JPH04299286A - Operating method of core for fast reactor - Google Patents

Abstract

PURPOSE:To make power uniform with one kind of enrichment degree fuel, though several kinds of enrichment degree are needed for the uniforming of a power distribution. CONSTITUTION:A fuel assembly where minor actinides are added 20 to 30% at weight ratio in uranium-plutonium mixed fuel for a fast reactor is loaded in and around the center of a core. The assembly is in order moved from the center side of the core to a peripheral direction in a fuel replacing time. The fuel is converted into a nuclide whose fission cross section is larger, by irradiation, and the uniforming of a power distribution is attained with one kind of fuel assembly.

Description

[Detailed description of the invention]

[Object of the invention]

0002

[Industrial Application Field] The present invention produces output by moving the loading position of the fuel assembly from the center of the core to the outer periphery every time the fuel is replaced, that is, from a position with high neutron density to a position with lower neutron density. This article relates to a method of operating a fast reactor core that flattens the distribution.

0003

2. Description of the Related Art In a fast reactor core, the neutron flux is generally high near the center of the core, and decreases as the core approaches the outer periphery of the core. Therefore, in order to flatten the output in the reactor core, it is necessary to load fuel assemblies containing several types of fissile nuclides, such as fuel assemblies with a high content of fissile nuclides and fuel assemblies with a small content of fissile nuclides. Prepare them and place them appropriately in the core so that the power distribution is flat. In other words, as shown in Figure 4, fuel assemblies with a low content of fissile nuclides are placed in the central region of the reactor core where neutron flux density is high (
The power distribution within the core is flattened by loading fuel assemblies containing more fissile nuclides into the inner core 41) and into the outer core region (outer core 42) where neutron flux density is lower. 1 is a horizontal sectional view showing the center plane of a two-zone homogeneous core as an example of a normal fast reactor. In the figure, reference numeral 41 is an inner core loaded with fuel assemblies containing a small amount of fissile nuclides. area, code 4
2 is the outer core, which is a region where fuel assemblies containing a large amount of fissile nuclides are loaded. In this manner, the conventional operating method uses two types of fuel assemblies to flatten the power distribution.

0004

[Problems to be Solved by the Invention] However, when adopting such a core configuration, it is necessary to prepare several types of fuel assemblies with different proportions of fissile nuclides. In particular, for a core with an electrical output of 200,000 KWe class or higher, it is essential to prepare several types of core fuel assemblies in order to flatten the power distribution. Generally, when manufacturing a fuel assembly, it is necessary to accurately control the amount of fissile nuclides in the nuclear fuel pellets that make up the fuel element, and the more types there are, the more steps are required to manufacture the nuclear fuel pellets. This increases the procurement cost of fuel assemblies.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a method for operating a fast reactor core that can achieve flattening of power output with one type of fuel assembly. [Structure of the invention]

[0006]

[Means for Solving the Problems] The present invention provides a method for operating a fast reactor core in which a portion of fuel assemblies are loaded into the core in each cycle. A fuel assembly made by adding 20% or more by weight of minor actinide elements to uranium-plutonium mixed fast reactor fuel is first loaded into the central region of the reactor core, and then added to the fuel at each subsequent fuel exchange. The fuel assembly is characterized in that the loading position of the fuel assembly is sequentially moved from the central region of the core to the outer peripheral region as the loaded nuclide is converted to a more fissile nuclide within the assembly.

[0007]

[Operation] A fast reactor can be constructed so that fissile nuclides added to the reactor are converted to more fissile nuclides as the nuclear fuel burns. Therefore, a fuel assembly with high conversion characteristics is prepared from the configuration described below, and it is first loaded into the central region of the reactor core where neutron density is high, and as combustion progresses, the fissile nuclides become more fissile nuclides. At each fuel change, the loading position is sequentially moved to a position with lower neutron density. That is, the power distribution is flattened by moving the fuel assembly from the center of the core toward the outer circumference. With such an operating method, it is possible to prepare fewer types of fuel assemblies with different proportions of fissile nuclides than before, and it is possible to reduce the procurement cost of fuel assemblies. This fuel assembly with high conversion characteristics converts transuranic elements extracted from the spent fuel of light water reactors, especially so-called minor actinide elements such as neptunium, americium, and curium, into uranium-plutonium mixture fuel assemblies, which are ordinary fast reactor fuel. This is possible by adding it to. The fission cross sections of these elements are 235U, 239Pu, 2
Although its fission cross section is smaller than that of commonly used fissile nuclides such as 41Pu, it is converted to a nuclide with a larger fission cross section by absorbing neutrons.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the fast reactor core operating method according to the present invention will be described with reference to the drawings. Note that Examples showing the effectiveness of the present invention were carried out under the following conditions. <Basic core specifications> Core thermal output 2
600 MWth (electrical output
1000 MWe) core height
100 cm aggregate pitch 16.2
cm Core fuel volume ratio Fuel: Coolant:
Structural material = 41.5: 36.5: 22.0 Number of core fuel assemblies 366 Number of fuel elements in assembly 271 Main operation cycle length 1
Number of fuel exchange batches per year: 5 cores Average withdrawal average burnup: Approximately 200,000 MWd/to
FIG. 1 is a horizontal cross-sectional view for explaining a reactor core according to the operating method of the present invention. In the figure, the fuel assembly constructed by adding a minor actinide element has a code of 1 when loaded.
It is loaded in the first region of the core center region indicated by 1. Minor actinide elements in the proportions shown in Table 1 were initially added to this fuel assembly in a weight percentage of 25%. As the core burns, the minor actinide elements are converted to fissile nuclides.

