JPS5934999B2 - Reactor decontamination methods - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decontaminating a nuclear reactor, and particularly to a method for decontaminating a nuclear reactor suitable for removing corrosion-generated particulates (crud) accumulated in a nuclear reactor. be.

Most of the crud brought into the reactor pressure vessel of a nuclear power plant, such as a boiling water nuclear power plant, adheres to the surface of the fuel rods that make up the fuel assembly loaded in the reactor core and becomes radioactive. be done.

Of this activated crud, loosely attached crud peels off and adheres to the inner surface of the piping connected to the reactor pressure vessel.

Therefore, the radiation dose rate of the piping increases.

Accumulation of crud within the reactor pressure vessel requires consideration of radiation exposure for workers and causes maintenance inspections to take longer.

An object of the present invention is to eliminate the above-described drawbacks of the prior art and to efficiently remove crud in a nuclear reactor.

The decontamination method of the present invention reduces the flow rate of the coolant in the coolant recirculation piping system to 20 -
It is characterized by repeatedly performing an operation of changing the temperature within a range of 40%.

A preferred embodiment of the present invention applied to a boiling water reactor will be described with reference to FIG.

The reactor core 2 exists within the reactor pressure vessel 1 .

A recirculation system piping 3 connected to the reactor pressure vessel 1 opens through a recirculation pump 4 to a nozzle portion of a jet pump 10 located within the reactor pressure vessel 1 .

Piping 9 communicates recirculation system piping 3 and water supply piping 12 .

Water supply pipe 12 is connected to reactor pressure vessel 1 .

A pump 5 , a regenerative heat exchanger 6 , a non-regenerative heat exchanger 7 and a filtration demineralizer 8 are installed in the piping 9 .

The piping 9 and the equipment installed in the piping 9 constitute a furnace purification system.

After the nuclear reactor is shut down, the flow rate of the coolant flowing through the reactor core 2 is repeatedly increased or decreased without opening the upper cover of the reactor pressure vessel 1.

The flow rate of the cooling phase flowing through the reactor core 2 is increased or decreased by the recirculation pump 4.
This is done by increasing or decreasing the number of revolutions.

The flow rate of the coolant flowing through the reactor core 2 can also be changed by installing a flow rate control valve in the recirculation system piping 3 and adjusting the opening degree of this flow rate control valve.

The coolant discharged from the recirculation pump 4 is injected into the jet pump 10.

At this time, the coolant present around the jet pump 10 is simultaneously sucked into the jet pump 10.

The coolant flows through the jet pump 10 into a plenum located in the lower part of the reactor core 2 and further reaches into the reactor core 2 .

As shown in FIG. 2, the flow rate of coolant discharged from the recirculation pump 4 is changed.

In other words, the lower limit of the discharge flow rate of the recirculation pump 4 is 20%.
The upper limit is limited to 40% to prevent cavitation during the process of increasing the rotation speed, and changes every 30 minutes per cycle.

Along with this, the coolant flow rate in the core 2 also increases and decreases at 30 minute intervals.

Due to repeated increases and decreases in the flow rate of coolant in the reactor core 2, crud is loosely attached to the surfaces of the structures in the reactor pressure vessel 1, including the fuel assemblies loaded in the reactor core 2. exfoliation is promoted.

Even after the nuclear reactor is shut down, the pump 5 of the reactor purification system continues to be driven.

Therefore, the coolant in the reactor pressure vessel 1, i.e.
The coolant containing the peeled cladding is cooled by the recirculating heat exchanger 6 and the non-regenerative heat exchanger 7 via the recirculation system piping 3 and the piping 9, and then guided to the filtration demineralizer 8.

The crud is removed in a filter demineralizer 8.

The purified coolant passes through the regenerative heat exchanger 6 and is returned into the reactor pressure vessel 1 through the water supply system piping 12.

FIG. 3 shows the relationship between the crud concentration in the coolant and the crud removal coefficient of the filtration demineralizer 8 in the filtration demineralizer 8 population.

The higher the concentration of crud in the coolant supplied to the filter demineralizer 8, the higher the crud removal rate.

FIG. 4 shows a comparison of crud concentration in eight filter demineralizers of this embodiment and the conventional example.

Characteristic A shows the cladding density of this embodiment, and characteristic B shows the cladding density of the conventional example.

In this embodiment in which the discharge amount of the recirculation pump 4 is varied as shown in characteristic C, the crud concentration is 2% lower than that in the conventional example.
It increased by 0 times.

This indicates that the crud accumulated in the reactor pressure vessel 1 has peeled off and the amount of crud in the reactor pressure vessel 1 has significantly decreased.

These cruds are efficiently removed by the filtration demineralizer 8.

This embodiment can be implemented at any time while the nuclear reactor is shut down.

In particular, if it is carried out after the reactor has shut down and before the replacement of fuel assemblies loaded in the reactor core begins, the clarity of the coolant in the reactor pressure vessel 1 will be high due to the removal of crud. Therefore, the fuel assembly replacement work becomes easy.

The present invention is applicable not only to boiling water reactors but also to pressurized water reactors and heavy water reactors.

Furthermore, by injecting a chemical decontamination agent into the coolant inside the reactor pressure vessel 1, crud attached to the fuel assemblies, control rods, and reactor internals inside the reactor pressure vessel 1 can be effectively removed. can be removed.

By combining the addition of such a chemical decontamination agent with the above-described increase/decrease in the coolant flow rate, the efficiency of removing crud within the reactor pressure vessel 1 is further improved.

According to the present invention, crud can be efficiently removed by varying the flow rate of coolant in the core.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of a boiling water nuclear reactor to which a preferred embodiment of the present invention is applied; FIG. 2 is a characteristic diagram showing fluctuations in the discharge flow rate of a recirculation pump in an embodiment of the present invention; The figure is a characteristic diagram showing the relationship between the crud concentration in the coolant at the inlet of the filtration demineralizer and the crud removal coefficient of the filtration demineralizer, and Figure 4 shows the degree of crud removal in the reactor pressure vessel of the present invention and the conventional example. FIG. 1... Reactor pressure vessel, 2... Reactor core, 3... Recirculation system piping, 4... Recirculation pump, 8... ...Filtration demineralizer, 9...Piping.

Claims (1)

[Claims] 1. A nuclear reactor is characterized in that after the rated operation of the reactor is stopped, the flow rate of the coolant in the coolant recirculation piping system is repeatedly varied within a range of 20-40% of the rated operating output. How to decontaminate a nuclear reactor. 2. The method of decontaminating a nuclear reactor according to claim 1, wherein the operation of changing the flow rate of the coolant is an operation of switching the coolant circulation capacity of the pump. 3. The method for decontaminating a nuclear reactor according to claim 1, wherein the operation of changing the flow rate of the coolant is an operation of adjusting the opening degree of a valve provided in the recirculation piping system.