RU2147148C1 - Tightness check technique for cans of fuel elements before placing them in dry storage - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: after spent nuclear fuel has been kept in water for long time, bag holding fuel bundles is placed in container pre- filled with demineralized water, and water is drained from refueling pool and from container. In the course of draining, water samples are taken in several steps. First sample is taken as soon as water drain is started. Then container is held for a day to allow decay of radioactivity. After that samples are taken during further emptying of container upon infusion for a day. Increment of specific radioactivity of Cs-134 and Cs-137 radionuclides is measured in water samples which is not to exceed 2,2•10^-7 Ci/l. After container is fully emptied, it is decayed and then evacuated; air discharged from container is conveyed to system that has air cooler, aerodynamic filter, and beta-activity gas sensors of noble gases Kr-85 and Xe-133. If increment of specific activity of Cs-134 and Cs-137 radionuclides in the last water sample does not exceed specified value and air evacuated is free from radioactive noble gases, cans are considered tight. EFFECT: facilitated procedure.

Description

 The invention relates to nuclear energy, in particular to systems for monitoring the tightness of the claddings of fuel rods after long-term storage of spent nuclear fuel (SNF) in water in order to prevent contamination of process media or transport equipment with fission products and fuel composition coming out of unsealed fuel rods.

 The most common is a method based on the detection of cesium-134 and cesium-137 (reference fission products) emerging from fuel rods through loose shells into an isolated volume of water during self-heating of fuel - the "water" method [1].

 At the same time, fuel assemblies of fuel assemblies are transported from the reactor to special containers located in the holding pool and filled with demineralized water, where the fuel is heated by residual energy or using heaters. After that, a water sample is taken and the activity of reference fission products present in the water sample is measured.

 The “dry” method is also known, in which the activity of xenon and krypton is measured in air samples taken from a canister in which fuel assemblies are placed [2]. This method is more sensitive than water. However, in the implementation of control in both cases, the heating of the fuel assembly is required.

 The disadvantages of this method include its lack of reliability and the need to heat the fuel assemblies to the required temperature in the case of determining the tightness of the shells later than 7 months after shutting down the reactor.

 At present, the “water-gas” method for monitoring the tightness of the fuel assembly shells has found wide application, in which the radioactivity of a water sample is determined from the canister in which the fuel assembly is placed, and then the water from the canister enters the degasser, where it is degassed by spraying in air then measure the radioactivity of the gaseous medium released from the water of the pencil case [2].

 The above methods are quite effective in determining the tightness of the cladding of the fuel rods in the case of their storage for 6-7 months after shutdown of the reactor.

 The objective of the invention is to develop a reliable, reliable way to control the tightness of fuel rods after long-term storage of spent nuclear fuel (SNF) in water when it is transferred to dry storage in order to exclude the possibility of leaking fuel in the container. The absence of the release of radioisotope fission products into the container will ensure the subsequent safe storage of spent nuclear fuel in the container even if it is depressurized and, accordingly, will ensure a normal radiation situation in the storage and at the border of the sanitary protection zone of the facility.

 The method in accordance with the invention lies in the fact that in the reloading pool, a cover with bundles of fuel elements placed in it is loaded into a container pre-filled with chemically demineralized water. Next, drain the water from the overload pool and drain the container. In the drainage process, water samples are taken in several stages.

 The first water sample is taken at the beginning of the drain. After which the container is kept for a day.

Next, samples are taken during further emptying of the container after infusion during the day. In water samples, the increment of the specific activity of the radionuclides Cs-134 and Cs-137 is determined, which should not exceed the established criterion of 2.2 • 10 ^-7 Ci / l.

 After complete emptying, the container is aged and then the container is evacuated, directing the evacuated air to a system consisting of an air cooler, an aerodynamic filter, and a gas detector of β-activity of noble gases Kr-85, Xe-133.

 If the increment in the specific activity of Cs-134 and Cs-137 radionuclides in water in the last sample does not exceed the established value and there are no radioactive noble gases in the pumped-out air, then the shells are considered leakproof.

 If the increment in the total specific activity of Cs-134 and Cs-137 radionuclides in water exceeds the specified value and / or the presence of radioactive noble gases in the pumped air is detected, then the envelopes are recognized as leaky.