[0009]

[Table 1]

[0010] In the operating method according to the present invention, first, the fuel assembly is loaded into the core center region, that is, the first region 11;
At each subsequent fuel change, move the fuel assembly to the first area 1.
1 to the second, third...areas 12, 13... in the outer circumferential direction, and as the loaded nuclide is converted to a more fissionable nuclide within the fuel assembly, the loading position is changed to a position with a higher neutron density. By moving it to a lower position, the output distribution is flattened.

[0011] The movement order of the assemblies during fuel exchange is: first area 11 (new fuel) → second area 12 (loaded after staying in the reactor for one year) → third area 13 (one year in area 12) Loading after staying in the reactor, i.e. loading after staying in the reactor for 2 years) → Fourth area 14 (loading after staying in area 13 for 1 year, i.e.,
Loading after staying in the reactor for 3 years) → 5th area 15 (area 1
After staying in the reactor for one year, it is loaded (in other words, after staying in the reactor for four years, it is loaded) and then taken out.

[0012] Since the period of stay in each area is one year, it will stay in the reactor for a total of five years before it is loaded and taken out. Since the proportion of fissile nuclides in the loaded fuel assembly increases with irradiation of the fuel, the power distribution within the core is flattened. However, if the weight ratio of minor actinide elements added is too small, the conversion characteristics to fissile nuclides will be too poor, and the output in the outer region will decrease too much, making it impossible to achieve a flat output distribution with one type of fuel. I can't. The output peaking factor is a measure of the degree of flattening of the output distribution.

FIG. 2 shows the relationship between the weight ratio of minor actinide elements added and the in-core power peaking factor. Since the smaller the power peaking factor is, the greater the output can be obtained with the same core size, it is important to reduce the power peaking factor in core design. In order to suppress this output peaking factor to about 2.5 or less, as is clear from FIG. 2, the weight percentage of the minor actinide element added must be 20% or more. FIG. 3 shows the power distribution when the core operating method according to the present invention is adopted when minor actinide elements are initially loaded at 25% by weight. Figure 3 shows the power distribution diagram in relation to the distance from the core center and relative power.
The solid line indicates the initial stage of equilibrium, and the dashed line indicates the final stage of equilibrium. As is clear from FIG. 3, the output distribution is appropriately flattened. Table 2 shows the core characteristics of this core. The maximum line output is 40
It is within 0W/cm.

[0014]

[Table 2]

According to the above embodiment, a well-flattened power distribution can be achieved by using one type of fuel assembly simply by improving the loading method (the loaded nuclide is converted to a more fissile nuclide). This can also be achieved by moving the loading position to a location with lower neutron density. However, in the above embodiment, the output is flattened when the core reaches an equilibrium state, so the excessive period of time until the fuel loaded to the extent that it can be loaded into the peripheral area of the core is
Several types of conventional fuel assemblies must be prepared and operated. Further, according to the above embodiment, minor actinide elements with long half-lives generated from light water reactors, which are generally treated as a nuisance, can be burned while being used for power generation. Note that in the above embodiment, five regions were adopted in the radial direction of the core, but this is just an example, and the number of regions is as follows:
It is sufficient to select a number of regions corresponding to the value obtained by dividing the period of stay of the fuel assembly in the reactor (years) by the operating period (years).

[0016]

Effects of the Invention According to the present invention, the fuel in the fuel assembly made by adding minor actinide elements to the fast reactor fuel is converted into a nuclide with a large fission cross section by irradiation. Output flattening can be achieved in aggregates.

[Brief explanation of drawings]

FIG. 1 is a horizontal sectional view showing a core for explaining a loading movement procedure of fuel assemblies in an embodiment of the fast reactor core operating method according to the present invention.

FIG. 2 is a curve diagram showing the relationship between the weight percentage of minor actinide elements in the fuel assembly in FIG. 1 and the output peaking coefficient.

FIG. 3 is a power distribution diagram showing the relationship between distance from the core center and relative power in the core operating method according to the present invention.

FIG. 4 is a horizontal sectional view showing a conventional fast reactor core using two types of fuel.

Claims (1)

[Claims] Claim 1: In a fast reactor core operating method in which a portion of fuel assemblies are loaded into the core for each cycle, transuranium elements, particularly minor actinide elements such as neptunium, americium, and curium, are added to uranium. - A fuel assembly composed of plutonium mixed fast reactor fuel added at a weight ratio of 20% or more is first loaded into the central region of the reactor core, and each time the fuel is replaced thereafter, the nuclides in the fuel assembly become more fissile. A method for operating a fast reactor core, characterized in that the loading position of the fuel assemblies is sequentially moved from a central region of the core to an outer peripheral region in accordance with the conversion to a nuclide with high nuclides.