JPS6241354B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear fuel pellet, and more particularly to a nuclear fuel pellet for a light water reactor or a heavy water reactor, which has a two-layer structure in which an inner region and an outer diameter region are respectively formed of fuel materials with different characteristics. Conventionally, single-layer pellets in which the entire cross section of the pellet is made of homogeneous fuel material have been widely used as fuel pellets for light water reactors or heavy water reactors, but multi-layer fuel pellets in which the inner region is less enriched than the outer diameter region have been widely used. is also being considered. The latter type of pellet suppresses the generated output by relatively lowering the enrichment level in the center of the pellet, and reduces the amount of fission products (FP) generated during combustion.
The purpose is to improve fuel integrity by lowering FP gas pressure. However, in both cases, no measures have been taken to improve the fuel conversion ratio (amount of fissile material produced/amount of fissile material destroyed), which is an indicator of fuel economy. Both heavy water reactors and light water reactors have a conversion ratio of 0.6 to 0.7, and are inferior in terms of fuel economy and uranium resource conservation compared to fast reactors, which have a conversion ratio of 1.0 or more. An object of the present invention is to provide fuel pellets with excellent fuel economy by creating enrichment differences in the cross-sectional direction of the fuel pellets. That is, the present invention provides a nuclear fuel pellet having a two-layer structure in which an inner region of a cross-sectional shape and an outer diameter side region surrounding this inner region are formed of fuel materials having different nuclear properties. It is characterized in that the enrichment degree of the fuel material forming the outer diameter side region is lower than the enrichment degree of the fuel material forming the outer diameter side region. Hereinafter, first, the principle of the present invention will be explained in detail. In the structure of the nuclear reactor fuel rod shown in FIG. 1, a large number of fuel pellets 1 are stacked inside a tube 2. As shown in FIG. In conventional fuel pellets, the radial enrichment is uniform and is in the form of a single area, as shown in FIG. 2, which is a horizontal cross-section. Resonance absorption cross section D of ²³⁸ U, which has a large contribution to the fuel conversion ratio
(4.0 to 9.9 eV) in the radial direction (direction from the center O to the outer diameter R side) is shown in FIG. Near the outer surface R of the fuel pellet, the self-suppressing effect of resonance absorption is smaller than in the center O, so the resonance absorption cross section D
increases rapidly as it approaches the outer surface R, and the difference reaches about 10% at the outer surface. Next, the radial distribution of low-energy neutron flux Φ (10 eV or less) is the fourth
It will look like the figure. Neutrons that have been moderated in the moderator region outside the fuel flow into the fuel pellet, resulting in a distribution that is bulged by approximately 20% on the outer surface. In addition, as shown in Fig. 5, the fuel enrichment E (w/v)
In general, the lower the enrichment E, the higher the conversion ratio. This is mainly because ^{the 238} U resonance absorption effect is small and the neutron spectrum is soft (the proportion of low-energy neutrons is high). The general trends of the core characteristics of conventional single-area fuel pellets have been shown above, but the present invention has the above-mentioned three effects, namely, (1)
Considering that ^{the 238} U resonance absorption cross section is large, (2) the proportion of low-energy neutrons is high, and (3) the conversion ratio is higher for lower enrichment fuels, conversion is achieved by lowering the enrichment at the outer periphery of the fuel pellet. This increases the ratio. FIG. 6 shows changes in radial power distribution due to combustion. As the combustion progresses, the difference in the plutonium production rate increases, and the output (relative output ratio P) increases at the outer periphery R as the combustion progresses (burnup B). Figure 7 shows the power distribution at burnup B (30GWd/t) for fuel pellets of enrichment E3.0w/v and natural uranium (E0.7w/v). The output of part R is high. This tendency is also clearly shown in Figures 8 and 9, which show the changes in the relative power ratio P _M of the outermost layer of pellets (approximately 2% by volume) with enrichment E and burnup B. ing.
The dependence of the power sharing ratio of ²³⁹ Pu on the fuel pellet radial direction (minimum layer L _M and center O) and burnup B are shown in FIGS. 10 and 11.
Since it is considered that the output sharing ratio Ppu of ²³⁹ Pu corresponds approximately to the production ratio of ²³⁹ Pu, it can be confirmed from these figures that the production of ²³⁹ Pu increases as the enrichment level decreases at the outer periphery of the fuel pellet. The above description presents data for demonstrating the principle of the present invention. FIG. 12 shows the relative power distribution P _R normalized by the average power of fuel pellets of 1.0. In the present invention, a particularly suitable dividing point between the inner region and the outer diameter region is a point where the output exceeds the average pellet output level of 1.0 and the relative radius Φ is 0.8 or more. Even if low-enriched uranium is loaded inside this region, the effect of the present invention will hardly be obtained. On the other hand, if the volume of the inner region, which is highly enriched and has a large output sharing ratio, is drastically reduced, it becomes necessary to increase the enrichment of the inner region in order to generate a constant output from the entire fuel pellet, and the center temperature decreases. Rise and
This leads to an increase in FP gas pressure, which is unfavorable from the standpoint of fuel integrity. Conventional fuel pellets have a two-layer structure in which the enrichment relationship is reversed so that the enrichment in the inner region of the pellet is lower than in the outer diameter region, but measures have been taken to ensure fuel integrity. The fuel conversion ratio is inferior to single area fuel pellets for the reasons mentioned above. It is also possible to surround the outside of the fuel pellets with metallic natural uranium, but this would be difficult to reprocess in practice. The present invention will be described below with reference to Examples. Table 1 shows the basic specifications of the fuel pellet of Example 1 according to the present invention.

