JPH0611585A - Fuel rod - Google Patents

Abstract

(57) [Summary] [Objective] The amount of uranium isotopes produced by combustion that changes the minor actinide and uranium isotope compositions is made smaller than before, and more effective utilization of uranium resources is aimed at. [Structure] The fuel pellets 3 having a concentration of 1% and the fuel pellets 4 having a concentration of 5% are alternately mixed in the cladding tube 2 at a ratio of 1: 1 and loaded, so that the average concentration is 3%. Is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel rod loaded in a nuclear reactor core, and more particularly to a fuel rod with a small increase in minor actinide and uranium isotopes produced by combustion.

[0002]

2. Description of the Related Art Nuclear fuel in conventional nuclear power plants is
Mainly composed of fuel rods each having a predetermined uranium enrichment so that the heat output in the core becomes uniform,
Pellets of the same concentration are continuously loaded in one fuel rod.

The fuel pellets are manufactured by molding and sintering uranium dioxide powder. Natural uranium consists of three isotopes with mass numbers of 234, 235, and 238. Enriched uranium is the isotopic composition of which enriches the ratio of ²³⁵ U that causes fission by thermal neutrons (enrichment). . Thus, the uranium in the early fuel pellets is composed of 234, 235, and 238 mass isotopes of uranium.

[0004]

However, in the uranium fuel burned in the reactor core, various actinides produced from uranium by the neutron reaction exist.
Plutonium, which is the fuel for fast breeder reactors, is one of them. Among the actinides produced from uranium, nuctunium, americium, curium, and other nuclides that do not produce fuel and have a relatively small amount in the current nuclear fuel cycle are collectively referred to as minor actinides. Similarly, the neutron reaction produces uranium isotopes with mass numbers of 232 and 236. Therefore, the uranium isotope composition differs from the initial one due to combustion. Production of such minor actinides and uranium isotopes with mass numbers of 232 and 236 increases with increasing burnup.

By the way, since minor actinides are unlikely to be fuel and have a long half-life, they must be separated as radioactive waste by a fuel reprocessing step in the nuclear fuel cycle. Also, uranium isotopes with mass numbers of 232 and 236 are difficult to separate as single nuclides by the conventional enrichment method, and thus may be uranium with an isotope composition not suitable for reuse. When these uranium samples are used, they cannot be reused unless an operation such as mixing with uranium having another isotope composition is performed. Therefore, the smaller the amount of uranium isotope produced that changes the minor actinide and the uranium isotope composition, the better.
However, in conventional fuel rods, no particular consideration has been given to the control of the minor actinide produced from uranium by combustion and the amount of uranium isotope produced which changes the uranium isotope composition.

The present invention has been made in consideration of such a point, and by devising a method for loading fuel pellets,
An object of the present invention is to provide a fuel rod that can reduce the amount of uranium isotopes that change the composition of minor actinides and uranium isotopes produced by combustion to a smaller extent than before, and can make more effective use of uranium resources.

[0007]

That is, the fuel rod of the present invention is characterized in that two or more kinds of fuel pellets having different enrichments are loaded.

[0008]

[Operation] The amount of ²³² U produced by combustion in the core is
As shown in FIG. 2, it increases with burnup. Also, the amount of ²³² U produced differs depending on the difference in the initial concentration. In FIG. 2, the curve a is 1% enrichment and the curve b is 3 enrichment.
%, Curve c shows burnup and ^{232 at} 5% enrichment, respectively.
The relationship with the amount of U produced is shown. Generally, as the enrichment and burnup increase, the amount of nuclide produced also increases. However, as shown in FIG. 3, which shows the relationship between the initial enrichment and the amount of ²³² U produced when the burnup is constant, the amount of ²³² U produced is the maximum near approximately 3% of the enrichment.

[0009] As apparent from the relationship between such initial enrichment and ²³² U produced amount, the average enrichment is desired enrichment using less enrichment of the fuel pellets of ²³² U produced amount of two or more By thus mixing and loading, the amount of nuclides generated by combustion can be made smaller than that of fuel pellets having the same enrichment.

[0010]

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

FIG. 1 shows an embodiment of a fuel rod 1 of the present invention. As shown in an enlarged cross section of a part of the fuel rod 1, a fuel having a concentration of 1% is contained in a cladding tube 2. Pellets 3 and fuel pellets 4 having a concentration of 5% are alternately mixed and loaded at a ratio of 1: 1. As a result, the fuel rod 1 having the average enrichment of 3% is formed.

Next, the production amount of nuclides, for example, ²³² U, produced by combustion in the fuel rod having the above construction will be described in comparison with a conventional fuel rod made of fuel pellets having the same enrichment.

In the case of a conventional fuel rod composed of fuel pellets having the same enrichment, as shown in FIG. 2, the amount of ²³² U produced by combustion in the core increases with the burnup. However, the amount of ²³² U produced due to the difference in initial enrichment is shown in Fig. 3.
As shown in, the maximum value is around 3%. This is because ²³² U is generated through a complicated process as shown in FIG.

