JPH0478959B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a technology utilized in a nuclear reactor, and particularly relates to a core of a fast breeder reactor when the nuclear reactor is a fast breeder reactor. [Prior Art] As is well known, in a fast breeder reactor, neutrons generated by nuclear fission etc. in the reactor core are absorbed into the parent fuel material to produce new fissile material, which is called breeding. It has the feature of making efficient use of fuel. 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 containing a fuel parent substance as a main component. The reactor core is loaded with enriched uranium or plutonium-enriched uranium as fuel, and the blanket is loaded with enriched uranium or plutonium-enriched uranium as fuel parent material.
For example, natural uranium or depleted uranium is loaded. Useful fissile material is produced by this fuel parent material capturing neutrons leaking from the core, and by capturing neutrons by the fuel parent material within the reactor core. As a quantitative reference value for such proliferation effect,
These include growth 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. 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 the most effective way to improve fast breeder reactors. It is considered the main focus. By the way, the doubling time is generally inversely proportional to the specific power of the reactor (output per unit fuel load).
The maximum value is constrained by fuel usage limits. Therefore, the time can be shortened by suppressing the decrease in power at the outer periphery of the core, flattening the power distribution, and increasing the average power density while suppressing the maximum value. Therefore, various methods have been devised to flatten the output distribution. A conventionally known measure for flattening the output distribution and improving multiplication properties is disclosed in Japanese Patent Laid-Open No. 16897/1983. For example, as shown in Fig. 1, four cylindrical internal blanket regions mainly made of natural uranium or depleted uranium are formed near the axial center of the core 1, and are thickened near the radial center of the core. , one was developed in which it became thinner near the periphery. Note that 2 in FIG. 1 is a radial blanket, and 3 is an axial blanket. According to this, the average neutron energy of core 1 is high,
In the reactor core 1, the ratio of the neutron capture reaction of the fissile material to nuclear fission decreases, and the number of neutrons generated per absorption reaction increases. On the other hand, in the internal blanket 4, the atomic density of natural uranium, depleted uranium, etc. is higher than in the reactor core 1, and the average neutron energy is lower, so that the nuclear fission reaction rate of natural uranium, depleted uranium, etc. decreases. As a result of this internal blanket being placed near the axial center, being thicker in the radial center and thinner at the periphery, the radial power distribution can be flattened. On the other hand, the axial power distribution can be flattened by the presence of the internal blanket 4 at the axial center of the core 1, which has a low power density and a small reactivity value. Safety is also improved due to the low sodium void reactivity and the presence of an internal blanket with low power density in the axial center of the core. [Problems to be Solved by the Invention] However, when a control rod is inserted during nuclear reactor operation, the flatness of the axial power distribution described above is broken due to the control rod insertion. This point will be explained with reference to FIG. In FIG. 2, the solid line shows the relationship between the axial distance and relative output when the control rod is inserted, and the broken line shows the relationship when the control rod is not inserted. In fast breeder reactors, control rods are often inserted from the top of the core, and their insertion depth is greatest at the beginning of combustion when reactivity is high, decreases as combustion progresses, and reaches its minimum at the end of combustion. When the control rods are partially inserted during the early to middle stages of combustion, the power density decreases because neutrons are absorbed by the control rods in the core, which is the fuel region between the upper axial blanket where the control rods are inserted and the internal blanket. lowered and, conversely, separated from the control rod.
Power density increases in the core, the fuel region between the inner blanket and the lower axial blanket. Therefore, the axial power distribution changes significantly depending on the insertion of the control rod. When determining the core configuration to avoid such drawbacks, it is necessary to ensure that the flatness of the axial power distribution due to control rod insertion is not lost. An object of the present invention is to provide a fast breeder reactor that can improve core performance by flattening the output, and in particular can shorten doubling time. [Means for Solving the Problems] A first means for achieving the above object is to provide a fast breeder reactor equipped with an internal blanket in the internal region of the reactor core surrounded by axial and radial blankets.
The internal blanket is provided in the core at a position where the center of the internal blanket is below the center of the core, and the internal blanket has a core configuration in which the thickness in the axial direction of the core is thick at the center in the radial direction of the core and thin at the periphery. . A second means of achieving the above object is based on a core configuration characterized in that the periphery of the inner blanket of the first means is spaced from the radial blanket inside the interior region of the core. [Operation] According to the first means, the internal blanket with a low nuclear fission reaction rate existing in the reactor core is distributed thicker in the radial center and thinner in the periphery, so that the radial power distribution is flat, being high in the center and low in the periphery. be converted into Furthermore, when the control rods are partially inserted into the reactor core, the peak of the axial power distribution falls downward;
Since the internal blanket with a low fission reaction rate is built into the core below the core center, the peak is suppressed, while the volume of the upper core region above the internal blanket is It increases relatively compared to the lower part, which compensates for the decrease in power caused by the control rods inserted in the increased upper core region, suppressing the drop in the power distribution. These synergistic effects flatten the power distribution in the axial direction. For this reason, the core power distribution is flattened in the three-dimensional directions of the axial direction and the radial direction, thereby improving core performance such as multiplication time. According to the second means, in addition to the effect of the first means, the peripheral portion of the internal blanket is separated from the radial blanket, so that the power reduction effect of the internal blanket is centered in the radial direction within the core region (inner blanket thickness is maximum). It acts gently from the inner blanket to the periphery (area where no internal blanket exists), reducing output fluctuations during the combustion period and further flattening the output distribution. Furthermore, although the core output decreases as it approaches the radial blanket, a core region may remain between the internal blanket and the radial blanket so that the decrease does not occur excessively. Therefore, the effect of further shortening the multiplication time can be obtained. [Example] Hereinafter, an example of the present invention will be described with reference to FIG. 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 upper axial blanket 9 and a lower axial blanket 7 each having a cylindrical reactor core 5 mainly composed of a fuel parent substance.
and is surrounded by a radial blanket 8. Between the axial center of the core 5 and the lower axial blanket 7, there is provided a cylindrical region extending in the radial direction of the core, ie, an internal blanket 6, whose main component is natural uranium or depleted uranium. This inner blanket 6 is thick near the radial center of the core and thinner at the periphery. A core 5 is defined between the periphery and the radial blanket 8. Also, 10
is the control rod. An embodiment of the present invention in a nuclear fuel element will be described with reference to FIG. In the reactor core 14, which is a fuel region consisting of a mixed oxide of plutonium and uranium, surrounded by an upper axial blanket 13 and a lower axial blanket 16, each containing a fuel parent substance as a main component, natural uranium or uranium as shown in FIG. An internal blanket is provided in which the position of the internal blanket 15 whose main component is depleted uranium or the like is lowered. In the following, the effects of the present invention in a reactor core configuration, which is an assembly of nuclear fuel elements, will be described as effects of the nuclear fuel element of the present invention. The core design parameters and reactor operating conditions are shown in Table 1. That is, the reactor thermal output is approximately 2500 MW, the electrical output is approximately 1000 MW, and the equivalent core diameter and core height are 325 cm and 95 cm, respectively.

