JPH0479434B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for use in a boiling water nuclear reactor. [Background of the Invention] In a conventional boiling water reactor, a fuel assembly containing enriched uranium of about 3% by weight on average is taken out and burned until the burnup reaches about 30 GWd/mt.
An example of a conventional fuel assembly is shown in FIG. The fuel assembly 3 is composed of a plurality of fuel rods 1 and a channel box 2. The control rods 4 or the neutron detector instrumentation tubes 5 are arranged between the fuel assemblies 3 with the fuel assemblies 3 loaded in the reactor core. For this reason, the interval between each fuel assembly 3 is widened to the extent that devices such as the control rods 4 are inserted, and the periphery thereof is filled with cooling water. The upper and lower ends of the fuel rod 1 are supported by upper and lower tie plates. The channel box 2 is fixed to the upper tie plate and is connected to the fuel rod 1.
surrounding a bunch of 6 is Waterrod, 7
is the coolant region through which the coolant flows. The coolant is
It flows inside the channel box 2 from the bottom to the top. During combustion, (i) fissile materials (uranium-235, plutonium
(ii) conversion of parent fuel materials (uranium-238, plutonium-240) into fissile materials, and (iii) accumulation of fission products. Fissile material and fuel parent material are called nuclear fuel material. Figure 2 shows an example of changes in the weight percentage of uranium-235 due to combustion. As shown in the figure, the spent fuel contains approximately 0.8% uranium by weight.
235 still included. Therefore, as a method to effectively utilize uranium resources, recovered uranium (hereinafter abbreviated as recovered uranium) obtained by reprocessing spent fuel is supplied again to an enrichment plant and enriched to the required concentration. It is possible to reuse it as fuel. However, if the recovered uranium is re-enriched,
There are the following problems when reusing _UO2 as fuel. As combustion progresses in a nuclear reactor, uranium-235 is
As shown in (i) above, it is consumed by the nuclear fission reaction and is converted to uranium-236 by absorbing thermal neutrons. Furthermore, when neutrons are absorbed by uranium-236,
Subsequent β-decay produces neptunium-237. Figure 3 shows an example of changes in the weight ratio of uranium-236 and neptunium-237. Curve 8 is uranium-236,
Curve 9 shows neptunium-237. As shown in the figure, spent fuel (burnup approximately 30GWd/mt)
contains about 0.4% by weight of uranium-236 and about 0.03% by weight of neptunium-237. Both uranium-236 and neptunium-237 are neutron absorbers.
The absorption cross section for thermal neutrons is 2.7 burns for uranium-238, 5.2 burns for uranium-236, and 5.2 burns for neptunium-237 for neutrons at 2200 m/sec.
It's big at 170 barns. Therefore, when using UO ₂ fuel obtained from recovered uranium, there will be a reduction in reactivity (reactivity penalty) corresponding to the concentration of uranium 236 contained. To solve this problem, the concentration of uranium-235 must be increased, which requires new natural uranium. This results in the opposite of effective use of uranium resources. The concentration of uranium-236 is determined by the supply ratio of recovered uranium to natural uranium in the reenrichment plant, but increasing the amount of recovered uranium used to reduce the amount of natural uranium required will increase the amount of _UO2 fuel. The total amount of uranium-236 involved increases. the result,
The effect of reactivity penalties is also increased. [Object of the Invention] An object of the present invention is to provide a fuel assembly that can reduce the reactivity penalty and reduce the amount of natural uranium. [Summary of the Invention] The present invention is characterized in that the amount of uranium obtained by reprocessing spent fuel present in the lower region of the fuel assembly is greater than the amount of uranium present in the upper region. be. Uranium obtained by reprocessing spent fuel is called recovered uranium. The present invention was made based on the results of a detailed study of the phenomenon of reactivity penalty of recovered uranium. The results of this study will be explained below. Figure 4 shows the relationship between the weight proportion of uranium-236 and the reactivity penalty when the concentration of uranium-235 is the same. The reactivity penalty is not proportional to the concentration of uranium-236; the higher the concentration of uranium-236, the more
