JP2000193773A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the ratio recovered uranium used in a fuel assembly by using the recovered uranium obtained through the reprocessing of a used fuel for a fuel pellet containing a burnable poison such as gadolinia in the fuel assembly. SOLUTION: A fuel assembly has a natural uranium blanket at upper and lower ends. Fuel pellets of the highest enrichment are used for A in the enrichment distribution of fuel rods 11 and, after A, the enrichment of fuel pellets in descending order is: B, C, D and E. Types of a fuel are classified into 1 to 5 according to the difference of the enrichment distribution of each fuel rod. A type P of fuel rods are short fuel rods and a type G of them are fuel rods of gadolinia, a burnable poison. The fuel assembly also has water rods 16. If recovered uranium is used only for the gadolinia fuel rods, the ratio of the used recovered uranium in volume is about 15%. Moreover, all fuel rods are made of recovered uranium by using it for all other uranium fuel pellets and the ratio of the used recovered uranium in volume can be set at 100%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a light water reactor, and more particularly to a fuel assembly using uranium recovered by reprocessing spent fuel.

[0002]

2. Description of the Related Art In a conventional light water reactor, for example, a boiling water reactor, a fuel assembly having burnable poisons such as enriched uranium and gadolinia is used.

FIG. 1 shows an example of a conventional fuel assembly. The fuel assembly 13 includes a plurality of fuel rods 11 and a channel box 12. Since the control rods 14 or the neutron detector instrumentation tubes 15 are arranged outside the channel box 12, the intervals between the fuel assemblies 13 are widened as long as devices such as the control rods 14 are inserted. The surroundings are filled with cooling water. The upper and lower ends of the fuel rod 11 are supported by upper and lower tie plates (not shown). Channel box 12 is fixed to the upper tie plate and surrounds a bundle of fuel rods 11. 16 is a water rod, and 17 is a coolant region in which coolant flows between the fuel rods. The coolant flows in the channel box 12 upward from below.

[0004] The following phenomena mainly occur in the reactor core during combustion.

(1) Consumption of fissile materials (uranium 235, plutonium 239, plutonium 241).

(2) Conversion of fuel parent materials (uranium 238, plutonium 240) to fissile materials.

(3) Accumulation of fission products.

FIG. 2 shows an example of a change in the weight ratio of uranium 235 due to combustion. As shown in FIG. 2, the spent fuel still contains about 0.8% by weight of uranium 235. Therefore, in order to make effective use of uranium resources, uranium collected by reprocessing spent fuel (hereinafter referred to as "recovered uranium") must be supplied to the enrichment plant again, concentrated to the required concentration and reused. Can be considered.

On the other hand, in a boiling water reactor, neutrons are absorbed by fissile material to cause fission, and neutrons released with energy at this time continue to emit energy by a chain reaction causing the next fission. A certain amount of fissile material has been loaded in the fuel assemblies that constitute the reactor since the initial stage of combustion, and the fuel assemblies from the loading of the fuel assemblies into the core until the fuel assembly ends its combustion period and is removed from the core The life of a body is largely determined by the fissile material contained in the fuel assembly. Burnup is used as an index of combustion of the fuel assembly, and is determined by the amount of heat generated per unit weight of fuel contained in the fuel assembly.

[0010] In order to improve the economy of the fuel loaded in the nuclear reactor, it is necessary to increase the burn-up by extending the period in which the fuel is loaded in the nuclear reactor as much as possible. Since the burnup of the fuel assembly increases as the amount of nuclear fuel material contained in the fuel assembly increases, the fissionability of the fuel assembly must be increased in order to increase the burnup of the fuel and improve economic efficiency. Increasing the material, that is, increasing the average enrichment of uranium 235 in the fuel assembly, has been performed.

Thus, effective utilization of uranium resources requires:
In a fuel assembly whose average enrichment has been increased in order to increase the burnup, it has become necessary to increase the usage rate of recovered uranium obtained by reprocessing spent fuel in the fuel assembly.

FIG. 3 shows the transition of the volume ratio of the gadolinia fuel rods in the fuel assembly with respect to the average fuel output of the fuel assembly as the burnup increases. As shown in the figure, the reactivity in the early stage of combustion increases with the increase in the average enrichment of uranium 235 in the fuel assembly. Therefore, in order to secure a furnace shutdown margin and a thermal margin, it is a burnable poison. Fuel rod containing gadolinia (hereinafter referred to as gadolinia fuel rod)
Also tends to increase. In the past, it was customary to avoid mixing the same neutron poison into a fuel rod containing neutron poison, which is flammable but gadolinia. However, as shown in FIG. Since the ratio of gadolinia fuel rods increases, it has become necessary to increase the usage ratio by mixing recovered uranium into gadolinia fuel rods.

Although several patents have already been filed except for pellets containing gadolinia, Japanese Patent Application No. 9-342527 has been filed.
It provides pellet enrichment and loading location for uranium fuel rods used without gadolinia fuel rods, as in the example of the above-referenced application.

