JPS6025756B2 - Damaged fuel detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for detecting damaged fuel in a nuclear reactor.

Conventionally, damaged fuel in a boiling water reactor is detected by opening the lid of the reactor pressure vessel, sucking up the reactor water in the fuel channel using an in-reactor shipping device, and performing nuclide analysis on this sample water. The presence or absence of fuel damage is being determined.

However, with this method, it takes time to wait for fission products (FP gas) to diffuse into the reactor water from the damaged cladding, and detection is affected by the radioactivity level of the reactor water itself. There was a problem with accuracy. The present invention has been made based on the above circumstances, and an object thereof is to provide a method for detecting damaged fuel that has high detection accuracy and can shorten detection time.

The present invention will be described below based on an illustrated embodiment.

In the figure, numeral 1 indicates a fuel assembly installed in a boiling water reactor. 2 is a fuel channel of this fuel assembly 1, and this channel 2 is closed at its upper and lower ends.

3 indicates the fuel east, which is a bundle of fuel rods, and A indicates the reactor water.

Further, 4 is a damaged fuel detection device according to the present invention, and this detection device 4 will be explained below.

That is, 5 is a chamber that airtightly closes the upper opening □ of the fuel channel 2, and this chamber 5 is attached to and detached from the channel 2 by a drive mechanism (not shown). A gas storage tank 6 is connected to the chamber 5 via a pumping/exhausting mechanism 10, which will be described later. A gas supply source 8 is connected to this tank 6 via a gas supply pipe 7 having an on-off valve 14 . This gas supply source 8 is filled with a highly inert gas such as helium gas at high pressure. And the tank 6 and chamber 5
A gas supply/exhaust mechanism 10 is provided between the two. This air supply/exhaust mechanism 1 has an air supply pipe 1 connected to a tank 6.
and an exhaust pipe 12, and a supply/exhaust pipe 13 connecting these pipes 11, 12 to the chamber 5, and each pipe 11, 12, 13 is provided with an on-off valve 15, 16, 1, respectively.
7 and 18 are provided. Further, a compressor 20 is provided in the middle of the exhaust pipe 12. Furthermore, a gas extraction pipe 21 branched from the exhaust pipe 12 is connected to the tank 6 .

A radioactivity detector 22 is installed on the discharge side of this gas extraction pipe 21.
is provided. 19 is an on-off valve.

Next, a method for detecting damaged fuel using the above embodiment device will be explained.

After the reactor operation is shut down, the chamber 5 is placed over the upper end of the fuel channel 2 with all of the on-off valves 14 to 19 closed. On the other hand, the on-off valve 14 of the gas supply pipe 7 is opened, and the tank 6 is filled with helium gas from the gas supply source 8 at a predetermined pressure. Note that this filling pressure P is the capacity of the tank 6 as V,
, the capacity in channel 2 is V2, and the water pressure at the bottom of the channel is P
If c, P=pcN, 10V2) 10ba
shall be.

V, and enters the gas supply process. In this step, the helium gas in the tank 6 is fed into the channel 2 by closing the on-off valve 14 and opening the on-off valves 15 and 16 of the air supply pipe 11 and the air supply/exhaust pipe 13.
Then, the reactor water A in the channel 2 is pushed down by the gas pressure, and the water level decreases, and the fed gas replaces the reactor water in the channel 2. Then, as shown in FIG. 2, when the water level drops to near the bottom of the channel 2, the on-off valves 15 and 16 are closed to stop the gas supply. Then leave it for a certain period of time. At this time, channel 2
If there is any damaged fuel inside, the FP gas leaking from the break will be released into the gas space within the channel 2. After that, the process moves to the gas exhaust process, and in this process, the on-off valves 16 and 17 are
, 18 and operate the compressor 20,
The gas in channel 2 is again compressed and recovered in tank 6. As a result, the liquid level of the reactor water in the channel 2 rises again, and as the liquid level rises, the release of FP gas from the damaged fuel is promoted. Then, as shown in FIG. 3, once the channel 2 is filled with reactor water again, the on-off valves 16, 17, and 18 are closed. Then, the above operations, that is, the gas supply process and the exhaust process are repeated multiple times.

As a result, the FP gas concentration in the gas in the tank 6 gradually increases, and becomes a concentration that is less likely to cause measurement errors. Therefore, after repeating the above steps, the on-off valves 18 and 19 are opened, and the gas in the tank 6 is sent to the radioactivity detector 22 for measurement, thereby detecting the presence or absence of fuel damage. However, according to the method of the above embodiment, by repeatedly transporting the same gas between the tank 6 and the channel 2, the FP gas concentration in the tank 6 can be gradually increased. Since measurement can be performed using the radioactivity detector 22, measurement accuracy is improved.

Moreover, since the FP gas is released into the gas space in channel 2, in addition to the rapid release of FP gas from the damaged fuel, the pressure when the reactor water level rises and falls in channel 2 increases. Since the release of FP gas is further promoted by the fluctuation, the time to wait for gas release is shortened, and the regular inspection period is shortened. Furthermore, according to the device of this embodiment, the gas is recovered into the tank 6 by being compressed into the tank 6 by the compressor 20. It is possible to fill the gas with high compression.

Therefore, the capacity of the tank 6 can be reduced. In this case, there is an advantage that the FP gas concentration can be further increased because the amount of gas remaining in the tank 6 is reduced when the gas is supplied to the channel 2. Although this embodiment has been constructed as described above, in carrying out the present invention, specific aspects of the chamber, the tank, the piping structure connecting these, the radioactivity detector, etc. may be changed unless it goes against the gist of the invention. Of course, various configurations and implementations are possible.

Further, the gas used is not limited to helium gas, but various gases can be used. As explained above, according to the method of the present invention, by repeating the steps of supplying gas from a tank containing gas into a fuel channel to replace water and collecting the gas in the channel again into the tank, , the FP gas released from the damaged fuel can be concentrated and collected in the tank.

Therefore, by measuring the radioactivity of the gas concentrated in this way, fuel can be detected without being affected by, for example, the radioactivity level of the reactor water itself, and detection accuracy can be improved. Moreover, in addition to being released into the gas space in the channel, FP gas release is promoted by the vertical movement of the reactor water level, so the time required for FP gas release is short. It becomes possible to shorten the time. Therefore, the reliability of the reactor is improved,
In addition, by shortening the regular inspection period, the reactor operating rate can be improved.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention, and FIG. 1 is a structural explanatory diagram diagrammatically showing a fuel assembly and a damaged fuel detection device, and FIG. 2 is a schematic vertical cross-section of the fuel assembly in a gas supply state. The top view and FIG. 3 are schematic longitudinal cross-sectional views of the same fuel assembly in a gas exhaust state. 2... Fuel channel, 5... Chamber, 6... Tank, 20... Compressor, 22...
・Radioactivity detector, A... Reactor water. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. After the upper opening of the fuel channel in the reactor is airtightly closed, gas is fed into the channel from a tank containing gas to replace the water in the channel, and after a predetermined period of time, the gas in the channel is removed. A method for detecting damaged fuel, comprising repeating the step of recovering the gas into the tank, and then extracting gas from the tank and measuring radioactivity.