JPH0545482A - Fuel assembly - Google Patents

Abstract

PURPOSE:To obtain a fuel assembly which can be converted efficiently into another substance which causes nuclear fission easily or of which the half-life is short, by using MA nuclides (Np, Am, Cm) in a high-level waste. CONSTITUTION:In a fuel assembly of which a fuel bundle is constructed by arranging a large number of fuel rods each prepared by inserting a nuclear fuel substance into a covering tube, the fuel rod contains in the nuclear fuel substance at least one kind of element out of Np, Am and Cm taken out of a spent fuel. Besides, the element is contained more in the upper region 1 of the fuel rod than in the lower region 2 thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for use in a boiling water reactor.

[0002]

2. Description of the Related Art A nuclear reactor using ordinary water as a coolant (hereinafter,
The fuel element used in the LWR is a enriched uranium fissionable fuel in which natural uranium is enriched to a U-235 concentration of 3 to 4%. Such fuel is irradiated in the reactor for 3 to 4 years, then taken out of the reactor, cooled for several years, and then reprocessed in a reprocessing plant. Spent fuel contains uranium and plutonium, as well as other actinide nuclides and fission products. In the reprocessing, uranium and plutonium are extracted from the spent fuel by chemical treatment, and actinide nuclides and fission products other than these are made into high-level waste. The extracted uranium and plutonium are refined and processed, and are effectively used again as fuel. On the other hand, high-level waste is stored for a long time until its radioactivity level is sufficiently low.

[0003]

High level waste must be stored safely until its radioactivity level is sufficiently low. The storage period depends on the type, content and half-life of the nuclide contained in the spent fuel. As an example of actinide nuclides other than uranium and plutonium contained in spent fuel, fuel with a U-235 concentration of 4% was irradiated in the reactor and taken out at a burnup of 45 GWd / t.
Table 1 shows the data after one year. These are much smaller than the amounts of uranium and plutonium in the spent fuel, so
It is called a minor actinide (hereinafter abbreviated as MA).
The process of generation of these MA nuclides in a nuclear reactor is shown in FIG.

It is said that it takes about 6 million years to reduce the radioactivity level of such high-level waste containing MA nuclides to an allowable value. This is because Np has a particularly long half-life.
This is because -237. However, Am-241 also becomes Np-237 when α-decays, so the effective half-life is Np-237.
That is all. Although the half-life of Cm isotope is not so long, the amount of neutrons and heat generated is large, so that a more strict container is needed to confine it than other elements. Therefore, if these MA nuclides can be removed from the high-level waste, the storage period of the high-level waste can be shortened to hundreds to thousands of years, and the storage burden can be reduced.

[0005]

[Table 1]

For this purpose, it is considered that MA nuclides are separated and purified from high-level waste, added to fuel, and irradiated in a nuclear reactor to be converted into a substance having a short half-life. One of the methods is to convert into fission products with a short half-life (several hundred years) by fission reaction. FIG. 6 shows the neutron reaction cross section for Np-237, which is the major MA nuclide, but the fission cross section is small for low energy neutrons and large for high energy neutrons above MeV. Other MA nuclides have similar cross-sectional area characteristics. Therefore, in order to utilize the fission reaction, it is appropriate to use a fast reactor having a lot of high energy neutrons rather than a light water reactor having a lot of low energy neutrons.

However, it is meaningful to consider the disappearance of MA nuclides in a light water reactor at present, when it is expected that the commercialization of the fast reactor will be delayed for several decades. However, since there are many low energy neutrons in the light water reactor, the MA nuclide is more likely to cause the capture reaction than the fission reaction. For example, Np-237
Captures neutrons and becomes Np-238, which has a half-life of 2 days and becomes Pu-238. A part of Pu-238 is irradiated in the reactor to become Pu-239, but most of it is taken out as it is without burning. Since Pu-238 has a short half-life of 87.7 years, Np-237 can be converted into a substance having a short half-life even in a light water reactor. Similarly, Am-241 captures neutrons and Am-242m (excited state) or Am
-242 g (ground state). The former is prone to nuclear fission,
The latter immediately becomes Cm-242. Am-243 captures neutrons and becomes Am-244, and immediately becomes Cm-244.
Cm-242 immediately becomes Pu-238 as described above. C
m-244 captures neutrons and becomes Cm-245, which is prone to fission. Therefore, any MA nuclide can be converted into a nuclide that easily undergoes fission or has a short half-life by a neutron capture reaction.

The object of the present invention is to reduce the burden of storage of high-level waste by neutron capture reaction of MA nuclides (neptunium, americium and curium) in currently operating light water reactors, especially boiling water reactors. Using
An object of the present invention is to provide a fuel assembly suitable for efficient conversion into another substance that is likely to undergo fission or has a short half-life.

