JP2005164450A - Measuring method of radiation dose rate from spent fuel storage container and measuring system of radiation dose rate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce expose possibility to the workers in radiation dosimetry of spent fuel storage container. <P>SOLUTION: Radiation dose rate is measured near or at the surface of the storage container, such that a radiation sensor 1 with a detecting unit 4 formed from material changing its optical property reversibly in response to irradiation is placed near or at the surface of the storage container C and variations of optical property in the detecting unit of the radiation sensor are observed remotely by means of an imaging unit 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to radiation dose rate measurement, and more particularly to a method and system for measuring radiation dose rate in a spent fuel storage container.

  Spent fuel (spent fuel assembly) removed from the nuclear reactor is drained for a certain period in the cooling pool inside the nuclear power plant to attenuate the radiation dose and calorific value, and then sent to a treatment facility such as a fuel reprocessing plant. Transported. In recent years, spent fuel is transported to a centralized storage facility (dry storage facility) and stored overseas. Radioactive material storage containers are used to transport spent fuel from nuclear power plants to these facilities and to store spent fuel at these facilities. A typical example of the storage container is a metal cask.

In a facility for storing a metal cask, it is necessary to confirm from time to time that the shielding performance against radiation of the metal cask being stored is maintained. As a method for confirming the shielding performance, it is conceivable to measure the radiation dose on the metal cask surface or in the vicinity of the metal cask, more specifically, the radiation dose rate. As a dose rate measurement in this case, there is a method of measuring a dose rate on the surface of the metal cask or in the vicinity of the metal cask with a general radiation measuring instrument. The dose rate is measured for each of gamma rays and neutrons. Usually, a scintillation counter is used for gamma rays and a BF ₃ detector is used for neutrons. Also known is a method of laying an optical fiber on a metal cask and measuring the radiation dose through this optical fiber (Patent Document 1).

JP 2002-48898 A

  In order to confirm the shielding performance of the metal cask, there is a problem of exposure of workers engaged in the measurement of the radiation dose on the metal cask surface and in the vicinity of the metal cask. For example, in the case of a method in which the dose rate is measured on the metal cask surface or in the vicinity of the metal cask using a radiation measuring instrument, the worker will approach the metal cask, and the worker will be exposed at that time. Even when the radiation measuring device is fixedly installed on the surface of the metal cask or in the vicinity of the metal cask, the measurement is performed when the radiation measuring device is installed. Also, in the case of a method using an optical fiber laid on a metal cask, a work for laying or maintaining the optical fiber in a radiation environment is accompanied, and a worker engaged in the work is exposed to an explosion. However, such exposure of workers is not desirable, and it is desirable to eliminate them or to reduce them as much as possible.

  In addition, radiation dose measurement for confirming the shielding performance of metal casks has a problem in determining the dose rate of gamma rays and neutrons. Each gamma ray and neutron have different shielding materials. For this reason, the metal cask is provided with a shielding body for gamma rays (for example, an inner cylinder and an outer cylinder made of carbon steel) and a shielding body for neutrons (for example, a resin). Therefore, if the dose rate of gamma rays and neutrons can be determined and the shielding performance can be confirmed for each shielding body, the storage management of the metal cask can be performed more rationally.

  The present invention has been made against the background of the above-mentioned situation regarding the storage of spent fuel, and can greatly reduce the possibility of exposure of workers in radiation dose measurement from the spent fuel storage container. Providing a radiation dose rate measurement method for the first purpose, and providing a radiation dose rate measurement method capable of measuring the dose rate for each type of radiation while greatly reducing the possibility of exposure to workers. The third object is to provide a radiation dose rate measurement system used in these methods as a second object.

  For the first object, in the present invention, in a method for measuring a radiation dose rate of a spent fuel storage container, radiation having a detection part formed of a material whose optical characteristics reversibly change by irradiation of radiation. A sensor is installed on or near the surface of the storage container, and the radiation dose rate on or near the surface of the storage container is measured by remotely observing changes in optical characteristics of the detection unit with an imaging means. It is characterized by that.

