JPS6319032B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly for a fast breeder reactor.
A fast breeder reactor has the feature of allowing the fuel parent material to absorb neutrons generated by nuclear fission in the reactor core to produce new fissile material, so-called breeding, which allows for effective use of fuel. The core of such a fast 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 substance as a main component. In this core configuration, as shown in FIG. 1, a plurality of fuel rods 1 are connected to wire spacers 3.
It is made by arranging a large number of fuel assemblies in a cylindrical shape, which are bundled together and placed in a channel 2 while maintaining intervals. FIG. 2 is a diagram illustrating the configuration of fuel rods in a fuel assembly corresponding to the reactor core.
In conventional fuel rods, only fuel pellets 5 made of fissile material such as highly enriched plutonium or uranium are filled in the core part of the envelope tube 4, and natural uranium or uranium is added above and below the core. Blanket pellets 6 of a fuel parent substance made of depleted uranium are filled. When a fuel assembly composed of such fuel rods is loaded, the region filled with fuel pellets 5 forms the core, and the region filled with blanket pellets 6 forms an axial blanket. Useful fissile material is then produced by the parent fuel material capturing the neutrons leaking from the core.

Quantitative reference values for the above-mentioned proliferation effect 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. In addition, doubling time is the time required to reproduce the same amount of fissile material as initially loaded into the reactor, and it is desirable to shorten this time. It is considered the main focus.

By the way, the doubling time tends to be shorter as the breeding rate is higher, and also to be inversely proportional to the specific output (output per unit fuel load) of the reactor. Therefore, by flattening the power distribution within the core, the average specific power of the core can be increased without increasing the maximum value of power density, which can be suppressed by fuel design, and the doubling time can be shortened.

Conventionally, as shown in Fig. 2, in a fuel assembly consisting of fuel rods filled with highly enriched fuel pellets 5 over the entire length corresponding to the core of the fuel rod, the output in the axial direction of the core There was a problem with large peaking.

An object of the present invention is to provide a fuel assembly structure for a fast breeder reactor that can flatten the power distribution in the axial direction of the core and improve breeding performance.

In the fast breeder reactor fuel assembly of the present invention, in the fast breeder reactor fuel assembly configured by bundling a large number of fuel rods, a first fuel rod filled with fuel pellets and a blanket are placed in the middle part in the axial direction. It is characterized by a mixture of a plurality of blanket pellets filled with fuel pellets and a second fuel filled with fuel pellets sandwiching the blanket pellets from above and below. Flattening can be achieved.

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 3 shows a cross-sectional view (a) of a fuel assembly in the first embodiment of the present invention and a diagram (b) of pellet arrangement in the fuel rod. In this embodiment, the first fuel rods 8 and the second fuel rods 9 are uniformly arranged as shown in FIG. 3a. Since there is less fissile material in the portion filled with blanket pellets, the power is low and the average axial power distribution is flattened as shown in FIG. The reason why the fuel rods 8 and 9 are used together in this embodiment is as follows. When all the fuel rods 9 are loaded into the fuel assembly, the output will drop because there is less fissile material in the part filled with blanket pellets, and as combustion progresses, fissile material will accumulate due to the breeding effect and the output will increase. This is to prevent output fluctuations from becoming large due to combustion. In other words, by using the fuel rods 8 in combination, the output is distributed evenly to the fuel rods 8 in the early stage of combustion, and the output is distributed evenly to the fuel rods 9 in the advanced stage of combustion, thereby improving the axial power distribution accompanying combustion. fluctuations can be suppressed.

FIG. 5 shows a cross-sectional view of a fuel assembly in a second embodiment of the present invention. In this embodiment, the second fuel rods in the first embodiment are gathered in the center of the fuel assembly, and the first fuel rods are arranged in the periphery thereof. In this embodiment, in addition to the effect of flattening the axial output distribution, the following other effects can be expected. By separating the fuel assembly cross-section into a highly enriched region and a region consisting of parent material, the average neutron energy becomes higher in the highly enriched region, and the neutron capture reaction of the fissile material increases. The ratio to fission reactions decreases, and the number of occurrences per absorption reaction increases. On the other hand, in the region consisting of the parent fuel material, the atomic number density of the fuel parent material is higher than in the surrounding areas with high enrichment, and the average energy of neutrons is lower, so the fission reaction rate of the fuel parent material decreases; The neutron capture reaction rate increases. That is, by localizing the region made of the parent substance within the cross section of the aggregate, the growth performance is improved.

FIG. 6 shows a cross-sectional view of a fuel assembly in a third embodiment of the present invention. In this embodiment, contrary to the second embodiment, the second fuel rods are arranged at the periphery. In this embodiment, other effects can be expected in addition to the effects of the second embodiment, such as flattening the axial power distribution and improving proliferation performance. Since the overall output of the fuel rods filled with blanket pellets is small, the temperature rise of the coolant (usually liquid metal) in the axial direction of the core is small near channel 3, and the coolant temperature in contact with channel 2 is , lower than the temperature of the coolant flowing through the center of the assembly.
As a result, there is a possibility that thermal deformation of the channel can be suppressed.