The reactivity penalty per atom of 236 is becoming smaller. This is because uranium-236 has a large self-shielding effect in resonance absorption. Therefore, when using the same amount of recovered uranium (when UO ₂ fuel contains the same amount of uranium-236), the reactivity penalty can be reduced by localizing uranium-236. Uranium-236 is rarely found in natural uranium, but is found in large amounts in recovered uranium. Recovered uranium is nuclear fuel material obtained by reprocessing spent fuel. Figure 5 shows the hydrogen to uranium atomic ratio and the uranium
This figure shows the relationship between the decrease in reactivity due to 236.
The figure shows that the reactivity penalty decreases as the hydrogen to uranium atomic ratio increases (the aggregate average void fraction decreases). Based on the void ratio of 0% (point A in the figure), the reactivity penalty increases by about 35% at a void ratio of 70% (point B in the figure). In the neutron spectrum of a boiling water reactor,
Neutron absorption of uranium-236 is centered in the resonance energy region. FIG. 6 shows the resonance energy region neutron flux to thermal neutron flux ratio 10 and the fast energy flux to thermal neutron flux ratio 11. According to this, when the ratio of hydrogen to uranium atoms increases (the void fraction decreases), the ratio of neutron flux in the resonance energy region to thermal neutron flux (10 in FIG. 6) decreases. The reactivity penalty is uranium in the resonance energy region.
Since it is approximately proportional to the fission amount of 235, as shown in Figure 5, the reactivity penalty of uranium-236 decreases as the water to uranium atomic ratio increases. Similar to FIG. 5, A and B in FIG. 6 indicate points at which the average void ratio of the fuel assembly is 0% and 70%. From the above, in order to reduce the reactivity penalty of uranium-236 and reduce the amount of natural uranium required, it is necessary to
We found that it is desirable to localize 236.
The present invention has been made based on this basic principle. A boiling water reactor has an axial void distribution as shown in Figure 7. The lower part of the core, which is the coolant inlet, is in a subcooled state, and as the coolant rises in the core, it enters the subcooled boiling or saturated boiling region, and the upper part of the core, which is the coolant outlet, is in a subcooled state. In this case, the void ratio has reached around 70%. Therefore, in order to realize the above-mentioned principle, uranium in the upper axial region of the fuel assembly must be
It can be seen that the amount (for example, concentration) of 236 can be made lower than that of the lower region. Since a large amount of uranium-236 is contained in recovered uranium, the amount of recovered uranium in the lower region should be greater than that in the upper region. For this reason, the upper region contains more fuel material produced from natural uranium than the lower region. [Examples of the Invention] The present invention will be explained in detail below with reference to Examples. FIG. 8 is a cross section of a fuel assembly in which uranium 236 is mixed in a conventional fuel assembly in which rods are arranged in 8 rows and 8 columns so as to have a uniform uranium 236 concentration distribution in the axial direction. 1 is a fuel rod, 2 is a channel box, 4 is a control rod, and 6 is a water rod.
Numbers 12, 13, 14, 15 shown at fuel rods
and 16 indicate fuel rods with different enrichment levels. Table 1 shows fuel rods 12 to 16 with uranium-235
indicates the concentration of (W/O) indicates weight %.