  Further, in the present invention, for the second object, in the method for measuring the radiation dose rate of the spent fuel storage container, it is formed of a material whose optical characteristics are reversibly changed by irradiation of radiation, A radiation sensor having a detection unit that can selectively detect the type is installed on or near the surface of the storage container, and a change in optical characteristics of the detection unit is remotely observed by an imaging means. Thus, the radiation dose rate at or near the surface of the storage container can be measured for each type of radiation.

  Further, in the present invention, with respect to the radiation dose rate measuring method as described above, the detection unit selectively covers the detection unit with respect to the type of radiation by covering the detection unit with a plurality of types of shielding bodies having different shielding properties with respect to the type of radiation. To give sex.

  Further, in the present invention, with respect to the radiation dose rate measuring method as described above, the detection unit is provided with selective detectability with respect to the type of radiation by combining a plurality of types of materials having different responses of optical characteristic changes with respect to the type of radiation. ing.

  Further, in the present invention, for the third object, in the radiation dose rate measuring system for measuring the radiation dose rate of the spent fuel storage container, the material is formed of a material whose optical characteristics reversibly change by irradiation of radiation. A radiation sensor having a detection unit, an imaging means for remotely observing the radiation sensor installed on or near the surface of the storage container, and a surface or the vicinity of the storage container by processing data from the imaging means The data processing device for obtaining the radiation dose rate is provided.

  In the present invention, it is possible to measure the radiation dose rate at or near the surface of the storage container by remotely observing, with an imaging means, a state in which the detection unit of the radiation sensor changes its optical characteristics by radiation irradiation. I have to. For this reason, according to the present invention, the work that needs to be performed near the spent fuel storage container can be as simple as installing the radiation sensor on or near the surface of the storage container. It is possible to greatly reduce the possibility of worker exposure. In the present invention, the radiation dose rate can be measured for each type of radiation from the storage container. Therefore, according to the present invention, the shielding performance can be confirmed for each shielding body in the storage container, and the storage container can be managed more rationally.

  Hereinafter, preferred embodiments for carrying out the present invention will be described. FIG. 1 schematically shows the configuration of a system used in the radiation dose rate measurement method according to the first embodiment. This radiation dose rate measurement system includes a radiation sensor 1, a video camera 2 that is an imaging means, and a data processing device 3.

  The radiation sensor 1 has a structure in which a detection unit 4 and a shielding unit 5 for identifying a radiation type are combined in series in the radiation irradiation direction. The detection unit 4 is a plate using a material whose optical characteristics change upon irradiation, specifically, a radiochromic material whose light absorption characteristics change upon transition to an isomer (excited state) upon irradiation with radiation. Is formed. As the radiochromic material, for example, a material in which allylethenes are blended in a resin matrix in which, for example, an acrylic resin or a silicone resin is used can be preferably used. The thickness of the detection unit 4 and the blending ratio of allylethenes to the resin matrix affect the sensitivity of the light absorption characteristic change in the detection unit 4. Therefore, the thickness of the detection unit 4 and the mixing ratio of allylethenes to the resin matrix are determined in consideration of the light absorption characteristic change sensitivity and the rate at which the radiation incident on the detection unit 4 imparts energy to the radiochromic material.

  The shielding part 5 is formed by combining a plurality of types of shielding bodies having different shielding properties with respect to the types of radiation in parallel. In the example of FIG. 1, two types of shielding body 6 for shielding neutrons formed in a plate shape with a lead material and shielding body 7 for shielding gamma rays formed in a plate shape with a polyethylene resin material are combined. Accordingly, the detection unit 4 is divided into a neutron beam detection unit 8 that only reaches and detects neutron rays and a gamma ray detection unit 9 that only reaches and detects gamma rays. This makes it possible to detect the type of radiation.

