JPH0675077A - Fuel assembly for nuclear reactor - Google Patents

Abstract

PURPOSE:To totally reduce fission material amount and improve fuel economy by increasing the proportion of uranium 235 in fuel rods in at least one fuel layer to be larger than that in immediately inner layer. CONSTITUTION:The enrichment of fissile plutonium or the proportion of uranium 235 in each fuel rod is 4.8wt% of enriched uranium of 235 for the outer layer fuel rods 4 and natural uranium of ca. 7wt% uranium 235 added by fissile plutonium of 2.1wt% enrichment for the inner and medium layer fuel rods 2, 3. Thus, core average fissile material amount is 3.7wt%. As the outer layer fuel rods 4 are of enriched uranium, neutron absorption by plutonium is mitigated compared with MOX fuel and neutron penetrability to inner fuel assembly is improved. Therefore, fissile material amount in inner and medium layer 2, 3 can be reduced to get the same power and core average fissile material amount is saved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly using MOX fuel for a heavy water moderated boiling light water cooling pressure tube reactor, and more particularly, to an average fission material amount of the assembly (nuclear species that causes fission U -235, Pu-239, Pu-241 relative to the whole) and to improve fuel economy.

[0002]

2. Description of the Related Art A conventional fuel assembly for a pressure tube reactor is
As shown in FIG. 2, small diameter rod-shaped uranium fuel was plutonium-enriched around the central spacer support tube 1 (hereinafter, referred to as MO
X) A cluster type fuel assembly in which a large number of fuel rods 2, 3 and 4 are bundled in a cylindrical shape is common. In this type of reactor, high-speed neutrons generated by nuclear fission go out of the assembly once and are decelerated by heavy water, which is a moderator of neutrons, to an energy region where a fission reaction easily occurs, and then the fuel assembly Come back to.

Therefore, as a result of neutron absorption in the fuel region, the neutron flux (thermal neutron flux) after deceleration decreases in the order of the outer layer, the intermediate layer and the inner layer as shown in FIG. In the above case, the larger the plutonium loading ratio, that is, the smaller the uranium 235 ratio, the larger due to the strong neutron absorption effect of the plutonium fuel. Based on this, in the conventional example shown in FIG. 2, the ratio of uranium 235 is
All fuel rods are the same (0.7 wt% in this example because natural uranium fuel is used), but the power distribution within the fuel assembly (almost proportional to thermal neutron flux x amount of fission material) is as close as possible. As described above, the amount of fission material in the intermediate layer fuel rod 3 having a small thermal neutron flux is large, and the amount of fission material in the outer layer fuel rod 4 having a large thermal neutron flux is small. Although the amount of fission material is reduced in the inner layer fuel rod, which has a relatively small effect on the uniformity of power distribution, as described above, since the amount of fission material in the middle layer with a small thermal neutron flux is increased, the fuel economy is reduced. It is not preferable from the viewpoint of sex.

(Public example: Japanese Patent Laid-Open No. 61-76982: A fuel assembly in which all fuels are natural uranium MOX fuels and the plutonium enrichment in the inner layer and the outer layer are equalized).

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce the amount of fission material and improve fuel economy in a fuel assembly using MOX fuel of a heavy water deceleration boiling light water cooling pressure tube reactor. It is to provide a fuel assembly that can.

[0006]

In order to achieve the above object, the proportion of uranium 235 in the fuel rods of at least one fuel layer of the fuel assembly is made larger than the proportion of uranium 235 in the fuel rods of the immediately inner layer. The fuel rods are arranged.

[0007]

A fuel assembly having a three-layer structure of an outer layer, an intermediate layer and an inner layer will be described as an example.

In the MOX fuel assembly in which the uranium 235 ratio of the outer layer fuel rod is made larger than the uranium 235 ratio of the fuel rod of the inner layer, that is, the intermediate layer, the plutonium ratio of the outermost fuel rod is small. For the above-mentioned reason, the rate of thermal neutrons coming from the outside of the fuel assembly is smaller than that of the conventional MOX fuel assembly in which the proportion of uranium 235 is uniformly absorbed by the outer fuel rods. As a result, the degree of inclination of the neutron flux distribution becomes smaller than that of the conventional type, as shown in FIG.

Therefore, assuming that the power distribution in the assembly is the same as that in the case where the ratio of uranium 235 is uniform in all layers, the ratio of the amount of fission material in the fuel rods in the fuel layer immediately inside the fuel layer to the fuel layer fuel rods is determined. Can be made smaller.

When the fuel having the same amount of the average fission material of the aggregate is used, it is possible to achieve higher burnup than before. In other words, it was possible to reduce the amount of fissile material to obtain the same output. In other words, if the burnup is the same (equivalent to the integrated value of the output), the amount of fissionable material can be reduced.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIGS.
It will be described with reference to FIG.

