JP2008249445A - Measuring method of radiation in sealed container - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measuring method of a radiation in a sealed container capable of measuring a radioactive material or the like with high sensitivity without lowering a concentration of gas sealed in the sealed container. <P>SOLUTION: Sealed gas is taken from the sealed container 2 by putting the sealed gas in the sealed container 2 into a depressurized environment, and a dusty radioactive material existing in the sealed gas is recovered by a filter, and a radiation concentration in the filter is measured. The sealed gas after recovering the dusty radioactive material therefrom by the filter is returned again into the sealed container 2. Gas of the same quantity as the sealed gas taken out from the sealed container 2 may be supplied into the sealed container 2 during storage or inspection of the sealed gas recovered from the sealed container 2. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radioactive substance leakage monitoring method and apparatus in a storage facility for dry storage of spent fuel generated from a nuclear power plant and vitrified material generated by reprocessing of spent fuel.

  The spent fuel assembly after being used in the nuclear power plant is stored in the fuel storage pool of the nuclear power plant. Spent fuel assemblies stored in the fuel storage pool for a predetermined period are reprocessed in a reprocessing facility to recover reusable nuclear fuel materials such as uranium and plutonium. By this reprocessing, a vitrified body of high-level radioactive waste (hereinafter referred to as a vitrified body) is generated. The vitrified body is finally buried in a disposal site provided underground. However, since the vitrified body has a high calorific value immediately after production, it is stored for several decades while being cooled in a dedicated storage facility, and disposed of after being reduced to a disposable calorific value.

  A part of the spent fuel assembly is stored in an intermediate storage facility such as a metal cask or vault for several decades before being reprocessed in the reprocessing facility.

  An example of a storage facility for vitrified bodies is described in Patent Document 1. In this example, the vitrified material storage facility has a plurality of storage tubes for storing the vitrified material in the storage pit, and the ventilation tube concentrically surrounds the storage tube, and between the storage tube and the ventilation tube. Air is taken from outside the storage facility. The vitrified body is cooled by this air. The air that has cooled the vitrified body is heated and discharged to the outside through an exhaust passage formed in the storage facility. The cooling air flows through the storage facility by natural circulation using the force by which the warmed air rises.

  Further, as described in Patent Document 2, in order to efficiently remove the heat generated from the radioactive waste, it is considered to enclose helium gas having excellent thermal conductivity together with the radioactive waste in the sealed container. It is done.

  Since radioactive waste can be stored for several decades or more by this method, it is necessary to monitor that radioactive materials, etc. in the radioactive waste are not leaking outside the sealed container.

  Methods for monitoring leakage of radioactive material are described in Patent Documents 2 to 4. In Patent Document 2, a high heat conduction gas having a specific gravity smaller than that of air is blown into the storage tube from above the storage tube, and the gas is sucked from the lower portion of the storage tube. The sucked gas passes through a filter and is discharged to the outside if there is no problem after monitoring the radioactive material. Helium gas is applied as the high heat conduction gas. Thereby, it is possible to monitor the gas in the storage tube while maintaining the helium gas having high thermal conductivity. However, in this method, it is necessary to always supply helium gas, which increases the cost.

  Patent Document 3 discloses that the gas inside the storage tube is periodically sampled and the radioactivity concentration in the sampling air is measured. However, this method releases the sampled gas to the outside air. In the case where helium is used as the gas in the sealed container, it is necessary to replenish the amount of gas released each time sampling is performed in this method, which increases the cost.

  In Patent Document 4, by providing a radiation detection sensor in the storage tube, it is possible to remotely monitor the leakage of the radioactive substance without sampling the gas in the storage tube. However, in this method, it is difficult to increase the measurement accuracy because very strong radioactive waste exists in the vicinity of the sensor that measures the radioactive substance in the gas.

Japanese Utility Model Publication No. 3-125299 Japanese Patent No. 3314531 Japanese Patent Publication No. 5-11598 JP-A-8-15497

  In the above prior art, sufficient consideration has not been given to maintaining the gas concentration inside the storage tube during sampling or improving the measurement accuracy. In addition, when targeting a long container like a storage tube, both the piping to be sampled and the piping to return the sampled gas from the radiation detector to the storage tube are installed on one end side of the storage tube. If the work and the work of returning the sampled gas to the storage pipe are performed at the same time, only the gas near the two pipes will be sampled, and the sampling efficiency of the gas near the other end will decrease, It is difficult to improve sampling accuracy.

