JPS59220686A - Reactor operation method and nuclear fuel assembly - 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 [Field of Application of the Invention] The present invention relates to a method of operating a nuclear reactor, particularly a light water-cooled nuclear reactor, and a fuel assembly for a nuclear reactor used in this operation.

[Background of the invention]

Figure 1 is an explanatory diagram showing the overall configuration of a conventional fuel assembly for boiling water reactors, and Figure 2 is a cutaway sectional view of the main parts of the lower part, where 1 is a fuel rod, 2 is a water rod, and 3 1 indicates the lower tie plate, 4 indicates the fuel rod 1 end plug, 5 indicates the lower end plug of the water rod, 6 indicates the cooling water inlet of the water rod 2, and 7 indicates the cooling water outlet of the water rod 2. and water rods 2 are supported on the lower tie plate 3 through fuel rod lower end plugs 4 and water rod lower end plugs 5, respectively.

The cooling water inlet 6 and the cooling water outflow lower of the water rod 2 of this fuel assembly are constructed of holes of a size such that the cooling water can flow in to an extent that no steam voids are generated within the water rod 2.

Such boiling water reactors have void distribution in the axial direction, and as operation progresses, the neutrons in the lower core become more thermalized than in the upper core, causing the power peak position to shift to the lower core and become distorted. come.

However, when looking at the cross section of the core, the output peak is located around the fuel due to the moderator in the bypass section, so from the perspective of ensuring fuel integrity and improving plant utilization, it is necessary to reduce the output peak as much as possible and increase the linear power density. Designs have been taken to keep it low.

In contrast, as a result of recent fuel technology development, PCI (
It has become possible to use barrier fuels that have measures taken against (fuel-cladding effect), so there is no longer a need to flatten the power distribution, and linear power density is dependent on the health of the fuel. Since it is possible to raise the temperature within a range that can maintain the PC'I level, new core designs are being considered for cores in which PC'I countermeasures have been taken.

One of these is the spectral shift operation method, which weakens the neutron moderation function by increasing the proportion of steam voids in the core or decreasing the proportion of cooling water, and reduces the proportion of high-energy neutron flux. The goal is to harden the energy spectrum of neutrons, increase plutonium accumulation, and improve fuel economy.

Furthermore, in a pressurized water reactor, a water removal control rod that does not contain a high neutron absorption material and is used to remove cooling water is used as a control rod. These water removal control rods are inserted into the reactor core during the early stages of combustion to reduce the water-to-uranium ratio and increase the amount of plutonium produced through the spectral shift effect, and are withdrawn from the reactor core in the latter half of combustion to increase the water-to-uranium ratio. let me,
Operating methods that increase reactivity are being considered.

[Purpose of the invention]

An object of the present invention is to provide a method for operating a nuclear reactor that increases the neutron spectrum shift effect in a light water-cooled nuclear reactor and thereby improves fuel economy.

[Summary of the invention]

The present invention provides a method of operating a nuclear reactor that increases the proportion of steam voids in the reactor core and utilizes a neutron spectrum shift.
In the early stage of fuel combustion, the steam voids in the water rods in the fuel assembly are filled to increase the accumulation of plutonium through neutron spectrum hardening, and in the late stage of combustion, the water rods are filled with cooling water to increase the reactivity. In a nuclear reactor fuel assembly including at least one water rod through which cooling water can pass, the water rod adjusts the inflow amount of the cooling water to generate water inside the fuel assembly. The second feature is that it has a flow rate adjustment section that can adjust the amount of steam voids.

For example, in a boiling water reactor, voids are generated during reactor operation, and fuel assemblies operated with high void ratios have a harder neutron spectrum than fuel assemblies operated with low void ratios, so plutonium Accumulation increases. This effect increases proportionally with the duration of combustion at high void fractions.

The present invention focuses on this point, and by controlling the steam void ratio in the water rod, which is one of the constituent materials of the fuel assembly, the void ratio is increased in the early stage of fuel combustion, thereby accumulating si-plutonium. In the late stage of combustion, the void fraction becomes zero, the neutron moderator increases, and together with the effect of plutonium accumulation, the reactivity increases, providing a fuel assembly that can increase the fuel burnup, and achieve the desired goal. It is possible to provide a method for operating a nuclear reactor for the purpose of

[Embodiments of the invention]

FIG. 3 shows a cutaway cross section of the main parts of the water rod used in one embodiment of the nuclear reactor fuel assembly of the present invention. In this figure, 8 is the main body of the water rod, 9 is the lower end plug, and 10 is the lower part. A cooling water flow path provided in the end plug 9, 11 is a screw with a small flow path, and 12 is a cooling water outlet. In this water rod, cooling water flows in through the small passage of the screw 11 with a small passage, passes through the water rod main body 8, and flows out from the cooling water outlet 12. The cooling water is heated while passing through the water rod main body 8 and generates steam voids.If the amount of cooling water flowing in is small, the void ratio becomes small, and as the amount of cooling water flowing in is increased, the void ratio becomes smaller. It is also possible to make the void ratio O by further increasing the cooling water flow rate. If the small flow path of the screw 11 with a small flow path is closed, the inside of the water rod main body 8 is filled with water voids, and the amount of cooling water that flows out of the voids flows in through the cooling water outlet 12.

That is, when this water rod is fitted with the screw 11 with a small passage, the flow rate of cooling water is small and the void ratio inside the water rod main body 8 becomes large, and conversely, when the screw 11 with a small passage is removed, cooling It is possible to increase the water flow rate and reduce the void ratio to O.

