JPS5819591A - Fast breeder - 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 The present invention relates to improvements in fast breeder reactors, and in particular to improvements in core performance thereof.

A fast breeder reactor is characterized by the ability to effectively utilize fuel by absorbing neutrons generated by nuclear fission in the core of a nuclear reactor into a parent fuel material to produce new fissile material. The core of such a fast breeder reactor generally has a cylindrical shape, and around the core are provided axial and radial blankets mainly composed of fuel parent material. The reactor core is loaded with enriched uranium or plutonium-enriched uranium as fuel, and the blanket is loaded with, for example, natural uranium or depleted uranium as a fuel parent material. Useful fissile material is produced by capturing the neutrons generated in the fission reaction by the parent fuel material in the reactor core and blanket.

Proliferation rate and doubling time are quantitative criteria for such proliferative effects. The proliferation rate is expressed as the rate of neglect, and the higher the rate, the better. The doubling time is the time required to reproduce the same amount of fissile material as was initially loaded into the reactor, and the shorter it is, the better. It is the main focus.

By the way, in order to shorten the doubling time, it is necessary to increase the multiplication rate and reduce the amount of fissile material loaded. The upper limit of the thermal output that can be extracted from the reactor depends on the thermal limit at the highest temperature point (maximum unit length output of the fuel element), so even for a core with the same thermal output, the flatter the power distribution, the smaller the core volume. Well, generally the fissile material loading is also reduced. Therefore, the doubling time is reduced in a core with a flat power distribution.

Conventionally, the core that has been widely used is called a homogeneous core.As shown in Figure 1, the core is divided into an inner core 1 and an outer core 2, and the outer core contains more fissile material than the inner core. It is designed to be loaded with fuel containing

However, in such a core configuration, the reduction in reactivity due to combustion is relatively large compared to the reactivity that can be controlled by control rods. Therefore, if the initial surplus reactivity is increased in order to extend the combustion period, the reactivity to be controlled will be increased, so it is necessary to insert control rods with a large control rod value into the reactor core.

Furthermore, as the size of the reactor is increased in order to increase the economic efficiency of a power generation plant, there is also the problem that the average neutron energy within the reactor core decreases and the breeding rate decreases.

On the other hand, conventional reactor cores have a core configuration called a Pafney core, in which a disk-shaped internal blanket 5 with a constant thickness is inserted into the axial center of the inner core 1, as shown in FIG. 2, for example. was developed. In this core, inside the core;
Since there is a blanket with low power density and low reactivity in the reactor, the power distribution of the core can be flattened and the breeding rate can also be improved.

However, in this Pafney core, as combustion progresses, the output of the outer core 2 without the inner blanket 5 drops significantly except near the boundary with the radial blanket 4, and there is a problem that the output fluctuates greatly due to combustion. .

The present invention was devised to suppress such power fluctuations due to combustion and to prevent a decrease in breeding performance due to a decrease in average neutron energy in the reactor core, and is a fast breeder reactor capable of shortening doubling time. is intended to provide.

The present invention is characterized by the fact that natural uranium is placed near the axial center of the fast breeder reactor core.

Load fuel with a lower contamination rate of fissile material, such as depleted uranium, low enriched uranium, or uranium with low plutonium enrichment, than the core fuel, and further reduce the contamination rate of fissile material to the radial center of the core. This is because the height is low and the surrounding area is high.

Embodiments of the present invention will be described below with reference to FIGS. 3 and 4. The target core is h4, using a mixed oxide of plutonium and uranium as fuel and liquid sodium as a coolant, but the present invention is also applicable to cores using other fuels and coolants. It is possible.

First, the core configuration will be explained with reference to FIG. A cylindrical core is surrounded by an axial blanket and a radial blanket that are mainly composed of fuel parent material. Near the axial center of the reactor core, a disk-shaped region extending in the radial direction of the core consisting of depleted uranium with a low fissile material contamination rate and uranium with a low plutonium enrichment is provided. amount/(amount of fissile material+amount of parent fuel material)x-ioo%) is lower in the radial core center portion 11 and higher in the peripheral portions 12 to 15. In this example, the fissile material contamination rate is 14.8% in the core 10, and 0.3%, 3.2%, and 6% in regions 11 to 15, respectively.
．． 0%, 8.9% and 11.8%.

