JPS6228438B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a fast breeder reactor, and particularly to the structure of its core. The core of a fast breeder reactor generally has a cylindrical shape or a similar shape, and is often surrounded by a blanket. Here, the fuel loaded into the reactor core is enriched uranium or uranium enriched with plutonium, and the blanket is a parent material that absorbs neutrons leaking from the reactor core and converts them into useful fissile material. Loaded with natural uranium or depleted uranium. The upper limit of the heat output that can be extracted from the core depends on the thermal limitations of the fuel rods at the highest temperature point, so it is necessary to flatten the power distribution of the core as much as possible in order to improve the power density of the core as a whole. be. Therefore, in the conventional core, as shown in FIG. 1, the core 1 is divided into an inner core 1a and an outer core 1b.
By dividing the core into two or by providing another core between the inner core 1a and the outer core 1b in the radial direction,
It is divided into two to three regions in the radial direction, and the outer region is loaded with fuel with higher enrichment (fissile material/fissile material + parent material). Note that 2 in FIG. 1 is a radial blanket, and 3 is an axial blanket. In the core configuration shown in Figure 1, the reduction in reactivity in the core due to combustion is relatively large compared to the reactivity that can be controlled by control rods, and the combustion period is suppressed, so core fuel is removed approximately every year. I try to exchange a fraction of each. In addition, if the initial surplus reactivity is increased to extend the combustion period, the reactivity that must be controlled by the control rods will increase, so it is necessary to insert control rods with large control value into the reactor core. This has led to the problem that the power distribution of the core is greatly distorted. Furthermore, as the reactor is made larger in order to increase its economic efficiency as a power generation plant, there is also the problem that the average energy of the neutron flux within the reactor core 1 decreases, resulting in a decrease in the multiplication rate. As a conventional type, as shown in Figure 2, 2
There is also one in which a disk-shaped internal blanket 5 is provided at the axial center of the inner core 1a of the regional core. In this type of core, the flattening of the radial power distribution is achieved by loading fuel with different enrichments in each region (as mentioned above, low enrichment fuel is loaded into the inner core 1a, and high enrichment fuel is loaded into the outer core 1a). What is done is the same as the example shown in Figure 1, and the role of the internal blanket 5 is as follows:
The main objective is to reduce the decrease in reactivity due to combustion. Therefore, as the combustion progresses, the output in the area of the internal blanket 5 increases, so as the combustion progresses, the output near the center of the core in the radial direction, including the internal blanket 5, increases relatively, while the output in the periphery decreases. As such, there is a problem of large fluctuations. Furthermore, there are other conventional examples in which internal blankets are provided to reduce the sodium void effect or to increase the growth rate. However, these systems aim to divide the reactor core by internal blankets, effectively creating an assembly of small cores. Therefore, the internal blanket is either annular and passes through the core in the axial direction, or it is layered with the same diameter as the core. In the former case, that is, one in which the internal blanket is annular, the output of the internal blanket area increases due to combustion, so it is necessary to flow a large amount of coolant in advance; , there are problems such as difficulty in flattening the output distribution. An object of the present invention is to provide a fast breeder reactor with a high breeding rate. A feature of the invention is that the inner blanket area is thicker at the center and thinner at the periphery of the inner blanket area. The details of the present invention will be explained below with reference to the drawings. Third
The figure shows an embodiment of a fast breeder reactor according to the present invention. In this embodiment, a disk-like structure is provided near the axial center of the reactor core 6, with the outer circumferential portion 7b being stepped and thinner than the central portion 7a. An inner blanket 7 is provided. The fuel constituting the reactor core 6 is a single enrichment fuel, and the internal blanket 7 is mainly made of natural uranium or depleted uranium. In order to reduce the decrease in reactivity due to combustion, it is possible to raise the core to a certain level to reduce the leakage of neutrons to the outside of the core, or to increase the fuel volume ratio and density within the core so that neutrons are absorbed by substances other than fuel. There is also a method of lowering the critical enrichment of the fuel and increasing the internal conversion rate (amount of fissile material produced/amount of fissile material annihilated in the reactor core) by lowering the proportion of neutrons, but in this example, As shown in the figure, an internal blanket 7, that is, a region made of a parent material such as depleted uranium or natural uranium, is provided in the axial center of the core, which greatly affects the reactivity of the entire core. Plutonium accumulated in this region, that is, the inner blanket region, by neutron absorption by the parent material along with combustion makes a large contribution to the reactivity, and suppresses the reduction in reactivity due to combustion. for that purpose,
As described below, the thickness of the inner blanket 7 is 10
It is best to set it to around 50cm. Next, by providing this internal blanket 7, the output distribution is flattened. The axial power distribution within the core can be flattened by placing a layer of internal blanket 7 with low power density and low reactivity value near the axial center of the core, as shown. In this case, the thicker the layer, the greater the effect, but if the internal blanket, which has a low power density, is made too large, it will be necessary to significantly increase the volume of the entire core including the blanket in order to obtain the same thermal output. Become. From this point of view, the thickness of the internal blanket 7 should be 10 to 50 cm for a core height of 120 cm.
cm is considered appropriate. On the other hand, in order to flatten the radial power distribution,
