JPS59155783A - 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] [Field of application of the invention] The present invention relates to a fast breeder reactor with improved core performance.

[Prior art]

Like the Shuhei, a fast breeder reactor absorbs neutrons generated by nuclear fission in the core of a nuclear reactor into fuel R material to produce new fissile material, so-called breeding, which allows for effective use of fuel. Has characteristics. The core of such a fast breeder reactor is generally formed in a cylindrical shape, and the core is surrounded by axial and radial blankets mainly composed of parent fuel materials4. Uranium or plutonium-enriched uranium is loaded, and the blanket is loaded with, for example, natural uranium or depleted uranium as a fuel parent material. This fuel parent material captures neutrons leaking from the core, producing useful fissile material.

Quantitative reference values for such proliferation effects include proliferation rate and doubling time. The proliferation rate is expressed as the ratio of the amount of new fissile material produced to the amount of fissile material consumed.
It is desirable that it be as high as possible. Also, doubling time is the time required to reproduce the same amount of fissile material as was initially loaded into the reactor, and it is desirable that this time be short, and shortening this doubling time is important for improving fast breeder reactors. Liquid is also the main focus.

By the way, the doubling time generally decreases as the breeding rate of the reactor increases, and it also tends to be inversely proportional to the reactor's specific power (output J per unit fuel load). If this is done, the average specific power of the core can be increased without increasing the maximum specific power, which can be suppressed by fuel design, and the doubling time can be shortened.Therefore, various methods have been used to flatten the power distribution. It has been devised.

For example, in a commonly used core structure called a homogeneous core, the core is divided into two to three regions in the radial direction, such as an inner core IA and an outer core IB, as shown in Figure 1. Enrichment (fissile material/(fissile material + parent material)
) was loaded with high fuel. Note that 2 in FIG. 1 is a radial blanket, and 3 is an axial blanket.

However, with such a core configuration, there is a problem in that as the reactor becomes larger, the average energy of the neutron flux within the core 1 decreases, and the breeding rate decreases.

In contrast, conventional reactor cores have a core structure called a Buffney core, in which a disk-shaped internal blanket 4 with a constant thickness is inserted into the axial center of the inner core IA, as shown in FIG. 2, for example. It has been developed. This type of core uses axial blanket fuel as the internal blanket 4. According to this, the average neutron energy of the reactor core increases, the ratio of neutron capture reactions to fission reactions of fissile materials in the reactor core decreases, and the number of neutron absorption and response events increases. On the other hand, internal blanket 4
In this case, compared to the reactor core 1, the atomic number density of the fuel parent material is higher and the average energy of neutrons is lower, so the nuclear fission reaction rate of the fuel parent material decreases, and conversely, the neutron capture reaction rate increases. That is, in the inner blanket region, the output is low, but
The proliferative effect increases. As a result, when looking at the entire core,
As the proliferation rate increases, the power distribution may flatten.

However, in this Buffney core, as combustion progresses, the output decreases in the outer core region IB where there is no internal blanket region 4 because the breeding effect is small, and on the contrary, in the central part of the core where the internal blanket region 4 exists, new power is generated due to the breeding effect. There is a problem in that the output increases due to the fissile material produced during combustion, and the output fluctuates greatly due to combustion.

[Purpose of the invention]

An object of the present invention is to provide a fast breeder reactor that suppresses fluctuations in power due to combustion and flattens the power distribution within the core, thereby improving core performance and, in particular, shortening doubling time. It's about doing.

[Summary of the invention]

The present invention is characterized in that, in a fast breeder reactor in which a cylindrical reactor core is surrounded by axial and radial blankets containing a fuel parent substance as a main component, the axially central portion of the reactor core is A disk-shaped region which is mainly composed of a fuel parent material and which extends in the radial direction of the core is provided in the reactor core, and fuel pellets used in the core are mixed in the periphery of this fuel parent material region.

As a result, the ratio of fissile material to fuel parent material in the peripheral part of the fuel parent material region is increased compared to the central part, suppressing a drop in output in the peripheral part, and improving output flattening. [Examples of the Invention] The present invention will be explained below using Examples. The reactor core in question is one in which a mixed oxide of plutonium and uranium is used as fuel and liquid sodium is used as a coolant.

First, the core configuration will be explained with reference to FIG. An axial blanket 6 whose main component is a cylindrical core 5 and a fuel parent material
and is surrounded by a radial blanket 7. Note that the core 5 is composed of fuel of a single enrichment. At the axial center of the core 5, there are provided eight disc-shaped regions, ie, eight internal blankets, which extend in the radial direction of the core and whose main component is a fuel parent substance. The outer diameter D1 of this internal frinket 8 is set at a ratio of 0.7 to 0.9 of the core diameter D2. That is, a cylindrical region where the internal blanket 8 does not exist is provided at the radial peripheral edge of the core 5.

The internal blanket 8 is divided into five regions, and the region a at the center of the core contains fuel pellets (mainly composed of fuel parent substances).
These are hereinafter referred to as blanket fuel pellets). On the other hand, in the area from area to area e to the periphery, a large number of fuel pellets containing a large amount of fissile material used in the reactor core (hereinafter referred to as core fuel pellets) are mixed between the blanket fuel pellets. . Table 1 is
The mixed ratio of core fuel pellets and blanket fuel pellets in each region is shown. For example, in region C, fuel pellets are stored in the cladding tube at a ratio of one core fuel pensite to two blanket fuel pellets. FIG. 4 shows the upper end of the fuel bottle corresponding to area C;
A longitudinal section of the inner blanket section and the lower end section is shown. 9,
10.11 are blanket fuel pellets, core fuel pellets, and cladding tubes, respectively.

