JPS5897698A - Method for cleaning nuclear fuel - 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 Technical Field of the Invention The present invention relates to a method for cleaning nuclear reactor fuel that is applied when cleaning fuel for boiling water nuclear reactors and the like.

BACKGROUND ART AND PROBLEMS In nuclear reactors that use water as a coolant, such as boiling water reactors or pressurized water reactors, metal impurities contained in the coolant may leak into fuel assemblies (hereinafter referred to as It adheres to materials such as fuel (simply referred to as fuel) and becomes a metal corrosion product called cladding, which becomes a strong source of radiation pollution. Such adhered contaminants not only cause blockage of the fuel coolant passages, but also sometimes separate from the fuel and are carried throughout the reactor by the flow of coolant, where they accumulate in various parts of the reactor. This can cause an open turtle, such as becoming a source of radiation. Furthermore, if the above-mentioned radioactive contaminants are attached to the fuel, radiation control may become difficult during the fuel reprocessing process. Therefore, it is important to remove crud and the like adhering to the fuel.

Figure 1 shows the fuel used in boiling water reactors. This fuel a is obtained by covering the outside of a fuel bundle C made up of a large number of fuel rods b with a square cylindrical channel box d, and the coolant is supplied from an inlet nozzle e at the lower end of the channel box. d, passes between the fuel rods b... and exits from the outlet f at the upper end. Therefore, the crud mainly adheres to the surfaces of the fuel rods b...

Conventionally, this crud has been removed by removing the channel box d from the fuel bundle C and then, for example, spraying a water jet in a fuel pool or rubbing it with a brush or the like. However, with these methods, not only can only the outer periphery of the fuel bundle C be cleaned, but also the channel box d must be removed from the fuel bundle C.
Therefore, there was a problem that extra effort was required and the working time was increased accordingly.Objective of the Invention The present invention was made based on the above circumstances, and its purpose is to treat not only the surface part of the fuel but also the surface part of the fuel. To provide a method for cleaning nuclear fuel that is easy to work with and can evenly clean the entire fuel to remove adhering contaminants, and can also clean the inside of a channel box or the like without having to disassemble the cover itself. be.

Summary of the Invention In other words, the present invention involves storing reactor fuel in a container, filling the container with water and sealing the container, and then reducing the pressure in the container using a pressure gauge that causes the water in the container to boil. However, this is a cleaning method in which the reactor fuel is cleaned using bubbles generated by this reduced pressure boiling.

In other words, since vacuum boiling occurs everywhere in the water in the container, the impact force due to the generation and growth of bubbles can be applied not only to the surface of the fuel but also to every corner of the entire fuel, and these bubbles cause the water to adhere to the fuel. Adhering contaminants such as crud can be removed.

Embodiment of the Invention An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 shows an apparatus for carrying out the method of the present invention, and in the figure, 1 is a cleaning container containing fuel a. This container 1 is configured such that its upper end opening can be sealed with a lid 2 that can be opened and closed. 3 is a tightening tool for fixing the lid 2. Further, a discharge port 4 is provided at the upper end of the container 1, and a filter r is connected to the discharge port 4 via a pressure reducing valve 5 and a water pipe 6.

As mentioned above, a flow port 8 is provided at the bottom of the container 1. A pressurizer @11 is connected to this flow port 8 via an air valve y and a flow pipe 11.

This pressurizer 12 is a closed container, communicates with the container 1, and is configured to be able to store water therein, as well as to be able to heat the water inside with an electric heater IJ. .

Note that the above-mentioned flow port 8 is aligned with the inlet nozzle e of the fuel a,
It also has the function of supporting the fuel phoenix.

The upper end of the fuel 1 is supported by a support 14.

Next, a method of cleaning fuel a using the above-mentioned embodiment apparatus will be explained. First, open the lid 2 of the container 1 and fill it with fuel a.
is stored in container 1. Next, the lid 2 is closed and the fastener 3 is used to seal the container 1 and the pressurizer 12.
Fill the flow pipe 11 with water. Then close the pressure reducing valve 5,
The heater 13 of the pressurizer 12 is energized to heat the water in the container 1. Since the water confined within the container 1 undergoes thermal expansion, the pressure inside the container 1 is high and the boiling point of water exceeds 100C. That is, as shown in Figure 3, at 1.5 atmospheres, it is approximately 110.8c.