[0009]

SUMMARY OF THE INVENTION The present invention is a fuel assembly in which a fuel bundle is formed by arranging a large number of fuel rods having a nuclear fuel material inserted in a fuel cladding tube, the fuel rods being used in the nuclear fuel material. It is characterized in that it contains at least one element of neptunium, americium, or curium extracted from the fuel, and the element is contained in a larger amount in the upper region than in the lower region of the fuel rod.

[0010]

As described above, the disappearance of MA nuclides in a light water reactor is mainly conversion to other nuclides by neutron capture reaction rather than fission. This neutron capture cross section occurs not only by low energy neutrons but also by resonance absorption of medium energy neutrons. The neutron capture cross section for each neutron is shown in Table 1. As a result of the inventor's detailed examination of the contribution to the annihilation of each neutron, it was found that the latter contribution was greater than the former in light water reactors. .. further,
It has been found that it is better to increase the proportion of medium-speed energy neutrons in order to efficiently perform the neutron capture reaction of MA nuclides in a light water reactor by utilizing this fact.

In the boiling water reactor, as the coolant flows upward from the lower part of the core, the coolant is heated by the heat of the fuel and boils. As a result, in the upper part of the core, the void rate of the coolant is high and the amount of water as the moderator is reduced, so that the moderation of neutrons is not performed so much, and the ratio of neutrons with high energy increases. Therefore, by incorporating more MA nuclides in the upper part than in the lower part in the fuel assembly, the neutron capture reaction with high energy can be utilized, so that the MA nuclide can be efficiently converted to another nuclide. be able to.

[0012]

EXAMPLES First to fourth examples of a fuel assembly according to the present invention will be described with reference to FIGS. 1 to 4. Each example is an example applied to a fuel assembly for a boiling water reactor. is there. Each drawing conceptually corresponds to the axial distribution state of the nuclear fuel material of the fuel assembly in each embodiment. Although not illustrated in the drawing, the fuel assembly is formed by arranging elongated fuel rods in, for example, 8 rows and 8 columns, and assembling a fuel bundle with upper and lower tie plates and spacers, and enclosing the bundle with channels. The fuel rod is formed by burning low-enriched uranium or the like, which is a nuclear fuel material, into pellets, inserting the pellet into a fuel cladding tube, and sealing the fuel cladding tube at the upper and lower ends with end plugs.

Each of the figures conceptually shows the axial distribution of the nuclear fuel material in the case where a large number of fuel rods having the above construction are assembled to form a fuel assembly. Here, the nuclear fuel material is U-235, U-238, MA nuclide, fissile P
u, natural uranium, and other elements that are inserted as pellets in the fuel cladding tube, and each fuel assembly has an upper region 1 and a lower region 2, an intermediate region 3, or an upper natural uranium region along the axial direction. 4 and lower end natural uranium region 5
Is equipped with.

(First Embodiment) A first embodiment will be described with reference to FIG. As is apparent from FIG. 1, the fuel assembly according to the first embodiment is divided into a central region and an upper region 1 and a lower region 2. The upper region 1 and the lower region 2 both have U-235 enrichment of 4.0%, but the upper region 1 further contains 0.5% of MA nuclide.

On the other hand, as a conventional example, the U-235 enrichment is the same as that of the present example in FIG. 1, but only the fuel assembly (conventional example 1) containing MA nuclides in the total length of 0.25% and the lower region. Fuel assembly containing 0.5% in the fuel (conventional example 2)
think of.

Table 2 shows the extinction rates of MA nuclides in these fuel assemblies. Here, the annihilation rate is the proportion of the neutron capture reaction, the fission reaction, or the one that has disappeared due to its own α-decay to the weight when loaded. Any of the MA nuclides can be eliminated more in the fuel assembly according to the first embodiment than in the conventional example 1 or the conventional example 2.

In this embodiment, the U-235 enrichment is equal in the upper and lower parts. However, when the MA nuclide is contained, the reactivity of the fuel is lowered, and when 0.5% of neptunium or americium is contained as in this example, it is reduced by about 3 to 4% Δk, and in the case of curium, it is reduced by 0.7% Δk. Further, when the MA nuclide is contained only in the upper region 1 as in this embodiment, only the reactivity of the upper region 1 is lowered, so that the output distribution is distorted downward and the peaking value becomes large. Therefore, in order to compensate this decrease in reactivity by increasing the enrichment of U-235, it is necessary to increase the enrichment of the upper region 1 by about 0.3%. In addition, in the case of fuels that originally have a difference in enrichment, a difference of 0.3% in addition to the original enrichment difference is added.

[0018]

[Table 2]

(Second Embodiment) A second embodiment will be described with reference to FIG. The description of the second embodiment is based on that of the first embodiment. The second fuel assembly was applied to a so-called MOX fuel in which plutonium obtained by reprocessing spent fuel was mixed with tail uranium (U-235 concentration 0.2 to 0.3%) obtained in the concentration step. Here is an example. Similar to the first embodiment, it is divided into an upper region 1 and a lower region 2 from the center just above and below, and the fissile plutonium (Pu-239 + Pu-241) concentration in both the upper and lower regions is high.
5.7%, and 0.5% of MA nuclide is contained only in the upper region 1. On the other hand, as a conventional example, the plutonium concentration is the same as that of the present example in FIG. 2, but the fuel assembly (conventional example 3) containing 0.25% of MA nuclide over the entire vertical length and 0.5% only in the lower region. Consider the contained fuel assembly (conventional example 4).