 Radioactive noble gases are uranium fission products that are inert in their chemical nature, and their presence in the container clearly indicates their exit from the fuel rods, i.e., the presence of defects in the cladding of the fuel rods.

 The use of Kr-85 and Xe-133 is due to the fact that these radioisotopes have a longer half-life than are commonly used in the practice of monitoring the tightness of the cladding of fuel rods in a working reactor or immediately after unloading fuel rods from the reactor, radioisotopes J-131-135.

 Cs isotopes, which are also fission products, actively leave the leaky cladding of fuel elements, and the value of the activity of water in the container in the case of a leaky cladding will be significantly higher than the activity of other radionuclides in water.

 The necessity of holding the container after complete emptying is due to the need for a “tincture” of the container with SNF, which will contribute to the release of fission products into the cavity of the container in case of leakage of the shells.

The established criterion for the tightness of the cladding of the fuel rods, equal to 2.2 • 10 ^-7 Ci / l, was obtained experimentally by determining the minimum increment in the activity of the radionuclides Cs-134 and Cs-137 due to the presence of leaking cladding of the fuel rods.

 The novelty of the method lies in the combination of new techniques and operations for determining the tightness of the claddings of fuel elements. The developed method will allow to establish with full reliability the defectiveness of the cladding of the fuel rods to be dry stored.

 EXAMPLE.

 The post-reactor cycle of RBMK 1500 spent nuclear fuel included storage of fuel assemblies in a water-filled pool for 5-10 years.

 The determination of the tightness of the cladding of the fuel rods is carried out in the following sequence.

 In the reloading pool under water, a cover containing bundles of fuel rods is loaded into a container pre-filled with demineralized water.

 The specific activity of the radionuclides Cs-134 and Cs-137 is measured in a series of background samples in the container water.

 The specific activity of the radionuclides Cs-134 and Cs-137 is measured in a series of samples during the drainage of 200 l of water from the container cavity before the daily infusion.

 Measure the specific activity of the radionuclides Cs-134 and Cs-137 in a series of samples after "infusion" during the day.

The result of the experiment showed that the increment in the amount of specific activity of radionuclides of the background sample and the sample taken during the drainage was 1.7 • 10 ^-7 Ci / l, which does not exceed the established criterion.

The increment of the sum of the specific activity of radionuclides in water samples to the “infusion” and in water samples taken after the daily infusion amounted to 1.8 • 10 ^-7 Ci / l, which does not exceed the established criterion.

 Thus, at this stage, no leaking cladding with fuel rods was found in the container.

 Measurement of Kr-85 activity in the air of the container consisted of measuring the background in the room and taking measurements in the air of the container during vacuum drying.

 The background value was 11.7 + 0.8 Bq / L and is in the measurement limit of the device of the volumetric activity radiometer RGB-07.

 The background measurement at the end of vacuum drying amounted to 13.8 Bq / L, which is slightly different from the measurements made before the start of vacuum drying.

 Thus, we can conclude that there are no leaky shells.

Literature
1.Green T., Laurent M., Examining fuel for detects "Nuclear Plant Safety, 1984, No. 5, p. 16-18.

 2. Sedov V.K. et al. Experience in the assembly and transportation of spent fuel assemblies from Novovoronezh NPP. Report at a meeting of experts from the CMEA member countries, Leningrad, September 1978.

Claims (1)

A method of controlling the tightness of the cladding of fuel rods when they are transferred to dry storage, including placing the fuel rods in a container with water and determining the activity of radionuclides in water, characterized in that the container with the fuel rods placed in it is gradually emptied, followed by at least 24 hours exposure and further complete emptying, in the process of emptying, water samples are taken from the container in several stages: during partial emptying and after holding the container in water samples, the increment of specific activity of of Cs-134 and Cs-137 ionuclides, then the container is evacuated, determining the presence of radioactive noble gases Kr - 85, Xe - 133 in the pumped air, and if the increment in the specific activity of the Cs-134 and Cs-137 radionuclides in the container water does not exceed established criterion 2,2 • 10 ^-7 and no radioactive noble gases in the air evacuated, the hermetic shell recognize if the value Cs-134 and Cs-137 specific activity of total water radionuclide last sample exceeds the criterion and / or zafiksirova of the presence of radioactive noble gases in evacuated, then recognize the envelope unsealed.