〔Effect of the invention〕

According to the invention set forth in claim 1, the breeding property of the fast breeder reactor, particularly the flattening of the power distribution in both the radial direction and the axial direction, is well achieved,
This makes it possible to significantly shorten the doubling time, which has a great effect on fuel economy. According to the embodiment of claim 2,
In addition to the effect of the invention of claim 1, there is an effect of further improving proliferation.

[Brief explanation of the drawing]

Fig. 1 is a schematic longitudinal cross-sectional view of the core of a conventional fast breeder reactor, Fig. 2 is a diagram of axial power distribution in the core of a conventional fast breeder reactor, and Fig. 3 is a schematic longitudinal cross-section of the core configuration of the present invention. 4 is a longitudinal sectional view of a nuclear fuel element in a conventional example, FIG. 5 is a longitudinal sectional view of a nuclear fuel element adopted in the present invention, FIG. 6 is a schematic longitudinal sectional view of a reactor core in a conventional example, and FIG. The figure is a schematic vertical cross-sectional view of the core when the present invention is adopted in the core of Figure 6,
FIG. 8 is a schematic vertical cross-sectional view of a core of the type improved from the conventional core shown in FIG. 6, FIG.
Figure 0 is a schematic vertical cross-sectional view of another type of core that is an improved version of the conventional core shown in Figure 6, and Figure 11 is a schematic vertical cross-sectional view of a core in which the present invention is applied to the core shown in Figure 10. . 5...Reactor core, 6...Internal blanket, 7...
Lower axial blanket, 8... Radial blanket, 9... Upper axial blanket, 10...
control rod.

Claims (1)

[Scope of Claims] 1. In a fast breeder reactor equipped with an internal blanket in the internal region of the reactor core surrounded by axial and radial blankets, the center of the internal blanket is located below the center of the core. A fast breeder reactor characterized in that the internal blanket is provided in the reactor core, and the internal blanket has a thickness in the axial direction of the core that is thick at the center in the radial direction of the core and thin at the periphery. 2. A fast breeder reactor according to claim 1, characterized in that the periphery of the internal blanket is spaced from the radial blanket inside the internal region of the reactor core.