JPS6232385A - Fuel aggregate for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly for a nuclear reactor using recovered uranium.

[Technical background of the invention and its problems]

Conventional light water reactors 1For example, in boiling water reactors, enriched uranium with an average concentration of 3% is used, and the extraction burnup is approximately 30%.
It is burned to 0Wd/mt. Since the spent fuel still contains about 0.8% by weight of ``'U,'' it is reprocessed to recover uranium in order to effectively utilize uranium resources, and the recovered uranium is sent back to the enrichment plant. The idea is to supply it, concentrate it to the required concentration, and reuse it.

However, there are the following problems when re-enriching the recovered uranium and reusing it as υ02 fuel.

First, spent fuel contains t3''tJ, which is generated through a complicated process through combustion within the nuclear reactor.The degree of its purity is extremely subtle, but it is accompanied by its radioactive decay. daughter nuclides (""Bi, ""Tl
2) (7) Recovered uranium will increase the air dose rate more than natural uranium because of the high-energy gamma rays it emits, exceeding IMaV. For this reason, recovered uranium fuel causes problems in various processes of fuel manufacturing. This is done by measuring the intensity of nuclear fission gamma rays produced by nuclear fission, but if recovered uranium is present at this time, measurement accuracy is impaired due to gamma rays from daughter nuclides of 232U. In addition, due to the high air dose rate in terms of fuel manufacturing and assembly and quality control, it must be handled differently than natural uranium enriched fuel. Therefore, the presence of recovered uranium introduces various complications in each process.

Second, ``U'' is produced by neutron capture of ``U'' in the nuclear reactor, and this z3''U is approximately 0.0% in the recovered uranium.
Since @2@I), which contains 4% by weight, is a neutron absorber, re-enriched uranium obtained by re-enriching 1-recovered uranium has a higher concentration than enriched uranium of the same enrichment obtained by enriching natural uranium. Reactivity decreases. Therefore, to compensate for this decrease in reactivity! The concentration of 35U would have to be increased, reducing the benefits of using recovered uranium.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and is intended to minimize the complexity of each process and quality control in a fuel manufacturing plant in a fuel assembly using recovered uranium recovered from spent fuel.

Furthermore, the purpose is to minimize the decrease in reactivity due to 3''U.

[Summary of the invention]

The present invention provides a nuclear reactor fuel assembly in which a large number of fuel rods with different fuel enrichments are arranged in a lattice pattern, in which recovered uranium obtained by reprocessing spent fuel is contained in fuel rods with one type of enrichment. It is characterized by being loaded only in

In the present invention, the fuel rods loaded with recovered uranium fuel are of only one type of enrichment.

First of all, the types of fuel rods that contain recovered uranium fuel are reduced, and various complications associated with the production of recovered uranium fuel are reduced. Furthermore, since the fuel rods containing recovered uranium have only one type of enrichment, only the fuel rods with that enrichment are removed from the production series.

It becomes possible to manufacture only the recovered uranium by a method suitable for the recovered uranium, and the above-mentioned complications associated with recovering uranium for each fuel rod of individual enrichment do not occur in each manufacturing process as in the past.

Further, in the present invention, it is desirable that the fuel rod loaded with the recovered uranium fuel be a fuel rod of the highest enrichment. The reason for this will be explained next.

Let the weight percentages of "'U and 23@U in the recovered uranium be R and νR, respectively.The weight percentages of 23"U and ""U in the enriched uranium obtained by re-enriching this recovered uranium are respectively yP, 1sU in enriched waste uranium
Let the weight ratio of . At this time, if R dazzle of recovered uranium is supplied to obtain Pkg of enriched uranium, the total amount of 23"U in the enriched uranium yp' P and the total amount of 23"U in the recovered uranium.
The ratio of Uf to R is given by the following equation.

According to the above formula x)2=0.008. xy=0.002
Figure 2 shows the relationship between Notoki's work and yp''P/y)1-R.As can be seen from the figure, when recovered uranium is re-enriched, The total 23@uf brought in decreases as the 20U enrichment of re-enriched uranium increases. Therefore, in the present invention, in the fuel rod loaded with recovered uranium, the enrichment of the recovered uranium is increased to 23@uf. By using the highest U enrichment, it is possible to reduce the negative effects of decreased reactivity due to !31 U, and it is possible to limit the number of fuel rods loaded with the above-mentioned recovered uranium to only fuel rods with the same type of enrichment. Combined with its effectiveness, recovered uranium can be used more advantageously.

