear power density during operation.
(2) When the recovered uranium is disposed on the inner peripheral portion, the reactivity loss can be reduced as compared with the case where the recovered uranium is disposed at the outermost peripheral position or a position adjacent to the water rod. It does not reach the effect of optimizing the axial arrangement of recovered uranium. On the other hand, the arrangement of the recovered uranium on the inner periphery further increases the relative output of the fuel rod adjacent to the water gap, and increases the maximum linear power density. Conversely, by arranging the recovered uranium on the fuel rods adjacent to the water gap, the relative output of the outermost fuel rods can be reduced and the margin of the maximum linear power density with respect to the allowable limit can be increased.
[0019]
As described above, from the viewpoint of reducing the reactivity loss when loading recovered uranium, the effect of optimizing the axial arrangement is greater than optimizing the radial arrangement of the recovered uranium. In the present invention, since a large number of recovered uranium is disposed in the lower part in the axial direction, the loss of reactivity due to the use of recovered uranium can be minimized.
[0020]
Incidentally, the maximum linear output density becomes severe in the lower region. From the viewpoint of securing the operation margin of the maximum linear power density, in the upper region, even if the local peaking coefficient is slightly increased due to the effect of the recovered uranium, it can be tolerated. From the above, in the present invention, it is proposed that priority is given to minimizing the reactivity loss, and that the most recovered uranium in the upper region is arranged in the second layer.
[0021]
Here, the region where the maximum linear power density becomes severe is generally a region which is lower than approximately 1/3 from the bottom of the axial effective fuel region. As described above, it is not desirable from the viewpoint of reducing the reactivity loss to arrange a large amount of recovered uranium in the upper region in the axial direction, particularly in a region approximately one-third from the top where the void ratio is saturated at a high void ratio. Therefore, from the viewpoint of reducing the maximum linear output and reducing the reactivity loss, it is effective that the upper region and the lower region are approximately 1/3 from the top and approximately 1/3 from the bottom in the axial fuel effective region. Thus, the upper and lower regions are preferably approximately one-third from the top and approximately one-third from the bottom of the axial fuel active area, respectively.
[0022]
In general, the addition of burnable poisons reduces the thermal conductivity of fuel pellets (hereinafter, pellets) and increases the fuel temperature. Therefore, increasing the output too much is not preferable in terms of thermomechanical soundness. For this reason, fuel rods containing burnable poisons are generally designed to suppress the increase in power during the combustion period by setting the enrichment to an appropriate value or less, or by arranging them at a radial position where the relative output is difficult to increase. You. As described above, since the fuel rods containing burnable poisons have a small output share, the loss of reactivity when using recovered uranium in pellets containing burnable poisons is the same as when using recovered uranium as fuel without adding burnable poisons. Smaller than. In the present invention, the recovered uranium can be used for the pellets containing the burnable poison, so that the reactivity loss can be minimized.
[0023]
【Example】
Example 1
As a first embodiment of the present invention, FIG. 4 shows a fuel assembly in which a square large-diameter water rod W is arranged in an area occupying nine fuel rods in a central part 3 × 3 in a 9 × 9 lattice array. . In the enrichment distribution of the fuel rods, A uses the fuel enrichment of the highest enrichment, and B, C, D, E and the enrichment of the fuel pellets decrease in the following order. The fuel rod types are set to 1 to 8 depending on the difference in the enrichment distribution of each fuel rod and the presence or absence of burnable poisons. Fuel rod types 7 and 8 are burnable poison-containing fuel rods, and the mixture ratio of burnable poison is α <β <γ. Also, a low-enrichment natural uranium blanket is provided at each of the upper and lower nodes of each fuel rod. Here, the shaded portion is the region containing the recovered uranium.