Figure 4 shows an example of the configuration of a reactor core loaded with reactor fuel assemblies that shoulder such water rods, and an embodiment of the method of operating a nuclear reactor according to the present invention will be explained using this diagram. do. Among the fuel assemblies that make up this core, 1 is the fuel assembly whose cycle has passed (less than) the first cycle after loading, and 2, 3, and 4 are similarly in the 2, 3, and 4th cycle (less than).
In this fuel assembly, the water ronds in fuel assemblies 1 and 2 are fitted with screws with small passages, and the water ronds in fuel assemblies 3 and 4 are fitted with screws with small passages. It has been removed.

FIG. 5 shows the infinite multiplication factor of a core configured in this way, with the number of cycles plotted on the horizontal axis and the infinite multiplication factor plotted on the vertical axis. A is an example, and B is a conventional operation method in which the water to uranium ratio is constant.On the left side of the straight line A water rod with a threaded screw removed is used. That is, the first fuel assembly has a high reactivity. Up to the second cycle, the inflow of cooling water is restricted by a screw with a small passage, steam voids are generated in the water rod body, plutonium is accumulated, and plutonium is accumulated in the water rod body.
By removing the small flow passage screw from the 1st cycle, the water rod main body is filled with cooling water to increase the reactivity, and at the same time, the reactivity can be increased due to the effect of plutonium accumulation in the 1st and 2nd cycles. It looks like this.

In the above embodiments, a screw with a small passage was used, but a screw without a small passage may be used.

Further, this water rod can be similarly used in a fuel assembly of a pressurized water reactor to obtain the same effect.

According to this example, it is possible to increase the neutron spectrum shift effect in a light water-cooled nuclear reactor,
This increases the discharge burnup of the fuel assembly,
Fuel economy can be improved.

In addition, special fillers are used to reduce neutrons □
Therefore, there is no reduction in economic efficiency due to neutron absorption of the filler, and there is no increase in waste due to the use of the filler.

Other examples of water ronds include using a blind stopper without a screw, or providing only a small hole, and drilling a hole to increase the amount of cooling water inflow in the -3rd cycle. Alternatively, the pores may be made larger.

FIG. 6(a) shows a cutaway cross section of the main part of another embodiment of the water rod. In this figure, 8 is the main body of the water rod, 13 is the lower end plug; 14 is the cooling water inlet, 15 is the outlet for cooling water or steam, and 17 is a cylinder whose appearance is shown in FIG. 6(b). A shaped partition plate 17 indicates a hole provided in the partition plate 16 for the inflow and outflow of cooling water or steam voids.

In the water rod having such a structure, the tip of the cooling water inlet passage 14 is closed during the initial stage of fuel combustion, so that the water rod is closed in response to the steam flowing out of the water rod body 8 from the cooling water or steam outlet 15. The amount of cooling water flows in from the entrance/exit 15, but since a partition plate 16 is provided in the water rod main body 8, the cooling water evaporates while falling along the inner wall of the water rod.

In the latter stage of combustion of the fuel assembly, the water rod is filled with cooling water from the cooling water inlet passage 14 by making a hole at the tip of the lower end 13.

The tip of the F section end frame may be closed with a screw.

In this embodiment, the void fraction within the water rod can be further increased, resulting in an even greater spectral shift effect.

〔Effect of the invention〕

The present invention increases the neutron spectrum shift effect in a light water-cooled nuclear reactor, thereby making it possible to provide a method of operating a nuclear reactor that improves fuel economy, and is an industrially effective dog.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the overall configuration of a conventional fuel assembly for a boiling water reactor, Fig. 2 is a cutaway sectional view of the main part at the bottom of Fig. 1, and Fig. 3 is a nuclear reactor of the present invention. FIG. 4 is an explanatory diagram of an example of the configuration of a reactor core loaded with the fuel assembly for a nuclear reactor shown in FIG. 3, and FIG. FIG. 6(a) is a characteristic diagram showing the effect of the reactor core in comparison with the effect of the core in the conventional structure, and FIG. b) is shown in Figure 6 (
It is an external view of the main part of (a). 8...Water rod main body, 9...Lower end plug, 1o...
・Cooling water flow path, 11... Screw with small flow path, 12...
Cooling water outlet. 10 Figure 2 Rod 3 Figure 4

Claims (1)

[Claims] 1. In a method of operating a nuclear reactor that increases the proportion of steam voids in the reactor core and utilizes neutron spectrum shifts, water rods in a fuel assembly are filled with steam voids during the initial stage of fuel combustion. A method for operating a nuclear reactor, characterized in that the accumulation of cyplutonium is increased by neutron spectral hardening, and the reactivity is increased by filling the water rod with cooling water in the late stage of combustion. 2. In a nuclear reactor fuel assembly including at least one water rod through which cooling water can pass, the water rod can adjust the amount of steam voids generated inside by adjusting the inflow amount of the cooling water. A fuel assembly for a nuclear reactor, characterized in that it has a flow rate adjustment section. 3. The fuel assembly for a nuclear reactor according to claim 2, wherein the water rod is a water rod that collapses the flow rate adjustment section near the fuel lower tie plate. 4. The fuel assembly for a nuclear reactor according to claim 2 or 3, wherein the flow rate adjustment section is a means for adjusting the size of a flow path provided in an end plug section at the lower end of the water rod. . 5. The water rod has a partition plate that allows the cooling water to flow from the cooling water inlet/outlet portion at the upper end along the inner wall surface of the water rod.
Nuclear reactor fuel assembly as described in Section.