[Table] On the other hand, as shown in Fig. 9 and Table 2, the fuel assembly according to the embodiment of the present invention contains uranium in the axial direction of the fuel rods.
The 236 concentration changes between the upper and lower parts (the uranium-236 concentration is higher in the lower part). The fuel assembly is axially
It is divided into 24 equal parts and divided into two areas, an upper area and a lower area, between the 12th and 13th areas from the bottom. The enrichment distribution of uranium-235 is shown in the fuel assembly (Figure 7).
is the same as the total amount of uranium-236 in the fuel assembly,
In other words, the total amount of recovered uranium used is also the same as that of the fuel assembly. The fuel assembly contains a large amount of fuel material made from recovered uranium in the lower region, and a large amount of fuel material made from natural uranium in the upper region.

【table】

[Table] Table 3 shows the difference in the infinite neutron multiplication factors of fuel assemblies when the fuel assemblies are used as a reference.

[Table] From this, on average for fuel assemblies, this example (fuel assembly) is approximately 0.2% lower than the fuel assembly.
ΔK neutron infinite multiplication factor increases. As a result, the enrichment of the uranium 235 in the fuel assembly can be lower than that of the fuel assembly, and the amount of natural uranium required can be reduced by about 1%. When the void fraction of the coolant is 40%, the difference in the infinite neutron multiplication factor between the upper and lower regions of the fuel assembly is:
It is approximately 1.3% ΔK, which is higher in the upper region.
When this fuel assembly is loaded into a boiling water reactor,
Due to the void distribution occurring in the axial direction, the difference in infinite neutron multiplication factors between the upper and lower core regions is canceled out, and a relatively flat axial power distribution can be achieved in the axial direction. In this example, the concentration of uranium-236 in the upper and lower regions of the reactor core is changed to increase the infinite neutron multiplication factor in the upper region, so the difference in the infinite multiplication factor between the upper region and the lower region is does not change throughout the operating period. Therefore, the power distribution in the axial direction can always be maintained flat throughout the operating period of the nuclear reactor, as described above. Other embodiments of the invention will be described below. In order to further reduce the required amount of natural uranium than in the fuel assembly, the fuel assembly of this example increases the axial distribution of uranium 236 in the fuel assembly and promotes localization of uranium 236. Table 3 shows the distribution of uranium-236 in the fuel assembly. The uranium-235 enrichment distribution and the total amount of uranium-236 are equal to the fuel assembly.

[Table] The difference in the infinite neutron multiplication factor of fuel assemblies based on the fuel assembly is approximately
The amount of natural uranium required can be reduced by approximately 2%. A fuel assembly according to another embodiment of the present invention uses both an axial distribution of uranium-236 and a cross-sectional distribution of uranium 236 in order to further increase the effect of the fuel assembly. A cross section of the fuel assembly is shown in FIG. Table 4 shows the concentrations of uranium-235 and uranium-236 in fuel rods 22-26.

【table】

[Table] The uranium-235 enrichment distribution and the total amount of uranium-236 are the same as in the fuel assembly. Table 5 shows the difference in the infinite neutron multiplication factors of the fuel assemblies when the fuel assemblies are used as a reference.

〔Effect of the invention〕

According to the present invention, the amount of recovered uranium present in the lower region of the fuel assembly is made larger than the amount of recovered uranium present in the upper region, so that the reactivity penalty due to recovered uranium can be reduced, and the required amount of natural uranium can be reduced. made it possible to reduce

[Brief explanation of drawings]

Figure 1 is a horizontal sectional view of a conventional fuel assembly;
Figure 3 is a diagram showing changes in U ²³⁵ due to combustion, Figure 3 is a diagram showing changes in uranium 236 and neptunium 237 due to combustion, and Figure 4 is a graph showing changes in uranium 236 concentration and reactivity penalty. Figure 5 is a diagram showing the relationship between the hydrogen to uranium atomic ratio and the reactivity penalty of uranium-236. Figure 6 is a diagram showing the relationship between the hydrogen to uranium atomic ratio and the resonance energy region and fast energy region neutron flux. 7 is a diagram showing the void distribution in the core axis direction, FIG. 8 is a horizontal cross-sectional view of a conventional fuel assembly, FIG. 9 is a cross-sectional view of Example 1, and FIG. 10 is a cross-sectional view of Example 3.

Claims (1)

[Scope of Claims] 1. In a fuel assembly having a plurality of fuel rods containing nuclear fuel material, uranium obtained by reprocessing spent fuel is added to a lower region than an upper region of the fuel assembly. A fuel assembly characterized by having a large amount of fuel. 2. The fuel assembly according to claim 1, wherein the uranium obtained by reprocessing the spent fuel contains uranium-236. 3. The fuel rod according to claim 1, wherein more uranium obtained by reprocessing spent fuel is present in the lower region than in the upper region of the fuel rods arranged in the center of the cross section of the fuel assembly. fuel assembly. 4. The fuel assembly according to claim 3, wherein the uranium obtained by reprocessing the spent fuel contains uranium-236.