JPS5819590A - 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 produce new fissile material by absorbing neutrons generated by nuclear fission etc. in the core of the nuclear reactor into the parent material of the fuel. It has the characteristics of being able to be used effectively. Such fast! The core of a breeder reactor is generally formed in a cylindrical shape, and the periphery of the core is surrounded by axial and radial blankets containing a fuel parent material as a main component. 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 this fuel parent material capturing neutrons leaking from the core, and by capturing neutrons by the fuel parent material within the reactor core.

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 the most effective way to improve fast breeder reactors. It is considered the main focus.

Incidentally, the doubling time is generally inversely proportional to the specific output (output per unit fuel load) of the nuclear reactor, but the maximum value is limited by the fuel usage limit. 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.

For example, as shown in Fig. 1, a cylindrical internal blanket 4 area made mainly of natural uranium or depleted uranium is placed near the axial center of the core 1, thicker near the radial center of the core and thinner near the periphery. A system was developed to make it possible. Note that 2 in FIG. 1 is a radial blanket; 3. is an axial blanket. According to this, the average neutron energy in the reactor core 1 is high, and in the reactor core 1, the ratio of the neutron capture reaction to nuclear fission of the fissile material decreases, and the number of neutrons generated per absorption reaction increases. In the internal blanket 4, the nuclear fission reaction rate of natural uranium or depleted uranium decreases because the atomic density of natural uranium or depleted uranium in the reactor core 1 is high and the average neutron energy is low. As a result of this internal blanket being placed near the axial center, 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.

However, when inserting control rods during reactor operation,
The flatness of the axial power distribution described above is broken by 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 the case of 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 the reactivity is high, decreases as combustion progresses, and reaches its minimum at the end of combustion. When the control rods are half-inserted during the early to middle stages of combustion, the power density increases 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. conversely, the power density increases in the core, which is the fuel region between the inner blanket and the lower axial blanket, away from the control rods. Therefore, the insertion of the control rods significantly changes the axial power distribution. When determining the core configuration to avoid these 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 axial output, and in particular can shorten doubling time.

A feature of the present invention that achieves these objects is that natural uranium, depleted uranium, etc. are The main component of the internal blanket is a cylindrical region extending in the radial direction of the core, and its axial center is provided within the core below the axial center of the core.

Hereinafter, one embodiment 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. A cylindrical reactor core 5 is surrounded by an upper axial blanket 91, a lower axial blanket 7, and a radial blanket 8, each of which has a fuel parent substance as its main component. Between the axial center of the reactor 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 internal blanket 6 is thick near the center of the core in the radial direction and thin near the periphery. Further, 10 is a control rod.

An embodiment of the present invention in a nuclear fuel element will be described with reference to FIG. Surrounded by an upper axial blanket 13 and a lower axial blanket 16 mainly composed of a fuel parent substance, an inner blanket mainly composed of plutonium and uranium, natural uranium or depleted uranium as shown in FIG. An internal blanket is provided below the position 15.

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 2500M
W, the electrical output is approximately 10,100 O, and the equivalent core diameter and core height are 325c1r1 and 95crn, respectively.

Table 1 Key Design Parameters Upper and lower axial blanket thickness is 403° and radial blanket thickness is 40 cm. The fuel smear density is 87% theoretical density in the core and 911% theoretical density in the blanket. The core fuel is enriched (fissile material/fissile material +
Assume that the parent substance) is single, and the axial blanket fuel is used as the internal blanket fuel for that ``1''! use.

The refueling period will be one year, the capacity factor will be 80%, and the number of refueling batches will be three for both core and blanket. The maximum linear power density at rated time is 430 W/cW1, and the surplus reactivity at the end of combustion is about 0.7%.

Next, effects based on the above configuration will be explained. As for the radial power distribution, by making the inner blanket 6 thicker near the center in the radial direction and thinner at the periphery, it is flattened to the same extent as the conventional one. This is due to the following reason. The average neutron energy of the reactor core 5 increases, 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 6, compared to the core 5, the number density of natural uranium or depleted uranium atoms is higher, and the average energy of neutrons is lower, so the fission reaction of natural uranium or depleted uranium, etc. is reduced, but the neutron capture reaction rate is increases.