  By using such a radiation dose rate measurement system, it is possible to remotely measure the radiation dose rate on the surface of the spent fuel storage container or in the vicinity thereof. Specific procedures in the measurement are as follows. First, the radiation sensor 1 is installed on or near the surface of the metal cask C that is a spent fuel storage container. If the radiation sensor 1 installed in this way emits radiation from the metal cask C, the radiation R is irradiated. Radiation from the metal cask C is mainly neutron rays and gamma rays. The neutron beam reaches the neutron beam detection unit 8 shielded by the shielding body 7, but does not reach the gamma ray detection unit 9 shielded by the shielding body 6. Although it reaches the shielded gamma ray detector 9, it does not reach the neutron beam detector 8 shielded by the shield 7. The neutron beam detection unit 8 that has received the neutron beam and the gamma ray detection unit 9 that has received the gamma ray change the optical characteristics in accordance with the dose rate (dose per unit time) of each irradiation radiation.

  The change in the optical characteristics in the detection unit 4 is captured by the video camera 3 that remotely observes the detection unit 4. Then, by processing the image data from the video camera 3 with the data processing device 3, the radiation dose rates of neutron rays and gamma rays at or near the surface of the metal cask C can be obtained. For the processing in the data processing device 3, reference data created in advance and stored in the data processing device 3 are used. That is, the dose rate is obtained by deriving the relationship between the optical characteristic change of the detection unit 4 and the dose rate captured by the image from the video camera 3 from the reference data.

  Hereinafter, creation of reference data to be stored in the data processing device 3 will be described. In the case of the present embodiment, since the detection unit 4 is formed using a radiochromic material whose light absorption characteristic changes, the change in the optical characteristic in the detection unit 4 is the color state (color and its density) due to the change in the light absorption characteristic. ) Changes. This change is caused by transition to an isomer upon irradiation with radiation as described above, and returns to the original ground state within a certain time. The speed of returning to the ground state depends on the temperature state of the radiation sensor 1. In other words, the light absorption characteristic of the detection unit 4 due to radiation irradiation correlates with the temperature atmosphere in which the radiation sensor 1 is placed. Data on the temperature atmosphere in which the radiation sensor 1 is placed can be obtained by a temperature monitoring device (not shown) that is normally installed in the metal cask C.

  Assuming the relationship as described above, the radiation sensor 1 is individually irradiated with gamma rays and neutrons at a known dose rate, and the change in light absorption characteristics in each case is measured. The relationship between the light absorption characteristic change (dose rate and color state change) is obtained by correlating with the temperature, and reference data is created from this.

  Here, a large number of metal casks are stored in the spent fuel storage facility. In such a spent fuel storage facility, a number of radiation sensors installed for each of a number of metal casks are remotely photographed. In that case, it is preferable to prevent a blind spot by installing a plurality of video cameras or by providing a moving mechanism for video cameras in the facility.

  According to the radiation dose rate measurement system or the radiation dose rate measurement method as described above, the dose rate of radiation on the surface of the metal cask or in the vicinity thereof can be determined and determined for each type of radiation. The shielding performance of the metal cask can be confirmed. For example, while the dose rate of gamma rays is reasonably attenuated considering the half-life of the radionuclide in the spent fuel stored in the metal cask, the neutron dose rate has been shown to increase due to the analysis that the dose rate of neutrons shows an increasing trend. It is possible to obtain information such as aged deterioration of the shielding resin.

  Moreover, in the radiation dose rate measurement from such a metal cask, the possibility of exposure of workers engaged in the measurement can be greatly reduced. That is, in the radiation dose rate measurement system or radiation dose rate measurement method according to the present invention, the work that needs to be performed near the metal cask is only to install the radiation sensor on or near the surface of the metal cask. The work may be a simple work such as placing the radiation sensor on an appropriate base or installing the radiation sensor on a metal cask. This means that the amount of work in the vicinity of the metal cask is greatly reduced, for example, compared to the case where installing a general radiation measuring instrument in the vicinity of the metal cask involves complicated work such as wiring of a power cable and a signal cable. This means that the possibility of exposure to workers is greatly reduced.