Table 1 The core design parameters and reactor operating conditions are shown in Table 1.
As shown in the table. In other words, the reactor thermal output is approximately 250
0MW% electrical output is approximately 10100O.

Core diameter and core height are 325 crr1 and 95 crr, respectively.
It is r1. Fuel exchange period is 1 year, design utilization rate Fi80
%, and the number of refueling batches is 3 for both the core and blanket. The rated maximum linear power density is 430 W/on.

Next, the operation based on the above configuration will be explained. The axial power distribution of the core has low power density and low reactivity fuel (low fissile material contamination rate)1 in the axial center of the core.
1 to 15, it can be easily flattened. On the other hand, the radial power distribution is flattened by making the fissile material mixing rate of the fuels 11 to 15 low at the center in the radial direction of the core and high at the periphery.

Further, by extending the fuels 11 to 15 in the radial direction of the core to near the radial blanket 4, output fluctuations due to combustion, which were a problem in the Pafney core, are suppressed.

Moreover, since the average neutron energy of the reactor core 10 is higher than that of a conventional reactor core, and the neutron flux at the boundary between the reactor core 10 and the surrounding axial blanket 3 and radial blanket 4 is increased, the axial blanket 3 And the amount of neutrons leaking into the radial blanket 4 increases. As a result, the neutron capture reaction and nuclear fission reaction of the fuel parent material are significantly improved.

Such flattening of the output distribution and change in reaction rate are effective in improving proliferation, particularly in shortening doubling time.

In addition, fast breeder reactors are equipped with control rods for reactivity control and power control. Typically, control rods are inserted into the reactor core from the top. In the case of Jin, fuel with low fissile material dust is loaded near the axial center of the reactor core, so if the control rods are inserted halfway, the power output of the upper half of the core will be low. In this case, it is preferable that the loading position of the fuel with a low amount of fissile material dust is not centered in the axial direction of the reactor core, but rather shifted to the lower side.

In addition, since the initial loading core is composed only of new fuel,
The output generated by the fuel with low fissile material dust is low, and the flatness of the output distribution is poor when viewed from the entire core.

Therefore, it is effective to flatten the power distribution by setting the fissile material-containing dust as the fuel for the initial loading core to be higher than that for the balanced core loading.

In the above example, the fissile material mixture rate of the fuel with a low fissile material mixture rate was gradually changed, but as shown in Figure 4, for simplicity, the fissile material mixture rate was changed to 2. It may be limited to types (16 and 17).

As explained above, according to the fast breeder reactor of the present invention, it is possible to significantly shorten the breeding property, especially the doubling time, and the fuel economy j is greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of the equalization core of a conventional fast breeder reactor. Figure 2 is a vertical cross-sectional view of the Pafney core of the company's fast breeder reactor. FIG. 3 is a vertical sectional view of the core of a fast breeder reactor that is an embodiment of the present invention. FIG. 4 is a vertical sectional view of the core of a fast breeder reactor according to another embodiment of the present invention. 3... Axial blanket, 4... Radial plancher No. /[1] Fig. 3 No. l, -m

Claims (1)

[Claims] 1. In a fast breeder reactor, natural uranium is placed near the axial center of a reactor core loaded with enriched uranium or plutonium-enriched uranium as the main fuel. Loading fuel with a lower mixing rate of fissile material than the core fuel, such as depleted uranium, low enriched uranium, or uranium with low plutonium enrichment, and reducing the mixing rate of fissile material to the radial center of the core. A fast breeder reactor characterized by a low height and a high peripheral area. 2. Claim 1 is characterized in that the axially central plane of the region loaded with fuel with a low fissile material content is located below the axially central plane of the reactor core. Fast breeder reactor. 3. In claim 1 or 2, the mixing ratio of the fissile material to the fuel for initial loading is higher than that for the fuel for equilibrium core loading.