JPS6138491A - 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 that utilizes recovered uranium obtained by reprocessing spent fuel taken out from a nuclear reactor.

[Technical background of the invention and its problems]

Since the spent fuel of a nuclear reactor contains substances useful as nuclear fuel, it can be reprocessed and reused as nuclear fuel. The isotopic composition of the recovered uranium obtained through this reprocessing varies depending on various factors such as the reactor type, the initial enrichment of the fuel, the burnup at the time of removal, and the cooling period from removal to reprocessing. An example of this in comparison with natural uranium is shown in Table 1 below.

Table 1 The recovered uranium shown in the above table is fuel for boiling water reactors (initial enrichment: 3.0 wt%) produced by enriching natural uranium.
) with a burnup of 28,000 MWd/l i
It was taken out after cooling, cooled for 10 years, and then reprocessed.

As is clear from Table 1, normally recovered uranium contains a concentration of υ equal to or higher than that of natural uranium. Therefore, by reusing recovered uranium, uranium resources can be used effectively and natural uranium resources can be saved.

However, since recovered uranium contains isotopes that do not exist naturally, caution is required when recycling it. Among these isotopes, tss U is rare and does not pose any particular problem, and fuel containing recovered uranium has inferior reactivity compared to natural uranium fuel when the J1 degree is the same. However, this problem can be solved, for example, by increasing the U enrichment. On the other hand, although the content of U is extremely small, the daughter nuclides (Bi, T/) produced as a result of its radioactive decay are 1Me.
Since it emits high-energy r-rays of V or more, it increases the air dose rate and causes the following. For this reason, fuel produced by enriching or mixing recovered uranium requires injection in large amounts for handling purposes. For reference, the production process of 2"U in the reactor is shown in Figure 5, and the radioactive decay sequence of U is shown in Figure 6, 1.

[Object of the Invention] An object of the present invention is to reduce the increase in radiation exposure that occurs when recovered uranium is reused in a nuclear reactor fuel assembly, thereby effectively and safely using recovered uranium. The goal is to make it usable as nuclear fuel.

[Summary of the invention]

The present invention provides a fuel assembly for a nuclear reactor using recovered uranium as a part of nuclear fuel, in which at least the fuel rods located at the outermost periphery of the fuel rods arranged in a lattice pattern constituting the fuel assembly are provided. The fuel rods are loaded with fuel pellets produced by enriching only natural uranium, and the fuel pellets containing recovered uranium are loaded on fuel rods at positions other than the outermost periphery.

That is, the present invention relates to a fuel assembly in which recovered uranium is utilized by not containing recovered uranium in the outermost fuel rod, and recovered uranium is used in this way.
This arrangement suppresses the increase in surface dose rate due to recovered uranium, allowing the recovered uranium to be used effectively and safely.

The reason why the recovered uranium is arranged as described above in the present invention will be explained next.

In general, substances with a large atomic number tend to absorb r-rays. Therefore, since the r-rays generated from the recovered uranium in the fuel pellets are easily absorbed by uranium (atomic number 92), the recovered uranium is filled inside the fuel assembly as described above. By arranging the loaded fuel rods, the r-rays emitted from them are absorbed by the uranium of the outermost fuel rods, making it difficult for them to reach the outer surface of the fuel assembly.

Therefore, according to the present invention, the dose rate on the outer surface of the fuel assembly does not become too high, and the recovered uranium can be used effectively and safely.

In the present invention, the fuel pellets loaded on fuel rods other than the outermost periphery include recovered uranium, as follows: (1) Uranium enriched by mixing natural uranium and recovered uranium (2) Only recovered uranium Enriched uranium (3) Mixed uranium of uranium enriched from natural uranium and recovered uranium (4) Mixed uranium of uranium enriched from recovered uranium and natural uranium (5) Uranium enriched from natural uranium and enriched recovered uranium Examples include uranium mixed with uranium.

[Embodiments of the invention]

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

FIG. 1 is a sectional view showing an example of the fuel rod arrangement of the fuel assembly of the present invention. Fuel rods 2 filled with pellets made only from natural uranium enriched uranium are arranged at each corner position of the entire outermost periphery and one row inside the outermost periphery, and the fuel rods 4 filled with pellets containing recovered uranium are arranged inside of these, and the fuel rod 4 is surrounded by the fuel rods 2 as a whole. 3 is a water rod, and l is a channel box. This fuel assembly has 30 fuel rods 4 and 32 fuel rods 2, and contains about 1.4 ppb/U of 2''U as a whole.