[0024]
In the first embodiment, recovered uranium is used in the lower region of the fuel rod type 3 and the upper region of the fuel rod type 2. Here, the number of fuel rod types 3 is 24, of which 20 are located in the outermost peripheral area (a) and 4 are located in the area (b) adjacent to the water rod. The number of the fuel rod type 2 is 12, of which 8 are located in the area (c) other than the outermost periphery and not adjacent to the water rod, and 4 are located in the area (b) adjacent to the water rod.
[0025]
In the above configuration, the recovered area of uranium is used more in the lower region (the fuel rods 3 are more numerous than the fuel rods 2), so that the reactivity loss can be reduced. Furthermore, in the upper region where the reactivity loss is large, more recovered uranium is used in the region of the inner peripheral portion other than the outermost periphery and not adjacent to the water rod, than the outermost peripheral region and the region adjacent to the water rod. In addition, it is possible to more effectively reduce the reactivity loss associated with the use of recovered uranium.
[0026]
FIG. 5 shows the ratio between the local peaking coefficient in the lower region and the local peaking coefficient in a case where recovered uranium is not used in the same design in this embodiment. In the lower region of the present embodiment, the recovered uranium is arranged more in the outermost peripheral region, so that the local peaking coefficient is reduced as compared with the case where the recovered uranium is not used through combustion. That is, with the above configuration, it is possible to reduce the reactivity loss due to the use of the recovered uranium while ensuring a sufficient thermal operation margin.
[0027]
In the present embodiment, the burnable poison is added to the pellets with low enrichment, and the position in the inner diameter direction of the aggregate is the inner peripheral portion where the relative value of the output hardly increases. Therefore, the relative output of the burnable poison-containing fuel rod is small, and the effect of the reactivity loss can be minimized even if the recovered uranium is used for the pellet containing the burnable poison.
[0028]
In this embodiment, recovered uranium is used for the pellet having the second highest enrichment. When the enrichment of the uranium 235 is high, the amount of the uranium 236 contained in the recovered uranium increases in proportion thereto, and the loss of reactivity increases. When the recovered uranium is used for pellets with high enrichment as in this embodiment, the effect of reducing the reactivity loss by the above configuration is particularly large.
[0029]
Example 2
As a second embodiment of the present invention, two large-diameter water rods W are arranged in an area occupying seven fuel rods at the center in a 9 × 9 lattice array, and partial-length fuel rods are used for some of the fuel rods. FIG. 6 shows the fuel assembly used. In the enrichment distribution of the fuel rods, A uses the fuel enrichment of the highest enrichment, and B, C, D, E, F and the enrichment of the fuel pellets decrease in the following order. Further, the fuel rod types are set to 1 to 8 depending on the enrichment distribution of each fuel rod, the difference in active fuel length, and the presence or absence of burnable poisons. Fuel rod type 7 is a partial length fuel rod, and fuel rod type 8 is a fuel rod containing burnable poison. Also, a low-enriched uranium blanket is provided at each of the upper two nodes and the lower one node of each fuel rod except for the partial length fuel rods. Here, the shaded portion is the region containing the recovered uranium. Further, unlike the other fuel pellets B, the fuel pellets B of the partial length fuel rods 7 do not use recovered uranium, and have the same uranium 235 enrichment.
[0030]
In the second embodiment, recovered uranium is used in the lower regions of the fuel rod types 2 and 5 and in all regions other than the uranium blankets of the fuel rod types 3, 4, and 8. The total number of fuel rod types 2 and 5 is 20, and all of them are located in the outermost peripheral area (a). The number of fuel rod types 4 is eight, and all of them are located in the outermost peripheral area (a). Further, the total number of fuel rod types 3 and 8 is 20, and they are all located in the area (c) other than the outermost periphery and not adjacent to the water rod.
[0031]
In the above configuration, since the recovered uranium is used more in the lower region, the reactivity loss can be reduced. Furthermore, in the upper region where the reactivity loss is large, the amount of recovered uranium is increased in a region other than the outermost periphery and not adjacent to the water rod, so that it is possible to more effectively reduce the reactivity loss associated with the use of the recovered uranium. it can. Further, since a large amount of recovered uranium is used at the outermost periphery in the lower region, the local peaking coefficient at the lower portion can be suppressed.