[0014]

As described above, in order to improve the economy of fuel, when the removal burn-up is increased, the average enrichment of the fuel assembly increases, and the fuel assembly is a burnable poison. The content of gadolinia and the like also increases. An object of the present invention is to use recovered uranium obtained by reprocessing spent fuel in a fuel assembly in which the content of gadolinia or the like, which is a burnable poison, has increased with an increase in average enrichment. The object is to provide a fuel assembly that increases the proportion.

[0015]

In order to achieve the above object, according to the present invention, in a fuel assembly, fuel pellets containing burnable poisons such as gadolinia in the fuel assembly are provided.
The recovered uranium obtained by reprocessing spent fuel is used.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 shows an embodiment of the present invention. 4A and 4B have a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rod 11 is A
Uses the highest enriched fuel pellets, B,
The enrichment of C, D, E and fuel pellets is low.
Further, fuel rod types are set to 1 to 5 depending on the difference in enrichment distribution of each fuel rod. Where fuel rod type P
Is a short fuel rod, and G is a gadolinia fuel rod which is a burnable poison. Reference numeral 16 denotes a water rod.

In this embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 15%.

Further, by using recovered uranium for all other uranium fuel pellets, all fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

The fuel assemblies used in a boiling water reactor have two types of fuel assemblies having different numbers of gadolinia fuel rods (hereinafter, the number of gadolinia fuel rods is large in order to improve the operation flexibility. The fuel is referred to as high Gd fuel and the low fuel is referred to as low Gd fuel.) The loading ratio between the high Gd fuel and the low Gd fuel is changed according to the cycle operation period to optimize the thermal characteristics. . FIGS. 5A and 5B show the second
As an example of the first embodiment, a high Gd fuel in the case of using the low Gd fuel in the first embodiment is shown. However, when the recovered uranium is used only for the gadolinia fuel rod, the volume ratio of the recovered uranium used is about 19%.
%, Especially for high Gd fuels.

FIGS. 6A and 6B show a third embodiment to which the present invention is applied. The fuel assembly shown in FIG. 6 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rods 11 is such that A uses the highest enrichment fuel pellet, and B, C, D, E and the fuel pellet in the following order. Concentration is low. Further, fuel rod types are set to 1 to 5 depending on the difference in enrichment distribution of each fuel rod. Here, the fuel rod type P is a short fuel rod, and G is a gadolinia fuel rod which is a burnable poison. Reference numeral 16 denotes a water rod. In the present embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 20%. Further, by using recovered uranium for all other uranium fuel pellets, all the fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

FIGS. 7A and 7B show a fourth embodiment to which the present invention is applied.

The fuel assembly shown in FIG. 7 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rods 11 is such that A uses the highest enriched fuel pellets.
The enrichment of D, E, F and fuel pellets is low.
Further, the fuel rod types are set to 1 to 7 depending on the difference in the enrichment distribution of each fuel rod. Where fuel rod type P
Is a short fuel rod, and G is a gadolinia fuel rod which is a burnable poison. Reference numeral 16 denotes a water rod.

In this embodiment, recovered uranium is used for gadolinia fuel rods G1 and G2, which are two or more kinds of burnable poisons. Thus, in the present embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the recovered uranium usage volume ratio is about 20%. Furthermore, by using recovered uranium for all other uranium fuel pellets, all fuel rods are used as recovered uranium and the volume ratio of recovered uranium used is reduced to 10%.
0%.

FIGS. 8A and 8B show a fifth embodiment to which the present invention is applied. The fuel assembly shown in FIG. 8 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rod 11 is such that A uses the highest enrichment fuel pellet, and B, C, D, E and the fuel pellet in the following order. Concentration is low. Further, fuel rod types are set to 1 to 5 depending on the difference in enrichment distribution of each fuel rod. Here, G is a gadolinia fuel rod, which is a burnable poison, and 21 is a water box. In this embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 15%. Further, by using recovered uranium for all other uranium fuel pellets, all the fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

FIGS. 9A and 9B show a sixth embodiment to which the present invention is applied. The fuel assembly of FIG. 9 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rod 11 is such that A uses the highest enrichment fuel pellet, and B, C, D, E and the fuel pellet in the following order. Concentration is low. Further, fuel rod types are set to 1 to 5 depending on the difference in enrichment distribution of each fuel rod. Here, G is a gadolinia fuel rod, which is a burnable poison, and 21 is a water box. In this embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 1%.
9%. Further, by using recovered uranium for all other uranium fuel pellets, all the fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

FIGS. 10A and 10B show a seventh embodiment to which the present invention is applied. The fuel assembly shown in FIG. 10 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rod 11 is such that A uses the highest enrichment fuel pellet, and B, C, D and enrichment of the fuel pellet in the following order. The degree is low. Further, fuel rod types are set to 1 to 4 depending on the difference in enrichment distribution of each fuel rod. Here, the fuel rod type P is a short fuel rod, and G is a gadolinia fuel rod which is a burnable poison. Reference numeral 16 denotes a water rod.