[Embodiments of the invention]

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

FIG. 1 is a fuel rod arrangement diagram of a fuel assembly for explaining one embodiment of the present invention. As shown in FIG. 1, a fuel assembly 1 is surrounded by a channel box 2, in which fuel rods 3 and water rods 4 are regularly arranged in a grid pattern, and are arranged around control rods 5. There is. The fuel rods 3 are comprised of six types of fuel rods with different uranium enrichment levels, indicated by reference numerals 11 to 16.

Each fuel rod of the fuel assembly contains 235 U.

The respective concentrations of 23G U and Gd, 03 are shown in Table 1 below.

Table 1 As is clear from the above table, in this example, 239U is contained only in the fuel rod numbered 11, that is, the recovered uranium is used only in the fuel rod numbered 11.

The fuel obtained from recovered uranium used here is 23
Reconcentrated recovered uranium with 5U and 23'U concentrations of 0.8% and 0.4% by weight, respectively, was made into a product with a @3'IJ concentration of 3.9% by weight. Yes, fuel pellets for exactly 12 fuel rods can be produced from 208 kg of recovered uranium. 5 o fuel rods other than No. 11 are obtained by enriching 710 kg of natural uranium.

For comparison with this example, the following 8 examples using a mixture of recovered uranium and natural uranium are shown below.
Using 710 kg of recovered uranium and 710 kg of natural uranium,
These were mixed and concentrated to form a fuel assembly having the same 135 U concentration distribution as in this example shown in Table 2 below.

As is clear from Table 2, in the case of the comparative example, if a fuel assembly with the same 20U concentration distribution as the present example is manufactured, ! ”
The U concentration distribution is as shown above, with 20 U per fuel assembly.
The content of U varies.

In the above example, the amount of 11U in this example is reduced by about 6% compared to the comparative example, and therefore the amount of necessary natural uranium can be reduced.

5. In order to produce the fuel necessary for the fuel assembly of the present invention,
A specific enrichment cascade can be chosen for the recovered uranium, so that only the recovered uranium is enriched; centrifuges are suitable for this. Since the centrifugal separator has a large separation coefficient, a cascade of several tens of stages is sufficient, and the plant scale is relatively small, so it is suitable for cases where mono-enrichment is required, such as the recovered uranium fuel of the present invention. On the other hand, the gas diffusion method requires a cascade of several hundred stages because the separation coefficient per stage is small, resulting in a large plant size.
This method is suitable for cases where there are many types of enrichment, such as normal fuel, since fuels with a plurality of enrichments are extracted from the middle of the cascade, but this method is not suitable for the recovered uranium fuel of the present invention.

〔Effect of the invention〕

As explained above, according to the present invention, fuel production is possible by using fuel obtained from recovered uranium for only one enrichment fuel rod among fuel rods composed of many different enrichment levels. Various complications associated with recovered uranium are reduced, and the various processes and management of fuel production are simplified.

In particular, if one of these types of fuel is the highest enrichment fuel (in other words, if the recovered uranium is re-enriched to the highest enrichment of the fuel assembly and used as a loaded fuel), 21
There is an effect that the content of @U is reduced and the decrease in reactivity due to 23'U is also reduced accordingly.

[Brief explanation of the drawing]

Fig. 1 is a fuel rod arrangement diagram of a fuel assembly explaining one embodiment of the present invention, and Fig. 2 shows the concentration of "'U in re-enriched uranium when recovered uranium is re-enriched, and the concentration of ''U in re-enriched uranium". All 13 of
It is a graph showing the relationship between the 1U weight and the ratio of the total 23@U weight in recovered uranium.

Claims (2)

[Claims] (1) In a nuclear reactor fuel assembly made up of a large number of fuel rods arranged in a lattice with varying fuel enrichment, the recovered uranium fuel obtained by reprocessing spent fuel is fuel with one type of enrichment. A nuclear reactor fuel assembly characterized in that only the rods are loaded. (2) The fuel assembly for a nuclear reactor according to claim 1, wherein the recovered uranium fuel is loaded only in the fuel rods with the highest enrichment.