JPS6150276B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear reactor core. Boiling water reactors have a void distribution in the axial direction, so the power distribution skews downwards in the core, and the power peaking is relatively large. To suppress this power peaking, measures are taken such as changing the position of all the fuel in the core during fuel exchange to reduce power peaking in the radial direction, and inserting control rods shallowly from the bottom of the core. These methods have drawbacks such as increasing the time required for fuel exchange, sudden changes in output near the tips of control rods, and requiring a large amount of calculation to formulate a core operation plan. . An object of the present invention is to eliminate the drawbacks of the prior art described above, to provide a nuclear reactor that can flatten the power distribution in the axial direction with a simple fuel assembly using fewer types of fuel rod materials, and that does not require shuffling of the fuel assembly. The aim is to provide the core of A feature of the present invention is that the fuel assembly includes a plurality of first fuel rods having a uniform enrichment in the axial direction, arranged in the center in a plane perpendicular to the axis of the fuel rod assembly, and in the upper part in the axial direction. A plurality of second fuel rods are divided into a region and a lower region, and the enrichment in the upper region is higher than that in the lower region, and the enrichment in the axial direction is uniform, and the enrichment is equal to A third fuel rod, which is smaller than the largest first fuel rod, is arranged in a peripheral area adjacent to and surrounding the central part in the plane, and the enrichment in the upper region of the second fuel rod is increased. The fuel rod is configured so that the enrichment does not exceed that of the first fuel rod, which has the highest enrichment, and the only control rods for power distribution control are control rods inserted deeply into the reactor core in a concentric control rod pattern. A preferred embodiment of the present invention will be described below with reference to FIGS. 1 and 2.
This will be explained in detail based on FIGS. 3, 3, and 4. It is a boiling water reactor with a thermal output of 2400 MW, a core height of 146 inches, and a number of nuclear fuel assemblies of 560. The fuel assembly 11 shown in FIG. 1 has an 8×8 square lattice as shown in detail in FIG. 2, and has fuel rods 1, 2, 3, 4, 5, and 6 (62 in total).
It is made up of two water rods 7. In the axial direction of the fuel assembly, 11/24 of the total length from its lower end
Divide the region at the position of and calculate the average enrichment at the top as
2.15% by weight, and the average enrichment in the lower part is 1.93% by weight, making a difference in the infinite multiplication factor, offsetting the downward skewing of the power peaking in the core due to the void distribution in the axial direction, and reducing the power peaking in the axial direction. let As mentioned above, there are six types of fuel rods in the fuel assembly 11, and the uranium enrichment in the fuel rods varies as shown in Table 1.
1 is placed at the position shown in Figure 2.