FIG. 7 shows a pellet arrangement diagram in a fuel rod in a fourth embodiment of the present invention. In this example,
In the second fuel rod 10, the region filled with blanket pellets is located below the core from a position half the length L of the fuel portion of the fuel assembly. In fast breeder reactors, in order to control the excess reactivity of the core and flatten the radial power distribution,
Control rods are inserted from the top of the core. When the control rods are inserted, the axial power distribution is distorted downward in the core due to the neutron absorption effect of the control rods, and the peak value of power density occurs in the lower half of the core. By arranging the region consisting of the blanket pellets 7 in the lower part as in this embodiment, it is possible to suppress an increase in the axial power peaking when the control rods are inserted, and the core power distribution can be flattened.

FIG. 8 shows a diagram of the arrangement of pellets in a fuel rod in a fifth embodiment of the present invention. This embodiment is characterized in that the second fuel rod 11 is filled with blanket pellets 12, which are shorter in length than the other fuel pellets 5, as the blanket pellets. Short blanket pellets can be used because they have a lower power output, which releases less fission gas, and there is no need to reduce the surface area to volume ratio of the pellet to confine the fission gas within the pellet. This is because the length can be shortened. By distinguishing blanket pellets from other pellets, errors during production can be reduced, and during reprocessing, blanket pellets containing a large amount of fissile material can be separated from other pellets, making it easier to use during reprocessing. can be expected to improve collection efficiency.

As explained above, in a fast breeder reactor loaded with the fuel assembly of the present invention, the axial power distribution is flattened, and the fluctuation in power due to combustion is small, and an improvement in breeding performance can be expected.

Next, in a fast breeder reactor loaded with the fuel assembly of the present invention, the radial power distribution can be flattened. That is, in the fuel assembly structure of the present invention, the first
The ratio of the number of fuel rods to the second fuel rods, or the length of the area filled with blanket pellets in the second fuel rods is changed depending on the radial position of the core where the assembly is loaded.

FIG. 9a shows a cross section of a reactor core loaded with two types of fuel assemblies according to the third embodiment of the present invention, in which the proportions of the first fuel rods and the second fuel rods are different. Figure 9b shows the second
FIG. In this embodiment, the fuel assembly 1 including a large number of second fuel rods is
3 is loaded in the center of the reactor core, a fuel assembly 14 having a small proportion of the second fuel rods is loaded in the periphery, and a radial blanket fuel 15 surrounds it.
In the figure, 16 represents a control rod. In this core configuration, the average enrichment of fissile material in the radial direction of the core is low at the center of the core and high at the periphery, so that the power distribution can also be flattened in the radial direction.

FIG. 10a shows a third fuel rod according to the present invention in which the length of the blanket pellet area in the second fuel rod is different.
FIG. 10b shows a cross-section of a reactor core loaded with two types of fuel assemblies of the embodiment and a fuel assembly consisting of the first fuel rod, and FIG. 10b is a longitudinal cross-sectional view of each fuel assembly. . In this embodiment, a fuel assembly 17 with a long blanket pellet filling area is located at the center of the core, a fuel assembly 18 with a short blanket pellet filling area is located at the periphery, and the fuel assembly 17 consisting of the first fuel rods is located at the periphery. A body 19 is located at the outermost part of the core.
In this case as well, the average enrichment in the radial direction is low at the center of the core and high at the periphery, so that the power distribution can be flattened in the radial direction.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the fuel assembly, Fig. 2 is a vertical cross-sectional view showing the structure of the fuel rod, Fig. 3a is a cross-sectional view of the fuel assembly in the first embodiment, Fig. 3b 4 is a characteristic diagram showing the power distribution, FIG. 5 is a cross-sectional view of the fuel assembly in the second embodiment, and FIG. 6 is the third embodiment. FIG. 7 is a vertical cross-sectional view showing the pellet arrangement of the fuel rods in the fourth embodiment, and FIG. 8 is a cross-sectional view showing the pellet arrangement of the fuel rods in the fifth embodiment. FIG. 9a is a vertical cross-sectional view showing the core configuration, FIG. 9b is a vertical cross-sectional view illustrating the fuel rod configuration within the fuel assembly, and FIG. 10a is a cross-sectional view showing the core configuration. Figure 10b is a cross-sectional view showing the configuration of fuel rods within the fuel assembly.

Claims (1)

[Scope of Claims] 1. In a fast breeder reactor fuel assembly formed by bundling a large number of fuel rods, a first fuel rod filled with fuel pellets and a middle part in the axial direction filled with blanket pellets. A fast breeder reactor fuel assembly characterized in that a plurality of second fuel rods filled with fuel pellets are intermixed so as to sandwich the blanket pellets from above and below. 2. The fuel assembly according to claim 1, wherein the second fuel rod is disposed at the lateral center of the fuel assembly, and the first fuel rod is disposed at the periphery. body. 3. The fuel assembly according to claim 1, wherein the first fuel rod is disposed at the center in the lateral direction of the fuel assembly, and the second fuel rod is disposed at the periphery. body. 4. In the second fuel rod, the portion to be filled with the blanket pellets has a central portion that is
4. The fuel assembly according to claim 1, wherein the fuel assembly is installed below 1/2 of the height of the fuel rod in the axial direction of the core. 5. In the second fuel rod, the axial length of the blanket pellet is shorter than the axial length of the fuel pellet. The fuel assembly described in Crab. 6. Any one of claims 1 to 5, characterized in that the upper and lower ends of the first and second fuel rods are filled with blanket pellets to form axial blankets. Fuel assembly described in .