As shown in the figure, the internal blanket thickness is thicker at the core center where the fast neutron flux is large and the reaction rate is high, and conversely it is thinner at the periphery of the core. In addition, if the internal blanket is provided only near the center of the core, the output in the area outside the core tends to decrease relatively significantly due to combustion, but if the internal blanket 7 is located at the center as shown in the figure, By providing it not only in the vicinity but also throughout the entire radial direction of the core, it is possible to prevent a large drop in the output in the outer core region due to combustion. This is because when the thickness of the internal blanket 7 increases to a certain extent, the amount of plutonium generated and accumulated per unit volume becomes saturated.In other words, the thickness of the internal blanket 7 is not proportional to the increase in output in that part. Depends on nature. In this way, by providing the internal blanket 7, the configuration as shown in FIGS. 1 and 2,
That is, there is no need to divide the core into several regions in the radial direction and load fuel with higher enrichment in the outer regions, which simplifies fuel production. Generally, the allowable maximum output of a fast breeder reactor is controlled by the maximum power density (strictly speaking, the maximum linear output of the fuel pin), and even with the same volume of the core, the flatter the power distribution, the higher the output. On the other hand, the fuel doubling time (the time required to breed fissile material equivalent to the initially loaded fuel) is determined by As the rate of increase increases, the length becomes shorter in inverse proportion to the output. Therefore, by flattening the output distribution and increasing the output, the output can be increased, thereby shortening the doubling time. Also, by providing an internal blanket, the proliferation rate is also increased. Next, the results of calculations for a fast breeder reactor having the configuration shown in FIG. 3 having the core specifications shown in Table 1 will be explained. The size of the internal blanket 7, which is the basis for calculation, is
The thickness of the central part 7a of the core in the radial direction is 30 cm, and the thickness of the outer peripheral part 7 is 30 cm.
b is 10 cm, the diameter of the thick part is 234 cm, and the area ratio is 1/2 of the core diameter of 331 cm.
In addition, the outer diameter of the thin part was set equal to the core diameter. The calculation results are shown in Table 2 and FIGS. 4 to 6 in comparison with those shown in FIGS. 1 and 2. In these calculations, the fuel replacement interval is assumed to be one year, and it is assumed that one-third of the fuel is replaced each time. As can be seen from Table 2, the reduction in reactivity due to combustion is slightly larger in the case of the above example according to the embodiment than in the two-zone core with internal blanket, but
It is smaller than a conventional two-zone core. Therefore, compared to a conventional two-zone core, the number of control rods can be reduced,
Alternatively, the combustion period can be extended. Also, the core based on the present invention has the highest breeding rate. 4, 5, and 6 show the radial distribution of the output per fuel assembly at the initial stage (solid line) and the final stage (broken line) of combustion in each case, respectively. The range of power fluctuation due to combustion in the core according to this example is slightly larger than that of the conventional two-zone core, but it is much smaller than that of the two-zone core with an internal blanket. It can be seen that even though the blanket 7 is provided, the size is not large enough to cause a problem. Even when comparing cores including internal blankets, the characteristics shown in Figure 6 of this example (core of Figure 3) are better than those of Figure 5 (core of Figure 2) at the early stage of combustion (BOC). ) and the end-of-life (EOC) range are small because of the difference in the internal blanket. In other words, in the core of a fast breeder reactor, as combustion progresses in the plutonium-enriched core region (also called the driver region), the output decreases relatively, and conversely, plutonium is produced in the blanket region, increasing the output. Tend.
In this embodiment, the internal blanket 7 extends to the outer periphery of the core, unlike in the past. As the combustion progresses, plutonium is also generated and accumulated in the inner blanket in the outer circumferential direction, so the output of the aggregate in this area does not decrease much as the combustion progresses, making it more effective for EOC than for BOC. It doesn't get smaller even when it gets old. The total power of the reactor is the same in BOC and EOC (the power integrated in the radial direction in Figures 4 to 6 is constant in BOC and EOC), so if the decrease in power in the area on the outer periphery of the core is small, then , the increase in output at the center inside the core will be small, and the range of fluctuation will be small. However, in the core of FIG. 2, there is an internal blanket only at the center inside the core, so the range of fluctuation cannot be made as small as in this embodiment. In the example shown in Figure 3, the thickness of the internal blanket is changed stepwise in the radial direction of the core, but as shown at 8 in Figure 7, the thickness of the internal blanket is changed continuously. It is also possible to improve performance. As explained above, in this example, a disk-shaped internal blanket with a narrow outer circumferential side is provided at the axial center of the reactor core, which reduces the reduction in reactivity due to combustion and reduces the amount of fuel with a single enrichment. It has become possible to flatten the power distribution through loading alone, minimize power fluctuations due to combustion, and significantly shorten the fuel doubling time. In addition, according to this embodiment, since only one type of fuel is required for the enrichment level, there is no need to manufacture many types of fuels with different enrichment levels, and the fuel manufacturing cost can be reduced. This is a major advantage of the embodiment. According to the present invention, the multiplication rate of fuel can be increased and the fuel can be used effectively.

【table】

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a side sectional view of a conventional fast breeder reactor with a two-zone core; FIG. 2 is a side sectional view of a conventional fast breeder reactor with a two-zone core with internal blankets;
FIG. 3 is a side sectional view showing an embodiment of the fast breeder reactor according to the present invention, and FIGS. The radial power distribution diagram, FIG. 7, is a side sectional view showing another embodiment of the fast breeder reactor according to the present invention. 2... Radial blanket, 3... Axial blanket, 6... Core, 7, 8... Internal blanket.

Claims (1)

[Claims] 1. A reactor core consisting of a core region having fissile material, an outer blanket region surrounding the outside of the core region and having a fuel parent material, and an inner blanket region disposed within the core region and having a fuel parent material. A fast breeder reactor with a built-in fast breeder reactor, characterized in that the thickness in the axial direction of the internal blanket area is thicker at the center of the internal blanket area and thinner at the peripheral area of the internal blanket area.