  Further, the radiation dose rate measurement system or the radiation dose rate measurement method according to the present invention makes it possible to obtain the spatial radiation dose distribution data for the spent fuel storage facility, and more appropriately manage the exposure of workers based on this data. It can also be used to perform.

  FIG. 2 schematically shows the configuration of a system used in the radiation dose rate measurement method according to the second embodiment. This radiation dose rate measurement system is different from the radiation dose rate measurement system of FIG. Since the other configuration is the same as that of the radiation dose rate measurement system of FIG. 1, common portions are denoted by the same reference numerals, and description thereof will be omitted as appropriate by using the above description.

  The radiation sensor 11 in the present embodiment uses two types of radiochromic materials having different sensitivities (responsiveness) to neutron rays and gamma rays so that the types of radiation, specifically, neutron rays and gamma rays can be distinguished. The detection unit 12 is formed in combination. That is, the detection unit 12 has a structure in which a neutron beam detection unit 13 formed of a radiochromic material that is highly sensitive to neutron rays and a gamma ray detection unit 14 formed of a radiochromic material that is highly sensitive to gamma rays are combined in parallel. Has been. Such a radiation sensor 11 is particularly effective for accurately obtaining a dose rate in a space surrounded by a plurality of metal casks.

  In each of the above embodiments, a radiochromic material is used as a material whose optical characteristics change upon irradiation, but instead, for example, a scintillation material that emits fluorescence upon de-excitation from an excited state by irradiation with radiation. Etc. can also be used.

  The present invention greatly reduces the possibility of exposure of workers in the measurement of radiation dose from spent fuel storage containers, and shields the storage containers by measuring the dose rate for each type of radiation from the storage containers. It is possible to check the shielding performance for each body, and to enable more rational storage management of storage containers. The present invention is highly useful as a significant contribution to further safety improvement in the field of nuclear power generation.

It is a figure which shows the structure of the radiation dose rate measurement system by 1st Embodiment. It is a figure which shows the structure of the radiation dose rate measurement system by 2nd Embodiment.

Explanation of symbols

1 Radiation sensor 2 Video camera (imaging means)
3 Data processor 4 Detector 6 Shielding body for neutron beam 7 Shielding body for gamma ray C Metal cask (storage container)
R radiation

Claims (5)

  In the method for measuring the radiation dose rate of a spent fuel storage container, a radiation sensor having a detection part formed of a material whose optical characteristics reversibly change by irradiation with radiation is installed on or near the surface of the storage container. A radiation dose rate measuring method characterized in that a radiation dose rate at or near the surface of the storage container is measured by remotely observing a change in optical characteristics in the detection unit with an imaging means. .   In the method for measuring the radiation dose rate of the spent fuel storage container, it is made of a material whose optical properties reversibly change by irradiation and can be selectively detected for the type of radiation. A radiation sensor having a detection unit is installed on or near the surface of the storage container, and a change in optical characteristics in the detection unit is remotely observed by an imaging means, whereby the radiation dose at or near the surface of the storage container. A radiation dose rate measuring method characterized in that the rate can be measured for each type of radiation.   The radiation dose rate according to claim 2, wherein the detection unit is selectively covered with a plurality of types of shielding bodies having different shielding properties with respect to the type of radiation to selectively detect the type of radiation. Measuring method.   The radiation dose rate measuring method according to claim 2, wherein the detection unit is provided with selective detection with respect to the type of radiation by combining a plurality of types of materials having different responses of optical characteristic changes with respect to the type of radiation. In a radiation dose rate measurement system for measuring a radiation dose rate of a spent fuel storage container, a radiation sensor having a detection part formed of a material whose optical characteristics reversibly change by irradiation with radiation, An imaging means for remotely observing the radiation sensor installed on or near the surface, and a data processing device for processing the data from the imaging means to determine the radiation dose rate on or near the surface of the storage container Radiation dose rate measurement system.

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