[0032]
In the present embodiment, the burnable poison is added to the pellets with low enrichment, and the position in the inner diameter direction of the aggregate is the inner peripheral portion where the relative value of the output hardly increases. By using the recovered uranium for the burnable poison-bearing fuel rod disposed at this position, the effect of the reactivity loss can be minimized and the amount of recovered uranium used per aggregate can be increased.
[0033]
That is, with the above configuration, the amount of recovered uranium used per aggregate can be increased, and the reactivity loss due to the use of recovered uranium can be reduced while ensuring sufficient thermal operation margin. Thus, a fuel assembly contributing to the effective use of uranium resources can be provided.
[0034]
【The invention's effect】
As described above, the present invention minimizes the decrease in reactivity while loading recovered uranium in boiling water reactor fuel aimed at high burnup while ensuring sufficient thermal operation margin. This has the effect of providing a fuel assembly for a boiling water reactor.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross section of a fuel for a boiling water reactor aiming at high burnup.
FIG. 2 is an infinite multiplication factor where the enrichment of all the fuel rods is assumed to be a uniform distribution (4.4 wt%). FIG. 7 is an explanatory diagram showing a change in reactivity loss when four fuel rods 1b located in a second layer are loaded with recovered uranium.
FIG. 3 is plotted on the line connecting the midpoints of each side of the assembly to the infinite multiplication factor when the enrichment of all fuel rods is assumed to be a uniform distribution (4.4 wt%). FIG. 9 is an explanatory diagram showing a comparison of reactivity loss when recovered uranium is loaded on each of four fuel rods 1a adjacent to the rod, a fuel rod 1b located in the second layer from the outermost circumference, and a fuel rod 1c on the outermost circumference. is there.
FIG. 4 shows a fuel assembly in which a square large-diameter water rod W is arranged in an area occupying 9 fuel rods at a central portion of 3 × 3 in a 9 × 9 lattice array according to the first embodiment of the present invention. It is explanatory drawing which shows arrangement | positioning of a fuel rod.
FIG. 5 is an explanatory diagram showing a ratio between a local peaking coefficient in a lower region and a local peaking coefficient in a case where recovered uranium is not used in the same design in the embodiment of FIG. 4;
FIG. 6 shows a 9 × 9 grid arrangement according to the second embodiment of the present invention, in which a central portion occupies seven fuel rods, two large water rods W are arranged, and some fuel rods have partial length fuel. It is an explanatory view showing arrangement of each fuel rod in a fuel assembly using a rod.

Claims (4)

A fuel rod group filled with fuel pellets made of nuclear fuel material in a cladding tube is regularly arranged in a square lattice-like arrangement, and is a non-boiling region occupying a region corresponding to a plurality of fuel rods at a substantially central position of the lattice arrangement. A boiling water reactor fuel assembly comprising a large diameter water rod, and a portion of the fuel pellets containing recovered uranium obtained by reprocessing spent fuel,
In the upper region and the lower region of the axial fuel effective region, the amount of recovered uranium contained in the lower region is larger than that in the upper region, and further,
A fuel assembly for a boiling water reactor, wherein in a lower region, the amount of recovered uranium contained in an outermost peripheral region is larger than the amount of recovered uranium contained in other regions. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein in the upper region, the amount of recovered uranium contained in a region other than the outermost periphery and not adjacent to the large diameter water rod is the largest. 3. The boiling water reactor according to claim 1, wherein the upper region and the lower region are approximately 1/3 from the top and approximately 1/3 from the bottom, respectively, of the axial fuel effective region. 4. Fuel assembly. The fuel assembly according to claim 1, further comprising a fuel rod including a burnable poison-containing fuel pellet, wherein the burnable poison-containing fuel pellet contains the recovered uranium. Fuel assemblies for boiling water reactors.

Images