JPS6243154B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for detecting fuel damage in a nuclear reactor, and in particular, detects krypton-85, which is a fission gas in off-gas, to detect the time of occurrence of fuel damage and reactor operating parameters. The present invention relates to a method and device for detecting fuel damage that allows the user to know the relationship between

Conventionally, methods for detecting fuel damage in nuclear reactors have been based on water-soluble nuclides (e.g. iodine-131, iodine-132, cesium-
134, cesium-137) in reactor water, and gas component nuclides generated as a result of uranium fission (e.g. xenon-133, xenon-137).
There is a method to measure the concentration of 135, xenon-135m, xenon-138, krypton-85, krypton-87, krypton-88) in the reactor off-gas.

However, according to the former method, the radioactivity concentration in the reactor water increases rapidly due to fuel damage, so although it is easy to detect fuel damage the first time, even after the damaged fuel is removed from the reactor, the radioactivity concentration in the reactor water increases rapidly. The radioactivity concentration in the fuel does not decrease, so even if a new fuel failure occurs, it is difficult to detect it. Furthermore, it is impossible to know when a fuel failure has occurred.

On the other hand, the latter method is generally carried out using the apparatus shown in FIG. In the figure, main steam generated in a nuclear reactor 1 rotates a steam turbine 2, and the turbine 2 drives a generator. The steam is condensed in the main condenser 3 and sent to the reactor water supply system shown by the broken line. During this time, the fission product gas generated within the nuclear reactor 1 moves through the turbine 2 to the condenser 3 together with the main steam. Then, it is extracted from the condenser 3 by an air extractor 4 using main steam as a working medium.
The oxyhydrogen recombiner 5 combines hydrogen and oxygen in the gas. Thereafter, water vapor in the gas is condensed by the exhaust gas condenser 6 and separated from the gas. After such treatment, the gas is attenuated by an attenuator, such as a rare gas hold-up device, and then released to the atmosphere. During this time,
A portion of the gas is sampled by a sampling device 9 and sent to a monitoring device, where fuel damage is detected by component analysis.

However, even with this second method, once a fuel failure occurs, fission product gases continue to be released from the damaged fuel into the reactor system, making it difficult to detect whether a new fuel failure has occurred. Ta.

The present invention has been made in view of the above-mentioned shortcomings of the prior art, and it is an object of the present invention to provide a method and device for detecting fuel damage that can detect the occurrence and time of occurrence of fuel damage, and also trace the cause of fuel damage. purpose.

The present invention continuously measures krypton-85, which is one of the gases produced by nuclear fission, and detects the occurrence and time of fuel failure based on the peak that occurs when fuel failure occurs, and furthermore detects the occurrence and time of fuel failure based on the correlation with the operating status of the nuclear reactor. This allows the cause of fuel damage to be confirmed.

Hereinafter, the present invention will be explained in detail with reference to Examples.

An embodiment of the invention is shown in FIG. In the figure,
As in FIG. 1, off-gas discharged from the reactor system is treated in the same manner as in FIG. Detection of fuel failure is carried out by sampling a portion of the gas by means of a sampling device. The collected gas is led to a dryer 11 and dried, then the pressure is increased by a blower 18, and sent to an activated carbon column 12, held up for a predetermined time, and then led to a β-ray detector 13.
Measure the beta-ray dose.

Here, the gas component nuclides generated as a result of uranium fission and their half-lives are as follows.

Nuclides Xenon-133 5.27 days Xenon-135 9.14 hours Xenon-135m 15.6 minutes It is significantly longer than the others, so the amount that accumulates in the fuel plenum as combustion progresses is roughly proportional to the burn-up of the fuel. On the other hand, other nuclides have short half-lives, so the amount accumulated in the plenum depends on the period of operation at a certain output and the time since the operating condition changed.

When a fuel failure occurs, the fission product gases accumulated in the plenum leak from the fuel into the reactor system. Then, it is extracted by the air extractor 4 and guided to the β-ray detector through the above-mentioned off-gas detection system. Activated carbon column 12 is operated at approximately 10°C. At this time, the hold-up time is approximately 50 hours for krypton and approximately 60 days for xenon. Therefore, after leaving the activated carbon column, most of the nuclides other than krypton-85 are attenuated, and the detector 13 can effectively measure the β-ray dose of krypton-85. Fuel damage can be detected by continuously measuring the beta-ray dose of krypton-85. In other words, the krypton-85 that had been accumulated due to fuel failure is released all at once, and the peak of the krypton-85 beta ray dose is observed. It is possible to estimate the approximate burnup of the damaged fuel from the height and area of the peak.

Furthermore, by recording the β-ray dose using a recording device and comparing this record with reactor operating parameters such as control rod operations, output changes, flow rate changes, etc., the cause of fuel damage can be analyzed. Recording device 8
is a device for recording the above-mentioned driving data, and is normally provided. When the operating data is taken out from the recording device 8 and introduced into the cracking device 16 via the time delay element 15, the analysis device takes a correlation and investigates the cause of the damage.

The above apparatus is equipped with a reference krypton extraction device 10 capable of injecting a prescribed amount of krypton-85 in order to check the operation of the apparatus. By periodically emitting krypton-85 from this device, the operation of the detector 13 can be confirmed.

[Brief explanation of the drawing]

FIG. 1 shows a conventional method for detecting fuel damage using off-gas, and FIG. 2 shows an apparatus embodying the method for detecting fuel damage according to the present invention. 1... Nuclear reactor, 2... Steam turbine, 4... Air extractor, 9... Sampling device, 12... Activated carbon column, 13... Beta ray detector, 14... Recording device, 1
6...Analysis device.

Claims (1)

[Claims] 1. A method for detecting fuel failure by measuring the radiation dose of fissile gas in off-gas of a nuclear reactor, in which a part of the off-gas is sampled, and nuclear fission in the sampled off-gas is detected. A method for detecting fuel damage, comprising attenuating the radioactivity of the product and then measuring an increase in the amount of krypton-85 in the collected off-gas. 2. A means for collecting off-gas from a nuclear reactor, and a means for holding up the collected off-gas,
A fuel damage detection device comprising means for measuring the β-ray dose of the off-gas held up for a predetermined period of time by the hold-up means, and means for determining fuel damage based on the amount of increase in the β-ray dose. .