Next, consider the axial power distribution. When the control rods 10 are inserted from the top of the reactor core, neutrons are absorbed by the control rods, and as a result, the fission reaction of fissile material in the core between the upper axial blanket and the inner blanket decreases, reducing the power density. The power density increases in the core between the blanket and the inner blanket, and the axial power distribution has a peak. This power peak is suppressed by lowering the inner blanket of low power density and low reactivity value below the conventional axial center. In addition, by lowering the internal blanket axially downward, the core volume between the upper axial blanket and the internal blanket increases, the rate of neutron leakage from this core region to the surrounding blanket is suppressed, and fissile material causes nuclear fission. This increases the reaction rate and offsets the loss in power due to control rod neutron absorption. By raising the inner blanket downward, the core volume between the four lower axial blankets and the inner blanket is reduced, and the rate of neutron leakage from this core region to the surrounding blanket increases, suppressing the increase in power. As a result, even when the control rod is inserted, the axial power distribution can be flattened.

Such flattening of the axial and radial power distributions improves the multiplication property, that is, shortens the doubling time.

In other words, due to the high neutron energy of the reactor core 5 and the flatness of the power distribution, the reactor core 5 and the upper axial blanket 9 surrounding it. The neutron flux at the boundary between the lower axial blanket 7 and the radial blanket 8 is high. To that end, the upper axial blanket 9. The amount of neutrons leaking to the lower shaft blanket 7 and the radial blanket 8 increases, and the neutron capture reaction and nuclear fission reaction of the fuel parent material are significantly improved.

In the above embodiment, the thickness of the internal blanket changes stepwise in the radial direction, and the core structure is such that the internal blanket is separated from the radial blanket without being in contact with it. As shown in the figure, even in the case of a core structure in which the wall thickness of the internal blanket 19 changes stepwise in the radial direction and the internal blanket 19 is in contact with the radial blanket, the internal blanket 19 is moved downward as shown in FIG. By lowering the distance between 1 and 2, substantially the same effect as in the above embodiment can be obtained.

As shown in FIG. 8, the wall thickness of the internal blanket 21 is gradually changed, and even in the case of a core structure in which the internal blanket 21 is separated from the radial blanket 22 without contacting it, the internal blanket 21 is changed as shown in FIG. By lowering it downward, substantially the same effect as in the embodiment described above can be obtained.

Even in the case of a core structure in which the wall thickness of the internal blanket 23 is gradually changed as shown in FIG. 10 and the internal blanket 23' is in contact with the radial blanket 24, the internal blanket 23 is moved downward as shown in FIG. Substantially the same effect as in the embodiment described above can be obtained by using the above-described method.

In the case of a core structure having a cylindrical internal blanket extending in the radial direction of the core at the axial center of the single enrichment core 25 as shown in FIG. 12, the internal blanket 26 is lowered as shown in FIG. By lowering it in the opposite direction, substantially the same effect as in the embodiment described above can be obtained.

As shown in FIG. 14, if the core is divided into an inner core 28A and an outer core 28B, or another core is provided between the inner core 28A and the outer core 28B in the radial direction,
When the outer core is divided into 2 to 3 regions in the radial direction, and the outer region is loaded with highly enriched fuel, it has a cylindrical internal blanket 29 that extends into the inner core in the radial direction of the core. When the inner blanket 29 is lowered as shown in FIG. 15, substantially the same effect as in the previous embodiment can be obtained.

As shown in FIG. 16, in the case of a core structure having a cylindrical internal blanket 31 in the axial center of the core with a single enrichment that extends to a point where it touches the radial blanket in the radial direction of the core, By hanging the inner blanket downward as shown in the figure, effects substantially similar to those of the previous embodiment can be obtained.