Here, FIG. 4 shows the generation paths of minor actinides and uranium isotopes. Reactions with neutrons were made using three uranium isotopes of mass numbers 234, 235, and 238 existing in natural uranium as starting nuclides. In addition, it shows the process until ²³² U is generated due to the radioactive decay. This generation path can be roughly divided into four reaction paths (1), (2), (3), and (4). Of these, reaction paths (1) and (2) are (n, 2n)
The reaction paths (3) and (4) are mainly due to the thermal neutron capture reaction. ^{232 for} each reaction path
²³² U is mainly produced by these four reaction paths, although the ratio of contribution to the amount of U produced varies depending on the burnup.

By the way, generally, the amount A of nuclide produced is represented by the following equation. A = N · σ · Φ · t where N is the number of target nuclides σ; reaction cross section Φ; incident particle flux t; time.

Here, FIG. 5 shows a change in the neutron spectrum in the core due to the difference in the initial enrichment. In the figure, the solid line d,
Dotted line e and dash-dotted line f are neutron spectra at initial enrichment levels of 1, 3, and 5%, respectively. As is clear from this figure, there is almost no change due to the initial enrichment in the high energy region, but in the thermal neutron energy region on the low energy side, the neutron flux intensity decreases as the initial enrichment increases. That is, in the neutron spectrum that contributes to ²³² U production, the ratio of thermal neutrons to total neutrons decreases relatively as the initial enrichment increases. This means that the incident particle flux Φ in the above reaction formula changes and the ratio of the reaction paths (3) and (4) due to the thermal neutron capture reaction among the ²³² U generation paths shown in FIG. 4 decreases relatively. Indicates. However, as the enrichment increases, the number N of target nuclides in the reaction pathways (3) and (4) increases. On the other hand, since the (n, 2n) reaction is a reaction by high-energy fast neutrons (the hatched portion in FIG. 5), it is not affected by the change in the spectrum due to the difference in the initial enrichment, and the reaction path (1),
The amount of ²³² U produced by (2) is hardly affected by the difference in initial enrichment.

FIG. 6 shows the initial enrichment of 1, 3, and 5% when the reaction cross section σ and the time t are the same at the same burnup.
The ratio of the amount of ²³² U produced is shown. As shown in FIG. 6, it can be seen that the average value of the initial enrichment values of 1% and 5% (apparent enrichment value of 3%) is about 20% lower than the uniform value of 3%.

As is clear from the above description, by mixing two or more types of pellets having different enrichments in a fuel rod, the amount of ²³² U produced by combustion is constituted by pellets of the same enrichment. It can be reduced compared to conventional fuel rods. Therefore, the used uranium can be easily reused, and the uranium resource can be used more effectively.

Further, as shown in FIG. 7, among the daughter nuclides produced from ²³² U by natural decay, ²³² Tl is 2 Me.
²³² U to emit high-energy gamma rays of about V
By reducing the production amount of uranium, the daughter nuclide that causes exposure can be reduced, and the handling such as processing of the used uranium after reprocessing can be facilitated.

[0020]

As described above, according to the present invention,
The amount of minor actinides produced from uranium by combustion and the amount of uranium isotopes that change the uranium isotope composition can be made smaller than that of a conventional fuel rod composed of pellets of the same enrichment. This makes it possible to reduce the amount of radioactive waste generated by the separation of minor actinides during the reprocessing of used uranium, and to suppress the change in uranium isotope composition and facilitate reuse.

In addition, since the amount of ²³² U produced is reduced, the amount of uranium in the used uranium when operating at the same output is reduced.
^Since the amount of daughter nuclide of ²³² U is also reduced, when reprocessing and reusing, it is possible to reduce the exposure during handling such as processing of uranium, which facilitates handling such as processing.

[Brief description of drawings]

FIG. 1 is a diagram showing a partially enlarged cross section of a fuel rod according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in the amount of ²³² U generated by combustion in the core.

FIG. 3 is a graph showing changes in the amount of ²³² U produced depending on the difference in initial concentration.

FIG. 4 is a diagram showing a ²³² U generation path in the core.

FIG. 5 is a graph showing a neutron spectrum in the core at each initial enrichment.

FIG. 6 is a diagram showing a change in a ²³² U production amount ratio depending on an initial enrichment ratio under the same conditions of burnup, reaction cross-sectional area (σ) and time (t).

FIG. 7 shows a natural decay series of ²³² U.

[Explanation of symbols]

 1 ... Fuel rod 2 ... Cladding tube 3 ... Fuel pellets with 1% enrichment 4 ... Fuel pellets with 5% enrichment

Claims (1)

[Claims] 1. A fuel rod in which two or more kinds of fuel pellets having different enrichments of uranium 235 are loaded.