  An object of the present invention is to provide a radiation measurement method (radiation concentration measurement method) in a sealed container that can measure radioactive substances and the like with high sensitivity without lowering the concentration of gas sealed in the sealed container (storage tube). There is to do.

  The feature of the present invention that achieves the above object is achieved by sampling the sealed gas in the sealed container (storage tube), performing radiation measurement in the sealed gas, and then returning the sampled gas to the sealed container again. . Thereby, the density | concentration of the gas sealed in the sealed container can be kept constant even during measurement of the radiation concentration.

  In the gas sampling method, the sealed container is placed in a reduced pressure environment, the gas in the sealed container is temporarily stored in another container, the radiation concentration in the gas is measured, and then the gas is returned to the sealed container again. Thereby, since all the gas in a long container like a sealed container can be sampled, radiation measurement accuracy can be improved.

  In addition to the helium sealed in the sealed container, a helium tank is provided to immediately supply the same amount of helium gas as the sampled helium gas in the sealed container immediately after the sampling of the helium gas in the reduced pressure environment is completed ( refill. Accordingly, the helium concentration in the sealed container can be kept constant even during the measurement of the radiation concentration from sampling of the helium gas, and the temporary temperature rise of the vitrified body can be minimized.

  ADVANTAGE OF THE INVENTION According to this invention, a radioactive substance etc. can be measured with high sensitivity, without reducing the density | concentration of the gas sealed by the sealed container (storage tube).

  Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. Since the basic structure of the radioactive substance storage facility is shown in, for example, FIG. 1 and FIG. 3 of Patent Document 1, only the portion directly related to the present invention will be described here.

  As shown in FIG. 1, the vitrified body 1 is sealed in a storage tube (sealed container) 2 in which helium gas is sealed. A sealing plug 3 that shields neutrons and radiation emitted from the vitrified body 1 is provided on the upper portion of the storage tube 2, and is sealed by a lid 4 that seals the storage tube 2. Cooling air 5 flows outside the storage tube 2 and removes decay heat generated from the vitrified body 1. The pipe 7 and the pipe 8 pass through the sealing plug 3 and the lid 4 to connect the inside of the storage pipe 2 and the radiation measuring device 6. The pipe 7 is for sampling the sealed gas (helium gas) in the storage pipe 2, and the pipe 8 is for supplying (returning) the helium gas after radiation measurement into the storage pipe 2.

  Next, a specific structure of the radiation measuring apparatus 6 will be described with reference to FIG. The helium gas in the storage pipe 2 is sucked by the vacuum pump 9 through the sampling pipe 7. When a pressure boundary is established by providing a pressure valve 10 between the vacuum pump 9 and the storage tube 2 and helium gas in the storage tube 2 is not sampled, the helium gas flows out of the storage tube 2 to the vacuum pump 9 side, The gas flow from the vacuum pump 9 side to the storage tube 2 is prevented from occurring.

  A storage tank 11 for temporarily storing the helium gas sucked by the vacuum pump 9 is installed on the downstream side of the vacuum pump 9. Pressure valves 12 are provided on both the upstream and downstream sides of the storage tank 11 to ensure a pressure boundary. The helium gas sucked from the storage tube 2 by the vacuum pump 9 is temporarily stored in the storage tank 11 and then sent to the radioactive substance recovery device 13 where the dusty radioactive substance is recovered. The helium gas from which the dust has been removed is stored in the helium tank 15 via the three-way valve 14. A pressure valve 12 is provided between the radioactive substance recovery device 13 and the three-way valve 14 and between the three-way valve 14 and the helium tank 15 to ensure a pressure boundary.

  All the helium in the storage tube 2 is sucked and collected by the vacuum pump 9 and the valve 10 is closed, and then the same amount of helium gas as the sucked and collected helium gas is supplied (injected) from the helium tank 15 into the storage tube 2. . Thereby, the time when there is no helium gas in the storage tube 2 is shortened as much as possible so that the temperature of the vitrified body 1 in the storage tube 2 does not rise.

  The radioactive substance recovery device 13 includes a filter, and collects dust-like radioactive substance with the filter by blowing helium gas through the filter. After the helium gas blowing in the storage tank 11 is completed, the filter in the radioactive substance recovery device 13 is recovered, and the radiation of the filter is separately measured in the measurement chamber. For this purpose, the filter has a detachable mechanism (structure).

  As regards the frequency of radiation measurement, considering that the storage of vitrified materials may last for several decades or more, and the corrosion of the storage tube will proceed gradually even if it occurs, the health monitoring is not always continuous. There is no need to implement it. Specifically, it is considered sufficient even once a day to several months.

