JPS5821191A - 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 a fast breeder reactor with improved core performance.

As is well known, fast breeder reactors are characterized by the ability to effectively utilize fuel by absorbing neutrons generated by nuclear fission in the reactor core into the parent fuel material to produce new fissile material. . The core of such a fast breeder reactor is generally formed in a cylindrical shape, and the core is surrounded by regions called axial and radial blankets, which are mainly composed of fuel loading materials. 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.

This fuel parent material captures neutrons, 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, and is preferably as high as possible.

The 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 to shorten this doubling time by 4.7'L. The main focus is on fast breeder reactor design.

By the way, the doubling time is generally inversely proportional to the specific power of the reactor (output per unit fuel load), so it can be shortened by suppressing the decrease in power at the outer periphery of the core and flattening the power distribution. Therefore, various methods have been devised to flatten the output distribution.

For example, in a commonly used core called a homogeneous core, the core 1 is divided into an inner core IA and an outer core IB, as shown in Figure 1', and the concentration of fissile material in the outer core is lower than that in the inner core. It's expensive.

1, some were divided into more areas in the radial direction, and the outer areas were loaded with fuel with high enrichment (fissile material/fissile material-rich material). Note that 2 in FIG. 1 is a radial blanket, and 3 is an axial blanket.

However, in a reactor core with such a structure, there is a known problem that as the reactor is made larger in order to increase its economic efficiency as a power generation plant, the average energy of the neutron flux in the reactor core 1 becomes lower and the multiplication rate decreases. ing.

In order to solve the above problems, for example, as shown in FIG. 2, a core structure called a non-homogeneous core has been developed in which an internal blanket 5 is provided within the core. In the type of core, the fuel of the axial blanket 7 is used as the internal blanket fuel. According to this, the average neutron energy increases due to the high enrichment of fissile material in the reactor core, and the ratio of neutron capture reactions to fission in the fissile material decreases, resulting in a decrease in the number of neutrons generated per neutron absorption reaction. Becomes a dog. On the other hand, in the internal blanket 5, the atomic number density of the parent fuel material is higher than that in the reactor core 4, so nuclear fission does not occur much, and the neutron capture reaction rate increases. As a result, the power distribution is flattened and the breeding rate is increased compared to a homogeneous core.

Furthermore, since fissile material generated by capturing neutrons of the fuel parent material is accumulated in the inner AB blanket 5, there is an advantage that the reduction in reactivity due to combustion is somewhat small, and therefore the control rods can also be of small value.

Here, we have explained a core that has a disc-shaped internal blanket with a certain thickness, but in order to flatten the power distribution of the core, it is better to make the internal blanket thicker on the inside and thinner on the outside. preferable. For this reason, the types of cores shown in Figures 3 to 6 have already been considered. All of these cores have the above-mentioned advantages in common.

As mentioned above, by providing an internal blanket in the reactor core, the decrease in reactivity due to combustion is reduced, but the difference between the initial stage of initial loading and the final stage of equilibrium is still 4.5% ΔK /
There are also about K, and it is necessary to control this in some way.

SUMMARY OF THE INVENTION An object of the present invention is to provide a non-homogeneous fast breeder reactor in which the reduction in reactivity due to combustion during transition from an initially loaded core to an equilibrium core is small, and therefore surplus reactivity can be easily controlled.

To achieve this objective, the present invention uses natural uranium or depleted uranium enriched with fissile materials (such as uranium-235, uranium-233, and plutonium-239) as the internal blanket fuel for the balanced core. During the exchange, the fissile material is replaced with an unenriched initial charge internal blanket fuel.

Using this method, the decrease in reactivity associated with the combustion of the initial core is alleviated by the enrichment of fissile material in the inner blanket of the equilibrium core, and as a result, the It becomes easier to control excess reactivity when transferring to the reactor core. Apparently, the extent to which reactivity reduction is alleviated is approximately proportional to the concentration of fissile material enriched in the inner blanket fuel.

Thus, according to the present invention, the surplus reactivity to be controlled when transitioning from an initially loaded reactor to an equilibrium reactor core is reduced, and therefore, there is an advantage that the control rods installed in the reactor core can be of low value. Note that the present invention is applicable to all the cores shown in FIGS. 2 to 6 and 1. It is also applicable to a radially non-homogeneous core with several layers of concentric internal blankets as shown in FIG.

In addition, in the above embodiment, depleted uranium or natural uranium enriched with fissile material was used as the internal blanket fuel for the equilibrium core, but instead of this, spent fuel of light water reactors or spent fuel of fast breeder reactors could be used. It is also possible to use processed blanket fuel. This method has the advantage of eliminating the need to adjust the concentration of fissile material.

[Brief explanation of the drawing]

FIG. 1 is a core diagram of a homogeneous fast breeder reactor, and FIGS. 2 to 7 are core diagrams of a heterogeneous fast breeder reactor. 4... Core, 5... Internal blanket, 6... Radial blanket, 7... Axial blanket. Agent Patent Attorney Akio Takahashi (7) 6 no zu waka 2 zu 5 5 6 zu

Claims (1)

[Claims] 1. At the time of initial loading, there is a disk-shaped region of depleted uranium or natural uranium that spreads in the radial direction in the axial center of the core,
When replacing fuel, the above depleted uranium or natural uranium is
A fast breeder reactor is characterized by replacing fissile material with enriched depleted uranium or natural uranium. 2. A fast breeder reactor according to claim 1, characterized in that fuel assemblies made of depleted uranium or natural uranium are mixed in the reactor core at the time of initial loading, instead of the disk-shaped region. 3. A fast breeding method according to claim 1 or 2, characterized in that depleted uranium enriched with fissionability and minerals or processed spent fuel of a light water reactor is used instead of natural uranium. Furnace. 4. Claim 1 or 2 is characterized in that instead of depleted uranium enriched with fissile material or natural uranium, processed fuel from spent blanket (1) of a fast breeder reactor is used. fast breeder reactor.