FIG. 4 is a plan view of the target core in which the fuel assemblies are loaded. The total number of fuel assemblies is 648. Electric power 600,000 kW, 15 months continuous operation, 4-batch refueling core is assumed.

In the fuel assembly of the conventional example, as shown in FIG. 2, all fuels are MOX fuels (the base fuel that enriches plutonium is natural uranium fuel), and the plutonium enrichment of the outer layer and the inner layer is equal. Enriched.

In this fuel, as shown in FIG. 5, the amount of fission material in the outer and inner layers is 3.7 wt%, the amount of fission material in the middle layer is 5.2 wt%, and the average fission material amount in the core is 4%. It is 0.2 wt%.

On the other hand, FIG. 1 shows the fuel rod arrangement of the fuel assembly of the present invention, in which the outer layer is a concentrated uranium fuel, and the middle and inner layers are a natural uranium-rich plutonium-enriched MOX fuel. There is.

In this embodiment, since the outer layer is made of enriched uranium, the strong neutron absorption effect of plutonium is relaxed as compared with the case of using MOX fuel, so that the neutron permeability to the inside of the assembly is improved and The bundle distribution is
As shown in FIG. 5, the distribution becomes flatter. As a result, if the power distribution is almost the same as the conventional example, it is not necessary to increase the amount of fission material in the middle and inner layers more than necessary. This is wasteful and can be reduced in this embodiment).

Regarding the fissile plutonium enrichment of each fuel rod or the uranium 235 ratio in this embodiment, the outer layer fuel is uranium 235 and the enriched uranium fuel is 4.8 wt%, and the inner and middle layers are natural uranium (uranium 235). 0.7w in proportion
t%), which is a fuel enriched with fissile plutonium enrichment of 2.1 wt%. Therefore, the average nuclear fission material amount is 3.7 wt%. (In the present embodiment, uranium 235
The ratio is 4.8, 0.7, 0 in the order of outer layer, middle layer and inner layer.
It is 7 wt%. In the conventional example, all are 0.7 wt%. ) From the above, when the fuel assembly of the present example is loaded, since the outer layer is made of enriched uranium, the strong neutron absorption effect by plutonium is mitigated as compared with the case of using MOX fuel, so that the inner side of the assembly is reduced. As a result of the improved neutron permeability of the core, the fraction of fissionable material in the inner and middle layers to obtain the same output can be reduced. The amount of substance is about 12 from 4.2wt% to 3.7wt%
You can save%.

[0018]

According to claim 1, at least a part of the MO
When using X fuel, the amount of uranium 235 in the fuel rods of at least one fuel layer is made larger than the uranium 235 ratio in the fuel rods of the immediately inner layer so that the amount of fission material required to obtain the same burnup is obtained. Can be reduced and fuel economy can be improved. Further, according to claim 2, by using the enriched uranium fuel as the outer layer and the plutonium-enriched MOX fuel as the intermediate layer and the inner layer, the amount of fission material necessary for obtaining the same burnup can be reduced, and the fuel economy can be improved. The property is improved.

[Brief description of drawings]

FIG. 1 is a sectional view of a fuel rod of a fuel assembly according to the present invention.

FIG. 2 is a sectional view of a fuel rod of a conventional fuel assembly.

FIG. 3 is a distribution diagram of thermal neutron flux in a fuel assembly.

FIG. 4 is a plan view of a core loaded with a fuel assembly.

FIG. 5 is a fission material amount, thermal neutron flux, and power distribution map in the fuel assembly.

[Explanation of symbols]

 2 ... inner layer fuel rod, 3 ... middle layer fuel rod, 4 ... outer layer fuel rod.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunikazu Kanato 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd., Hitachi Works

Claims (4)

[Claims] 1. A heavy water decelerated boiling light water cooling pressure tube type reactor fuel assembly in which a large number of fuel rods are bundled in an annular shape, when a fuel enriched with plutonium is used as at least a part of the fuel rods. The fuel assembly for a nuclear reactor, wherein the uranium 235 ratio of the fuel rods of at least one fuel layer of the fuel assembly is larger than the uranium 235 ratio of the fuel rods of the immediately inner layer. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein the fuel rods in the outermost layer are uranium fuels not enriched with plutonium, and the remaining fuels are fuels enriched with plutonium in the uranium fuels. 3. The fuel assembly for a nuclear reactor according to claim 2, wherein natural uranium is used as uranium in the fuel rod enriched with plutonium in the uranium fuel other than the outermost layer. 4. The uranium in a fuel rod enriched with plutonium in the uranium fuel other than the outermost layer, as recovered uranium taken out by reprocessing spent fuel, or as a tail after enrichment of natural uranium. A fuel assembly for a nuclear reactor that uses the depleted uranium extracted.