  The radioactive substance recovery device 13 may be configured such that a radiation measuring device is installed in the vicinity of the filter and the radiation of the filter is continuously measured. In this case, it is not necessary to attach or detach the filter, and continuous measurement is possible, so that it can also be used for monitoring. Furthermore, in this case, the measurement target of the radioactive substance is not limited to dust-like radioactive substance, and measurement of gaseous or liquid radioactive substance is also possible.

  Next, a second embodiment of the present invention will be described with reference to FIG. In the first embodiment, one radiation measuring device is used to monitor one storage tube. In this embodiment, a plurality of storage tubes are monitored by one radiation measuring device. FIG. 3 shows an example in which two storage tubes 2 are monitored by one radiation measuring device 6. Specifically, the pipe 7 and the pipe 8 that connect the storage pipe 2 and the radiation measuring device 6 are shared by the two storage pipes 2, sampling helium gas from the storage pipe 2, and helium to the storage pipe 2. Implement gas supply in a lump.

  Three-way valves 16a and 16b are installed in two systems of pipes 7a and 7b connecting the radiation measuring apparatus 6 and the two storage pipes 2a and 2b. Three-way valves 16a and 16b are also installed in two systems of pipes 8a and 8b connecting the radiation measuring apparatus 6 and the two storage pipes 2a and 2b. The three-way valves 16a and 16b are used to identify the storage pipe in which the radiation leaks into the helium gas when radiation is detected by radiation measurement performed collectively on the two storage pipes 2a and 2b. Specifically, the flexible hose 17 that can be attached and detached is connected to the three-way valve 16a or 16b, and as described in the first embodiment, the radiation is measured one by one for each storage tube, and the leaked storage Identify the tube.

  INDUSTRIAL APPLICATION This invention can be utilized for the leakage monitoring of the radioactive material in the storage facility which dry-stores the spent fuel generate | occur | produced from a nuclear power plant, and the vitrified body generated by the reprocessing of a spent fuel.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic apparatus block diagram which implement | achieves the radiation measuring method of 1st Example of this invention. Detailed view of the radiation measuring apparatus of the first embodiment. The schematic apparatus block diagram which implement | achieves the radiation measuring method of 2nd Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Glass solidified body 2,2a, 2b Storage tube 3 Seal plug 4 Lid 5 Cooling air 6 Radiation measuring device 7, 7a, 7b, 8, 8a, 8b Pipe 9 Vacuum pump 10 Pressure valve 11 Storage tank 12 Pressure valve 13 Radioactive substance Recovery device 14, 16a, 16b Three-way valve 15 Helium tank 17 Hose

Claims (8)

  In a storage facility for storing a glass solidified body or spent fuel together with a sealed gas in a sealed container and cooling the outside of the sealed container, the sealed container is sealed from the sealed container by making the sealed container a reduced pressure environment by a decompression device. A method for measuring radiation in a sealed container, comprising: taking in gas, collecting dust-like radioactive material present in the taken-in sealed gas with a filter, and measuring the radiation concentration of the filter from which the radioactive material has been collected.   The radiation measurement method in the sealed container according to claim 1, wherein the sealed gas after the dust-like radioactive substance is collected by a filter is supplied to the sealed container.   2. The sealed container according to claim 1, wherein the same amount of gas as the sealed gas taken out from the sealed container is supplied to the sealed container during storage or inspection of the sealed gas recovered from the sealed container. Radiation measurement method.   2. The radiation measuring method in a sealed container according to claim 1, wherein the radiation concentration of the filter that collects the dust-like radioactive substance is measured on the spot.   In a storage facility for storing a glass solidified body or spent fuel together with a sealed gas in a sealed container and cooling the outside of the sealed container, the sealed container is sealed from the sealed container by making the sealed container a reduced pressure environment by a decompression device. A method for measuring radiation in a sealed container, which takes in a gas, collects a gaseous or liquid radioactive substance present in the taken-in sealed gas by an adsorption action of the filter, and measures the radiation concentration of the filter .   6. The sealing according to claim 1, wherein measurement of radioactive substances in the sealed gas in the plurality of sealed containers is collectively performed by taking in the sealed gas from the plurality of sealed containers. Radiation measurement method in the container.   5. The radiation measuring method in a sealed container according to claim 4, wherein the radiation concentration of the filter that collects the dust-like radioactive substance is continuously measured on the spot.   8. The radiation measuring method in a sealed container according to claim 1, wherein the sealed gas is helium.

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