[Table] In this example, the average enrichment of the pellet is 3.0w/v, the outer region is set to 10% by volume, and natural uranium is loaded. Since the outer region is natural uranium, the neutron spectrum is soft, and the thermal energy absorption cross section of ²³⁸ U is approximately 20% larger, and the thermal energy neutron flux is more than twice as large, improving the conversion ratio. In an example of application to a heavy water reactor, the average conversion ratio of fuel pellets is improved by approximately 10% compared to conventional fuel pellets with an enrichment of 3 w/o. Approximately 95% of the power generated by the fuel pellet is shared by the inner region, and the temperature at the center of the pellet increases by approximately 8%. FP release rate is approximately 2
%, but the increase in gas pressure remains at about 0.1 kg/cc and does not pose a problem. Table 2 shows the basic specifications of the fuel pellets of Example 2.

[Table] Pellet average concentration, outer region and composition were the same as in Example 1, and 10% by volume in the center of the pellet.
This is an example of a hollow pellet with a hollow area of 50%. The conversion ratio is improved by about 10% as in Example 1. The enrichment in the inner region increases by about 10%, and the fuel temperature increases by about 10%, but by using hollow pellets, the FP gas pressure is about half that of solid pellets, so there is no problem. Furthermore, by using hollow pellets, the amount of strain in the fuel pellets can be reduced to about 2/3, and the soundness of the fuel is greatly improved. The basic specifications of the fuel pellets of Example 3 are shown in Table 3.

[Table] Average pellet concentration and region division in Example 1
This example is the same as , but uses depleted uranium in the outer region. Further softening of the neutron spectrum results in a conversion ratio of approximately 15% compared to conventional fuel pellets.
%improves. The concentration in the inner region is about 2% higher and the center temperature increases slightly, but this is not a problem as described in Example 1. In addition, as shown in Example 2, there is also a measure to use hollow pellets, which makes it possible to achieve high performance without impairing the integrity of the fuel. As described above, according to the present invention, it is possible to improve the conversion ratio of nuclear fuel in a light water reactor or a heavy water reactor, thereby increasing fuel economy.

[Brief explanation of the drawing]

Figure 1 is a sectional view of the main part of a nuclear reactor fuel rod, Figure 2 is a view taken along the - arrow in Figure 1, and Figures 3 to 12 are diagrams showing various nuclear characteristics of fuel in nuclear fuel pellets. . 1...Fuel tube, 2...Fuel pellets, O
...Pellet center, R...Pellet outer periphery, D...
... ²³⁸ U resonance absorption cross section (burn), Φ...neutron flux distribution, B...burnup (GWd/t), C...conversion ratio, E...enrichment (w/v), P...relative Output ratio, P _M ... Outermost layer relative output, Ppu... ²³⁹ Pu output sharing ratio (relative value), P _R ... Relative output distribution, φ
...Pellet radius (relative value).

Claims (1)

[Claims] 1. In a nuclear fuel pellet having a two-layer structure in which an inner region of a shaped cross section and an outer diameter side region surrounding this inner region are formed of fuel materials having mutually different nuclear properties, the fuel forming the inner region The nuclear fuel pellet characterized in that the enrichment of the fuel material forming the outer diameter side region is lower than the enrichment of the substance.