119.6tZ" at 2 atm, 132.9C at 3 atm
It will be nearby.

Next, the pressure inside the container 1 is released by closing the air valve 10 and opening the pressure reducing valve 5.

At this time, the high-temperature water in the container 1 violently boils under reduced pressure, so that bubbles are generated from all parts of the water. The crud separated from the surface of the fuel rod b due to the impact force caused by the generation and growth of bubbles is discharged into the water, carried by the water flow generated by boiling, and sent from the discharge port 4 to the water pipe 6, where it is sent to the filter 1. is collected by.

In other words, according to the above method, it is possible to apply an impact force due to the generation and growth of bubbles not only to the surface portion of the fuel bundle C but also to the fuel rods b arranged inside, and the entire fuel bundle C is Can be washed thoroughly.

Since the channel box d can be cleaned while attached to the fuel bundle C, the unnecessary effort of removing the channel box d is eliminated, and the work is simplified accordingly.

In addition, in order to adjust the force of boiling, the speed of pressure drop within the container 1 may be increased or decreased by adjusting the opening degree of the pressure reducing valve 5. Alternatively, the intensity of boiling may be adjusted by changing the height of the pressure before boiling under reduced pressure and adjusting the degree of pressure drop during boiling under reduced pressure. Also,
As a means for increasing the pressure in the container 1, for example, a pressure generating device such as a pressure pump may be used.

In the above example, yong! The water in Il was once brought to high temperature and pressure, and then reduced in pressure and boiled. However, in the present invention, for example, a negative pressure generating mechanism such as a vacuum pump is connected to the container 1,
The negative pressure may be used to reduce the pressure inside the container 1 and bring it to a boil.

In other words, as shown in Figure 3, water has a pressure of 7 atm at 0.4 atm.
It boils at around 5.4C, and at 0.2 atm it boils around 60C, so if the design temperature is around 66C, such as fuel pool water in a boiling water reactor, pool water in container 1 boils at around 60C. It is also possible to boil the fuel directly under reduced pressure in the fuel pool by filling the container with air, sealing it, and reducing the pressure inside the container to, for example, 0.2 to 0.3 atmospheres. In this way, a heating means such as a heater is not required, the structure is simplified, and the cladding can be removed without increasing the size of the equipment.

Effects of the Invention As described above, the present invention removes adhering contaminants such as crud adhering to the fuel by boiling it under reduced pressure in a closed container and using the cooling caused by the generation and growth of bubbles. It is something. Therefore, not only the surface portion of the fuel but also every corner of the entire fuel can be thoroughly cleaned, resulting in a high decontamination effect. What's more, the interior of the channel box can be cleaned without removing the cover itself, which simplifies the work and reduces work time.It also has the advantage that cleaning can be done without using chemicals, so there is no risk of damaging the fuel cladding. It's thick.

[Brief explanation of the drawing]

Figure 1 is a partially cutaway perspective view of the fuel assembly;
The figure is a longitudinal sectional view showing an apparatus for carrying out the method of the present invention,
FIG. 3 is a characteristic diagram showing the relationship between water pressure and boiling temperature. 1... Container, 12... Pressurizer, a... Reactor fuel. Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 2

Claims (3)

[Claims] (1) Reactor fuel is stored in a container, and this container is filled with water and sealed, and then the pressure inside the container is reduced to a pressure level that causes the water in this container to boil, and this reduced pressure boiling is performed. A method for cleaning nuclear reactor fuel, characterized in that the reactor fuel is cleaned using bubbles generated by the above. (2) The method for cleaning nuclear reactor fuel according to claim (1), characterized in that the water in the container is once brought to a high pressure and then the pressure in the container is released to boil it under reduced pressure. (3) The method for cleaning nuclear reactor fuel as set forth in claim (1), characterized in that water in the container is boiled under reduced pressure by reducing the pressure in the container using a negative pressure generation mechanism.