The core design parameters and reactor operation supervision are as shown in Table 2. That is, the reactor thermal output is approximately 2500M
W, the electrical output is approximately 10,100 O, and the equivalent core diameter and core portion are 325 cm and 100 crrI, respectively. Axial blanket and radial blanket thickness are 35c each
rn and 40crr1. The fuel smear degree is 87% theoretical density in the core and 91% theoretical density in the blanket. The core fuel has a single enrichment level, and the axial blanket fuel is used as is for the internal blanket fuel. The refueling period will be one year, the capacity utilization rate will be 80%, and the number of refueling batches will be three for both the core and frinket. The maximum linear power density at rated time is 430w/crn, and the surplus reactivity at the end of combustion is about 0.7%.

Table 1 N1: Number of core fuel pellets N2: Number of blanket fuel pellets Table 2 Main design parameters Next, the operation based on the above configuration will be explained. First, considering the reaction, the axial power distribution of the reactor can be easily flattened by inserting an internal blanket 8 with low power density and small reactivity value in the axial center of the reactor core 5. On the other hand, the radial power distribution is such that core fuel pellets are mixed in the periphery of the inner blanket, and the outer diameter of the inner blanket 8 is 0.7 to 0.0.
By setting the ratio to 9, the average enrichment distribution in the radial direction becomes low at the center of the core and high at the periphery, as shown in FIG. The figure shows the radial average enrichment distribution at the beginning of the equilibrium cycle.

With such an enrichment distribution, the radial power distribution is flattened as shown in FIG. 6, and power fluctuations due to combustion can also be reduced.

The internal blanket 8 shown in FIG. 3 consists only of blanket fuel pellets in the center of the reactor core, where the high-energy neutron flux is large and the reaction rate is high, while in the periphery, the amount of blanket fuel pellets is small (10). Furthermore, a region without a fuel parent substance is formed in the peripheral portion of the core 5. Therefore, it is possible to improve the multiplication property by changing the neutron spectrum due to the separation of the fuel parent material and the fissile material to some extent within the reactor core 5, and as shown in Fig. 6, it is possible to reduce the output fluctuation due to combustion. can.

That is, in the fast breeder reactor according to the present invention, the average neutron energy of the reactor core 5 increases, the ratio of the neutron capture reaction of the fissile material to the fission reaction decreases, and the number of neutrons generated per absorption reaction increases. On the other hand, in the internal blanket 8, the atomic number density of the fuel parent material is higher than in the reactor core 5, and the average energy of neutrons is lower, so the nuclear fission reaction of the fuel parent material decreases, but the neutron capture reaction rate increases.

As described above, since the neutron energy of the reactor core 5 is high and the neutron flux at the boundary between the reactor core 5 and the axial blanket 6 and radial blanket 7 surrounding it is high, The amount of neutron leakage increases (11). As a result, the neutron capture reaction and nuclear fission reaction of the fuel parent material are significantly improved.

Such changes in the reaction rate and flattening of the output distribution can improve proliferation, particularly shorten the doubling time.

In addition, if the internal blanket 8 of the above shape is provided, there is no need to divide the core into several regions and fully load highly enriched fuel into the outer region, as in the conventional method.
It is the town's intention that the enrichment level of core fuel should be of one type. The Hfj fuel for the axial blanket 6 can be used as is for the fuel loaded into the internal blanket 8, so when the enrichment of the core fuel is set to one type,
The advantage also arises that the production of the fuel is simplified.

In addition, in the above embodiment, all the core fuel pellets were mixed in the radial peripheral part of the internal blanket 8, but by further mixing the core fuel pellets in the axial peripheral part,
It is also possible to flatten the axial power distribution.

〔Effect of the invention〕

(12) As explained above, according to the fast breeder reactor of the present invention,
It is possible to significantly shorten proliferation, especially doubling time,
This has a significant effect on fuel economy.

In addition, the decrease in reactivity due to combustion is reduced, the number of control rods can be reduced, and only one type of fuel is required, eliminating the need to produce multiple blocks of fuel with different enrichments as in the past. This also has the advantage of reducing fuel production costs.

[Brief explanation of the drawings] Figures 1 and 2 show conventional examples. Figure 1 is a vertical cross-sectional view showing a homogeneous core, Figure 2 is a vertical cross-sectional view showing a Buffney core, and Figure 3 is a vertical cross-sectional view showing a homogeneous core. FIG. 4 is a vertical cross-sectional view of the fuel rods used in the core, FIG. 5 is a characteristic diagram showing the enrichment distribution in the radial direction of the core, and FIG. 6 is a view in the radial direction of the core. FIG. 3 is a characteristic diagram showing output distribution. 5... Core, 6... Axial blanket, 7...
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Claims (1)

[Claims] 1. A cylindrical reactor core is surrounded by axial and radial blankets containing a fuel parent material as the main component, and a fuel parent material such as decapitated uranium or natural uranium is the main component in the axial center of the core. In a fast breeder reactor in which a cylindrical region extending in the radial direction of the core is arranged, a fuel plate used in the core is placed around the fuel parent material region in the axial center of the core.
A fast breeder reactor characterized by a mixture of