For comparison with the fuel assembly of the present invention described above, FIG. 2 shows a fuel assembly for a normal boiling water light water reactor that does not use recovered uranium, and FIG. A fuel assembly for a boiling water reactor is shown below.

In the case of the fuel assembly shown in Figure 3, the tst o daughter species TI contained in the recovered uranium of the fuel rods arranged on the outer periphery.
, The r-rays generated from Bi tend to leak out of the assembly, which poses a safety problem when handling, but in the fuel assembly of the present invention shown in Fig. 1, the r-rays are less likely to leak out. Safety can be increased.

As an expression of such effects, the relative surface dose rates of each fuel assembly shown in FIGS. 1 to 3 are shown in Table 2 below and FIG. 4.

Table 2 Figure 4 shows the U content (pp b/U) in the fuel assembly.
Or, it is a graph showing the relationship between the number of fuel rods using recovered uranium and the relative surface dose rate, where point a indicates the dose rate value of the fuel assembly in Figure 1, and point a indicates the dose rate value of the fuel assembly in Figure 2. The indicated point is indicated by b1, and the point indicating the dose rate value of the fuel assembly in FIG. 3 is indicated by C. The dose rate is proportional to the source intensity, which is
The content of recovered uranium is proportional to the number of fuel rods using recovered uranium.

Therefore, if the fuel rods using recovered uranium are arranged so that they are evenly distributed in the fuel assembly, the relationship between the number of fuel rods using recovered uranium and the relative surface dose rate is shown in Figure 4. As shown by the dotted line in , a straight line connecting point C becomes a straight line. Here, the reason why the surface dose rate does not become O even when the U° content is 0 is due to the γ-decay of U! This is because ``mPa emits r-rays.

However, in the fuel assembly of the present invention shown in Fig. 1, the r-rays are absorbed by the outer peripheral fuel rods that use only natural uranium, so the surface dose rate decreases, and as shown by point a, it is lower than the dotted line. It will be a low value.

In this example, the number of fuel rods using only natural uranium is only four in the outermost layer and each corner of the second layer, but if these are all in the outermost layer and the second layer, the fuel rods per aggregate are The glue content naturally decreases, but the surface dose rate further decreases due to the shielding effect of fuel rods made of only 1ζ natural uranium.

Although the above explanation has been made regarding a fuel assembly for a boiling water type nuclear reactor, the present invention is similarly applicable to a fuel assembly for a pressurized water type nuclear reactor.

〔Effect of the invention〕

According to the present invention, the r-rays emitted from recovered uranium are absorbed within the fuel assembly and the dose rate on the outer surface of the fuel assembly is reduced, so recovered uranium obtained from spent fuel can be reused safely and effectively. be able to.

[Brief explanation of the drawing]

Figure 1 is a fuel rod layout diagram showing an example of the fuel assembly of the present invention, Figure 2 is a fuel rod layout diagram of a normal fuel assembly that does not use recovered uranium, and Figure 3 is a fuel rod layout diagram showing an example of the fuel assembly of the present invention. Figure 4 is a fuel rod arrangement diagram of the fuel assembly used in the fuel rod, and Figure 4 is a graph showing the relationship between the U content in one fuel assembly or the number of fuel rods using recovered uranium and the relative surface dose rate. Figure 6 is a diagram showing the generation process of U in a nuclear reactor, and is a diagram showing the radioactive decay sequence of U. 1... Channel box 2... Fuel rod loaded with fuel pellets made only from natural uranium 3... Water rod 4... Fuel rod loaded with fuel pellets containing recovered uranium (8733) Agent Patent attorney Yoshiaki Inomata (
(and 1 other person) Figure 1 Figure 2 Figure 3 Figure 4 Figure length r, i-'e,, per side t zJ2u'Hd (pf/
u) No. 5 Figures 127-111Md

Claims (2)

[Claims] (1) In a nuclear reactor fuel assembly that uses recovered uranium as part of the nuclear fuel, at least the outermost fuel rod of the fuel rods arranged in a lattice that makes up the fuel assembly contains natural uranium. 1. A fuel assembly for a nuclear reactor, characterized in that fuel pellets produced by enriching only uranium are loaded, and the fuel pellets containing recovered uranium are loaded in fuel rods other than the outermost periphery. (2) All of the outermost fuel rods and the four fuel rods placed at the four corners of the inner row are loaded with fuel pellets made by enriching only natural uranium, and the other fuel rods are loaded with fuel pellets made by enriching only natural uranium. The fuel assembly for a nuclear reactor according to claim 1, wherein fuel pellets containing recovered uranium are loaded.