In this embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 17%. Further, by using recovered uranium for all other uranium fuel pellets, all the fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

FIGS. 11A and 11B show an eighth embodiment to which the present invention is applied. The fuel assembly of FIG. 11 has a natural uranium blanket at the upper and lower ends, and the enrichment distribution of the fuel rod 11 is such that A uses the highest enriched fuel pellet, and B, C, D, E and the fuel pellet in the following order. Concentration is low. Further, fuel rod types are set to 1 to 5 depending on the difference in enrichment distribution of each fuel rod. Here, G is a gadolinia fuel rod which is a burnable poison, and 22 is a water cloth.

In this embodiment, when the recovered uranium is used only for the gadolinia fuel rod, the used volume ratio of the recovered uranium is about 20%. Further, by using recovered uranium for all other uranium fuel pellets, all the fuel rods can be recovered uranium and the volume ratio of recovered uranium used can be 100%.

[0031]

According to the present invention, in a fuel assembly containing gadolinia, which is a burnable poison, it is possible to increase the ratio of recovered uranium obtained by reprocessing spent fuel in the fuel assembly. it can.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a conventional fuel assembly.

FIG. 2 is a characteristic diagram showing an example of a change in a weight ratio of uranium 235 due to combustion.

FIG. 3 is a characteristic diagram showing an example of a volume ratio of gadolinia fuel rods in a fuel assembly with respect to a fuel assembly average extraction burnup.

FIGS. 4A and 4B are a plan view showing an arrangement of uranium pellets according to the first embodiment, and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 5A and 5B are a plan view showing an arrangement of uranium pellets according to a second embodiment and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 6A and 6B are a plan view showing an arrangement of uranium pellets according to a third embodiment, and a distribution diagram showing examples of a fuel enrichment and a gadolinia distribution.

FIGS. 7A and 7B are a plan view showing an arrangement of uranium pellets according to a fourth embodiment and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 8A and 8B are a plan view showing an arrangement of uranium pellets according to a fifth embodiment and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 9A and 9B are a plan view showing an arrangement of uranium pellets according to a sixth embodiment and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 10 (a) and (b) are a plan view showing an arrangement of uranium pellets according to a seventh embodiment, and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

FIGS. 11A and 11B are a plan view showing an arrangement of uranium pellets according to an eighth embodiment and a distribution diagram showing examples of fuel enrichment and gadolinia distribution.

[Explanation of symbols]

1 to 7: fuel rod type, 11: fuel rod, 12: channel box, 13: fuel assembly, 14: control rod, 15 ...
Neutron detector instrumentation tube, 16: water rod, 17: coolant area, 21: water box, 22: water cross, P: short fuel rod, G: burnable poison-containing fuel rod.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuo Suzuki 3-2-1 Sachimachi, Hitachi-City, Ibaraki Prefecture Within Hitachi Engineering Co., Ltd. (72) Inventor Sadayuki Izutsu 3-1-1 Sachimachi, Hitachi-City, Ibaraki Prefecture No. 1 Inside Hitachi, Ltd. Hitachi Plant (72) Inventor Shingo Fujimaki 3-1-1 Kochicho, Hitachi City, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd. Hitachi Plant

Claims (7)

[Claims] 1. A fuel assembly including a nuclear fuel material loaded in a core portion of a nuclear reactor using light water as a coolant and having a plurality of fuel rods arranged in a lattice, wherein At least one of the fuel rods contains a fuel rod containing a burnable poison, such as gadolinia, and is recovered by reprocessing spent fuel into a fuel pellet containing a burnable poison, such as gadolinia, in a fuel assembly. A fuel assembly using uranium. 2. The fuel assembly according to claim 1, wherein recovered uranium obtained by reprocessing spent fuel is used for all uranium-enriched fuel pellets handled in the uranium processing facility. 3. The fuel assembly according to claim 1, wherein the average enrichment of the fuel assembly is increased by increasing the burnup of the fuel assembly.
3.5% by weight or more of the fuel assembly. 4. The fuel rod according to claim 1, wherein the plurality of fuel rods include at least two types of fuel rods having different fuel pellet filling lengths, wherein the fuel pellet filling length is at least two. A fuel assembly, wherein at least one short fuel rod is arranged. 5. The fuel assembly according to claim 1, wherein the fuel rods constituting the fuel assembly are arranged in a grid of 9 rows and 9 columns, and seven fuels in the center thereof can be arranged. A fuel assembly, wherein two large-diameter water rods are arranged in a suitable area. 6. The fuel assembly according to claim 1 or 2, wherein the fuel rods constituting the fuel assembly are arranged in a grid of 9 rows and 9 columns, and one fuel rod is provided in a central area where nine fuels can be arranged. A fuel assembly, wherein a water box is arranged. 7. The fuel assembly according to claim 1, wherein the fuel rods constituting the fuel assembly are arranged in a grid of 10 rows and 10 columns.