Table 3 shows the extinction rates of MA nuclides in these fuel assemblies. Any of the MA nuclides can be eliminated more in the fuel assembly according to the present embodiment than in Conventional Example 3 or Conventional Example 4. MOX
As for the fuel, MA is used as compared with the first embodiment, which is uranium fuel.
Although the extinction rate of nuclides is small, this is because plutonium has a larger neutron absorption cross section than uranium.
This is because the MA nuclide hardly absorbs neutrons. In addition, M
The method of compensating for the decrease in reactivity due to the nuclide A can be performed in the same manner as in the first embodiment.

[0021]

[Table 3]

(Third Embodiment) A third embodiment will be described with reference to FIG. The fuel assembly of the third embodiment is a uranium fuel, and as is clear from FIG. 3, a low enrichment (U-235 0.7%) is used to reduce neutron leakage at the upper and lower ends.
The upper and lower ends of the natural uranium regions 4 and 5 are provided. The inside is divided into upper, middle and lower three regions 1, 3 and 2. In order to flatten the output distribution which is likely to be the lower peak, the upper region 1 and the middle region 3 have a higher U-235 enrichment than the lower region 2. 0.2% higher. The MA nuclide is contained in 0.8% only in the upper region 1 having the highest void fraction. On the other hand, consider a fuel assembly (conventional example 5) containing 0.1% of MA nuclide over the entire length. MA for these fuel assemblies
The annihilation rate of the nuclide is shown in Table 4. The annihilation rate could be increased by this example. From the viewpoint of the void ratio, it is preferable that the natural uranium part at the upper end also contains the MA nuclide, but the effect of extinction is small because there are few neutrons at the end due to leakage.

In the present embodiment, the reactivity of the upper portion containing the MA nuclide is lowered, whereby the reactor shutdown margin can be increased. The reactor must be subcritical even if one control rod is pulled out during shutdown, and the subcriticality when the maximum value control rod is pulled out is the reactor shutdown margin. The neutron flux peak is at the top of the core during reactor shutdown. Therefore, according to the present embodiment, the reactivity of the upper portion of the fuel where the neutron flux becomes a peak can be reduced, and the reactor shutdown margin can be improved.

[0024]

[Table 4]

(Fourth Embodiment) A fourth embodiment will be described with reference to FIG. The fourth embodiment is similar to the third embodiment. The fuel assembly of the fourth embodiment is a MOX fuel, and plutonium is contained in the middle region 3 excluding the natural uranium regions 4 and 5 at the upper and lower ends, and the upper and middle regions 1 and 3 are lower than the lower region 2. Fissile plutonium concentration is 0.3% higher.
0.8% of MA nuclide is contained only in the upper part. On the other hand, consider a fuel assembly (conventional example 6) containing 0.1% of MA nuclide over the entire length. The annihilation rate of MA nuclides by these fuel assemblies is shown in Table 5. The annihilation rate could be increased by this example.

[0026]

[Table 5]

[0027]

According to the present invention, by containing a large amount of MA nuclides in the upper portion of the fuel assembly having a high void ratio of the coolant, the extinction by the medium energy neutron capture reaction of the MA nuclides can be efficiently performed. Therefore, the storage period of high-level waste can be shortened.

[Brief description of drawings]

FIG. 1 is an axial distribution diagram showing a distribution state of nuclear fuel substances in a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is an axial distribution diagram showing a distribution state of nuclear fuel material in a second embodiment of the fuel assembly according to the present invention.

FIG. 3 is an axial distribution diagram showing a distribution state of nuclear fuel material in a third embodiment of the fuel assembly according to the present invention.

FIG. 4 is an axial distribution diagram showing a distribution state of nuclear fuel material in a fourth embodiment of the fuel assembly according to the present invention.

FIG. 5 is a decay diagram showing the production process of MA nuclides in a nuclear reactor.

FIG. 6 is a graph showing a neutron reaction cross section of Np-237.

[Explanation of symbols]

1 ... Upper region, 2 ... Lower region, 3 ... Middle region, 4 ... Upper natural uranium region, 5 ... Lower natural uranium region.

Claims (1)

[Claims] 1. A fuel assembly in which a fuel bundle is formed by arranging a number of fuel rods in which a nuclear fuel material is inserted in a fuel cladding tube, wherein the fuel rods are neptunium or americium taken out of spent fuel in the nuclear fuel material. Alternatively, at least one element of curium is contained, and the element is contained in a larger amount in the upper region than in the lower region of the fuel rod.