Reliability and Traceability in Radiation Calibration

Transcription

1 Reliability and Traceability in Calibration Sadao Nakashima Teruo Ebato 1. Introduction Fuji Electric has a successful history of delivering radiation measuring equipment used in the controlled areas of facilities that handle radioactive matter and nuclear fuel materials. measuring equipment is required to have high measuring accuracy, and proper calibration, using a calibrator that provides traceability to national standards, is essential in order to ensure that accuracy. In order to calibrate radiation measuring equipment for various uses, Fuji Electric maintains and operates calibrator having different nuclides and irradiation strengths. 2. Traceability For radiation measuring equipment, establishing traceability linking the national standards to products, and maintaining the calibration accuracy are extremely important. Fuji Electric s calibration equipment for radiation measurement devices is based on Japanese domestic laws and standards, and is traceable to the national standards shown in Fig Calibrator The calibrator is measured at a location an arbitrary distance from the radiation supply, with a reference measuring device traceable to national standards, and is assigned a value. The device to be calibrated is placed in the same place with the calibrator, and the calibration is performed by comparing the measured values obtained with the values measured by the reference measuring device. The calibration of the device to be calibrated is performed using a radiation of the same nuclide, also by changing the irradiated dose equivalent rate. oreover, the energy calibration is performed using a radiation of different nuclide to irradiate at the same dose equivalent rate. Fig.1 Traceability system chart National Institute of Advanced Industrial Science and Technology National measurement standards Institute of easures (IR) Secondary measurement standards Neutron reference nf : 252 Cf nth : 241 Am-Be Exposure reference instrument Practical standards (Dosemeter) Practical standards (Dosemeter) (Rem counter) Evaluation Substitution Substitution Fuji s Tokyo Factory Gamma 137 Cs 60 Co Calibrator for gamma [High-level dose] [iddle-level dose] Beta 90 Sr- 90 Y Calibrator for beta [Low-level dose] Neutron 252 Cf Calibrator for neutron [Neutron] easuring devices for general use (Produced by manufactures) 118 Vol. 53 No. 4 FUJI ELECTRIC REVIEW

2 3.1 Gamma-ray calibrator (1) Gamma-ray calibrator (high dose) This calibrator is used for calibrating survey meters, area s, ionization chambers and the like. 137 Cs, 60 Co and 226 Ra radiation s are used for the calibration and for energy calibration. This calibrator is configured from an irradiator that houses the radiation, a calibration truck in which the device to be calibrated is placed, and a controller that controls the operation of these apparatuses. When an irradiated dose equivalent rate is set by the controller, the type Fig.2 Photograph of gamma-ray calibrator (high range) of radiation and distance are computed, the calibration truck moves automatically to a predetermined location, the radiation is selected, irradiation by the radiation is implemented, and so on. Figure 2 shows the external appearance, Fig. 3 shows the system configuration and Fig. 4 shows the calibration range of this gamma-ray calibrator. (2) Gamma-ray calibrator (medium dose) This calibrator uses a 137 Cs radiation to calibrate personal dosemeters and environmental dosemeters. The calibrator is configured from an irradiator and a controller. This calibrator varies the strength of the radiation and the time duration of the irradiation to change the irradiated dose and to perform the calibration. The devices to be calibrated are arranged in a circle around the radiation, and 50 devices can be calibrated simultaneously. Figure 5 Fig.4 Calibration range of gamma-ray calibrator (high level) 137Cs 4.44TBq 137Cs 370GBq 137Cs 37GBq 137Cs 3.7GBq 137Cs 370Bq 137Cs 37Bq 60Co 37GBq 60Co 370Bq 226Ra 1.85GBq Note : 226Ra corresponds to Gy/h only Fig.3 System configuration of gamma-ray calibrator (high level) obile truck camera for measurement value readout camera for room ing Device to be calibrated Collimator Laser marker Calibration table housing unit Turntable Thermometer Hygrometer Barometer Calibration truck Irradiation rod camera for positional readout Scale drive mechanism Rail Gamma-ray irradiator Sequencer indicator Reliability and Traceability in Calibration 119

3 shows the external appearance, Fig. 6 shows the system configuration and Fig. 7 shows the calibration range of this gamma-ray calibrator. 3.2 Neutron calibrator This calibrator uses a 252 Cf radiation to calibrate neutron area s, rem counters, personal dosemeters and the like. This calibrator is configured Fig.5 Photograph of gamma-ray calibrator (middle level) from an irradiator and an apparatus (automatic conveyor) in which the device to be calibrated is placed, and calibrates with the dose equivalent of fast neutrons and thermal neutrons. Personal dosemeters are conveyed automatically by an automatic conveyor to the calibration location, and 24 personal dosemeters may be calibrated consecutively. Figure 8 shows the external appearance, Fig. 9 shows the system configuration and Fig. 10 shows the calibration range of this neutron calibrator. The apparatus shown in the front in Fig. 8 is the automatic conveyor and the rear apparatus is the irradiator that houses the radiation. Fig.8 Photograph of neutron calibrator Fig.6 System configuration of gamma-ray calibrator (middle level) camera for room ing ax. 50 dosemeters housing unit Turntable drive mechanism Irradiation rod Fig.9 System configuration of neutron calibrator (middle level) Gamma-ray irradiator camera for room ing table ax. 24 dosemeters housing unit Fig.7 Calibration range of gamma-ray calibrator (middle level) 137Cs 370GBq 137Cs 37GBq 137Cs 3.7GBq 137Cs 370Bq Automatic conveyor Irradiation rod drive mechanism Neutron irradiator 120 Vol. 53 No. 4 FUJI ELECTRIC REVIEW

4 Fig.10 Calibration range of neutron calibrator (middle level) Fig.12 System configuration of beta-ray calibrator (middle level) 252Cf 200Bq : nf 252Cf 200Bq : nth housing unit Shutter part Inlet part Fig.11 Photograph of beta-ray calibrator Dosemeter Beta-ray calibrator Computer Fig.13 Calibration range of beta-ray calibrator 0.01 µ 0.1 µ 1 µ 10 µ 100 µ 1 msv/ 10 msv/ 100 msv/ 1000 msv/ 90Sr 3.3 Beta-ray calibrator This calibrator uses a 90 Sr radiation and this calibrator is dedicated for use with personal dosemeters (beta-rays). This calibrator is configured from a radiation housing unit, a part on which to mount the device to be calibrated, an operation part and a computer. The radiation is housed in the radiation housing unit and the dosemeter is inserted through an inlet to the mounting part. Then, by setting the irradiation time duration from the operation part or from the computer and starting the calibration, the shutter will open and the calibration begins. After the preset irradiation time duration has elapsed, the shutter closes automatically and the calibration is completed, and personal dosemeter measurements are verified with the computer. Figure 11 shows the external appearance, Fig. 12 shows the system configuration and Fig. 13 shows the calibration range of this beta-ray calibrator. 4. Postscript This paper has introduced Fuji Electric s calibrators. onitoring by radiation measurement devices is of critical importance for ensuring the radiation safety in nuclear power plant facilities and surrounding areas, and the calibration of these devices ensures the reliability of the abovementioned ing and is an indispensable part of maintenance. To complement efforts to design, manufacture and sell radiation measurement devices, Fuji Electric intends to continue to maintain and improve the reliability of calibration equipment that is traceable to national standards. Reliability and Traceability in Calibration 121

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