As shown in FIG. 18, by dividing the core into an inner core 33A and an outer core 33B, or by providing another core between the inner core 33A and the outer core 33B in the radial direction,
It is divided into 2 to 3 regions in the radial direction, and the outer region is loaded with highly enriched fuel, but has a cylindrical inner blanket 34 in the axial center that extends until it touches the radial blanket in the radial direction of the core. In the case of a reactor core structure, by lowering the internal blanket 34 as shown in FIG. 19, substantially the same effect as in the previous embodiment can be obtained.

According to the fast breeder reactor of the present invention, the doubling time can be significantly shortened due to the flattening of the breeding property, especially the axial power distribution, which has a great effect on fuel economy.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view of the reactor core, Figure 2 is a characteristic diagram showing the axial power distribution of the reactor, Figure 3 is a vertical cross-sectional view showing the core configuration, and Figures 4 and 5 are the structure of nuclear fuel elements. FIGS. 6 to 19 are vertical cross-sectional views of the reactor core. ,
9.° Upper axial blanket, 10. Control rod. Agent Patent Attorney Akio Takahashi 3 H O K Ht A /8 Figure jf1/'/'f5

Claims (1)

[Scope of Claims] 1. In a fast breeder reactor in which a cylindrical core is surrounded by axial and radial blankets containing a fuel parent substance as a main component, a core containing natural uranium or depleted uranium as a main component; A fast breeder reactor characterized in that an internal blanket, which is a cylindrical region expanding in the radial direction, is provided in the reactor core so that its axial center is below the axial center of the reactor core. 2. In claim 1, the inner blanket is provided such that the axial center of the inner blanket is below the axial center of the reactor core, and the fissile material concentration is slightly enriched at the time of initial loading. A fast breeder reactor is characterized by loading fuel and replacing it with natural uranium or depleted uranium when replacing the fuel. 3. In claim 2, the internal blanket is provided such that the axial center of the inner blanket is below the axial center of the reactor core, and when it is first loaded, a busy light water reactor is used instead of natural uranium or depleted uranium. A fast breeder reactor characterized by using fuel material processed from spent fuel. 4. According to claim 2, an internal blanket is provided such that the axial center of the inner blanket is below the axial center of the reactor core, instead of natural uranium or depleted uranium, etc. at the time of initial loading. A fast breeder reactor characterized in that it uses fuel material processed from spent blanket fuel of a fast breeder reactor. 5. Claim 2 provides that the fuel initially loaded into the inner blanket is removed in the latter half of the time, the inner blanket is provided so that the axial center of the inner blanket is below the axial center of the reactor core. A fast breeder reactor that uses natural uranium or depleted uranium as fuel. 6,' In claim 1 or 2, the portion of the radial or axial blanket close to the reactor core also includes:
A fast breeder reactor characterized in that fissile material is mixed in at the time of initial loading and replaced with fuel that does not contain fissile material at the time of fuel exchange. 7. In claim 6, the internal blanket provided so that the axial center of the inner blanket is below the axial center of the reactor core is loaded with the same fuel material as the reactor core at the time of initial loading, A fast breeder reactor characterized by replacing fuel with natural uranium or depleted uranium when exchanging fuel. 8. In a nuclear fuel element consisting of fuel surrounded at the top and bottom by an axial blanket whose main component is a fuel parent substance,
A nuclear fuel element characterized in that an internal blanket containing natural uranium or depleted uranium as a main component is provided in a fuel region such that the center of the internal blanket is below the center of the fuel. 9.% Allowance In claim 8, spent fuel from a light water reactor is processed into the inner blanket provided so that the center of the inner blanket is below the center of the fuel, instead of the original uranium or depleted uranium, etc. A nuclear fuel element characterized by the use of a fuel material that is 10. In claim 8, spent blanket fuel from a fast breeder reactor is used instead of natural uranium or depleted uranium in the inner blanket provided so that the center of the inner blanket is below the center of the fuel. A nuclear fuel element characterized by using fuel material processed by.