[Table] Yes. In the center of the fuel assembly 11, there is a fuel rod 1 with an enrichment of 2.5% by weight, and the fuel rods 2, 3, and 4 in the periphery are located at the top and bottom (11/24th of the total length from the bottom end of the fuel assembly 11). The fuel rods have different enrichment levels (divided into upper and lower parts), with the upper part being higher than the lower part. The fuel rod 6 contains Gd ₂ O ₃ . In the center is water rod 7. Figure 3 shows the average axial power distribution in the reactor core when the control rods are fully withdrawn. 12 is the output distribution of this embodiment, and it can be seen that the axial output distribution is greatly improved compared to the output distribution 13 of the conventional example. In the conventional reactor core, the enrichment of uranium in the axial direction does not change as in this embodiment. In this example, it is not necessary to flatten the power distribution by the control rods in the axial direction, but only in the radial direction, and only the control rods are deeply inserted, and a simple pattern with a constant control rod insertion depth is used. It's getting old. Figure 4 shows the control rod pattern at zero burnup. This is a horizontal cross-sectional view of the entire reactor core, with each square containing four fuel assemblies and one control rod placed in the center. The numbers listed are the control rod insertion rates, which divide the axial direction of the core into 24 equal parts, and indicate that the control rods are inserted from the bottom end of the core to the position indicated by the number. The larger the number, the deeper the control rod is inserted into the reactor core. Also, the control rods in the blank spaces are completely withdrawn. The maximum linear power density in this reactor core is kept very low at 11.3kw/ft despite the simple control rod pattern. For reference, FIG. 5 shows a conventional control rod pattern for a reactor core composed of fuel assemblies in one region in the axial direction (uranium enrichment is constant in the axial direction). The maximum linear power density at this time is 11.9kw/ft. The maximum power density of the example core is 0.6kw/ft lower than that of the conventional core, and as shown in Figure 5, the control rods for power distribution adjustment are arranged in a concentric control rod pattern inside the core. The control rods are inserted into the same depth, and the control rods are inserted deeply at the same depth. The most effective method is to divide the core into an upper and lower part between 1/3 and 7/12 of its height from the lower end of the core, and make the infinite multiplication factor of the upper part larger than that of the lower part. It is possible to achieve flattening of the output. In particular, the fuel assembly used in this example has fuel rods 1 with uniform enrichment in the axial direction arranged at the center of a plane perpendicular to the axis of the fuel assembly, and the enrichment in the upper region in the axial direction is a fuel rod 2 larger than that in the lower region,
3 and 4 are arranged at the periphery of the upper plane, and the enrichment in the upper and lower regions of the latter fuel rod is smaller than that of the former fuel rod, so that it is composed of fewer types of fuel material. A simple fuel assembly can flatten the axial power distribution of the core. In such a core structure, there is no need to shuffle the fuel assembly (move the fuel assembly within the core) when replacing the fuel assembly, as in the conventional case. Therefore, the time required to replace the fuel assembly is significantly reduced. In addition, the operation of the control rods is simplified, making it easier to control the operation of the reactor. The embodiment described above has a core structure in which the fuel assemblies 11 shown in FIG. 2 are arranged throughout the core.
Even if the fuel assembly 11 shown in FIG. 2 is arranged in a part of the reactor core and the conventional fuel assembly whose uranium enrichment is constant in the axial direction is arranged in the remaining part of the reactor core, the above-described implementation will not be possible. The same effect as in the example can be obtained. That is, even in this case, the infinite multiplication factor in the upper part of the core is higher than that in the lower part. According to the present invention, it is possible to flatten the power distribution in the axial direction of the reactor core with a fuel assembly having a simple structure using a small number of types of fuel materials. The time required to replace the assembly can be significantly reduced.

[Brief explanation of the drawing]

Figure 1 is a horizontal cross section of the core of a nuclear reactor.
FIG. 2 is an explanatory diagram showing the arrangement of fuel rods in a horizontal cross section of a fuel assembly applied to the present invention, and FIG. 3 is a diagram showing the core with the control rods fully withdrawn. FIG. 4 is an explanatory diagram showing the control rod pattern of the reactor core with burnup of 0 in an embodiment of the present invention;
FIG. 5 is an explanatory diagram showing a conventional control rod pattern in a reactor core. 1, 2, 3, 4, 5, 6... fuel rod, 11...
fuel assembly.

Claims (1)

[Claims] 1 In a nuclear reactor core in which a plurality of fuel assemblies are arranged and has a plurality of control rods, coolant flows from the bottom to the top, and voids occur in the top, the fuel assemblies are A plurality of first fuel rods having a uniform directional enrichment are centrally arranged in a plane perpendicular to the axis of the fuel rod assembly, and are divided into an upper region and a lower region in the axial direction, and the first fuel rods have a uniform directional enrichment. a plurality of second fuel rods in which the enrichment in the region is greater than that in the lower region, and the enrichment in the axial direction is uniform and the enrichment is greater than that of the first fuel rod having the highest enrichment; a third smaller fuel rod is disposed in the plane at a periphery adjacent to and surrounding the center, and the upper region of the second fuel rod has the highest enrichment. A nuclear reactor configured without exceeding that of the first fuel rod, wherein the control rods for power distribution control are only control rods inserted deeply into the reactor core in a concentric control rod pattern. core.