T. Kawano

National Institute for Fusion Science (NIFS-Repository)

  To develop an on-site radiation monitoring cart (monitoring cart), necessary apparatuses including a NaI(Tl) scintillation survey meter, 3He proportional neutron survey meter, alpha/beta measuring system, germanium spectrometer, HC collector, liquid scintillation counting system, dust sampler, and tritium gas monitor were selected from commercially available equipment. They were conceptually installed to construct a monitoring cart 1635 × 800 × 1500 mm.㻌 The cart was evaluated for its radiation detection limit. For monitoring radiation dose, a NaI (Tl) scintillation survey meter and neutron survey meter can be used. As shown in Table 1, the detection limit of a NaI (Tl) scintillation survey meter is 0.1 μSv/h for gamma ray radiation and that of the neutron survey meter is 0.01 μSv/h for neutron radiation. The ratios of the detection limits to the legal limits are 4.00E-03 for gamma ray radiation and 4.00E-04 for neutron radiation. These results suggest that the radiation detection limits of these survey meters on the monitoring cart are sufficiently lower than those stipulated by the law.         For monitoring radioisotope surface contamination, two methods are usually employed. These are direct measurement with survey meters (survey method) and sampling measurement by wiping with a smear paper (smear method). The survey method of the two is mainly used for checking qualitatively if contamination has occurred or has been removed. The radioactivity wiped on the smear paper is measured using the liquid scintillation counting system (3H and 14C), the alpha/beta radiation measurement system (alpha- and beta-emitting nuclides without 3H and 14C), or the germanium spectrometer (gamma-ray-emitting nuclides).     To estimate the performance, the alpha/beta measuring system was assumed to be regularly used. The detection limits are 0.05 Bq/cm2 for alpha-particle-emitting nuclides and 0.04 Bq/cm2 for other nuclides, as shown in Table 2.          Comparing these detection limits to the legal limits, the ratios are 0.013 and 0.001 for alpha-particle-emitting nuclides and other nuclides, respectively. For monitoring radioisotope concentration in the air, the tritium gas monitor is used to directly and continuously measure the tritium concentration in the air. In Table 3, the detection limit of the monitor is 1.1E-2 Bq/cm3 for 3H in the air. The ratios of the detection limits to the regulation limits are less than 2.20E-02.                  The radioactivity of the 3H and 14C in the air is also measured using the HC air sampler and the liquid scintillation counting system. Assuming that the sampling air volume is 30 L and the measuring time is 10 min, the detection limits are 5.8E-5 Bq/cm3 and 8.0E-6 Bq/cm3 for 3H and 14C, respectively. The detection limits are smaller than the legal limits, and the ratios are 2.90E-05 and 2.00E-04 for 3H and 14C, respectively. For monitoring radioisotope concentration of other nuclides in the air, two dust samplers can be used to collect the radioisotopes bound to the dust in the air. The filter papers in the dust samplers trap various radioisotopes, which are assumed to be 32P, 35S, 60Co, 131I, and 137Cs. The radioactivity of 32P and 35S can be measured with the alpha/beta radiation measurement system, and those of 60Co, 131I, and 137Cs can be measured with the germanium spectrometer. Assuming that the sampling volume of air is 900 L/30 min and the measurement time is approximately 4000 seconds, the detection limits of these measurements are estimated as shown in Table 3. The detection limits for 32P and 35S are 3.8E-6, and 2.00E-6 Bq/cm3, respectively. In addition, the ratios of the detection limits to the regulation limits for 32P and 35S are 5.43E-04 and 1.00E-04, respectively. The detection limits of these measurements with the germanium spectrometer are 5.6E-7, 1.68E-6, and 5.6 E-7 Bq/cm3 and the ratios are 5.6E-4, 1.68E-3, and 1.87 E-4 for 60Co, 131I, and 137Cs, respectively.  It is concluded that all apparatuses necessary for radiation monitoring can be compactly installed in an appropriately sized monitoring cart and that the cart has detection capability sufficient for radiation monitoring in a radiation workplace.  Radiation     Device          Detection    Detection limit/              installed         limit (Sv/h)    legal limit Gamma ray  NaI (Tl) scintillation     0.1      4.00E-03              survey meter  Neutron   Neutron survey meters   0.01      4.00E-04 Table 1 Comparison of detection and regulation limits of the radiation dose by using the cart. Nuclides    Devices          Detection   Detection limit/    installed        limit (Bq/cm3)   legal limit  3H   Tritium gas monitor 1.10E-02      2.20E-02     3H   HC sampler and  5.80E-05      2.90E-05 14C   Liquid scintillation   8.00E-06      2.00E-04 counting system                32P   Dust sampler and     3.80E-06      5.43E-04 35S   Liquid scintillation    2.00E-06      1.00E-04     counting system                       60Co  Dust sampler and     5.60E-07     5.60E-04 131I   Ge-spectroscopy     1.68E-06      1.68E-03 137Cs     system          5.60E-07     1.87E-04 Table 3 Comparison of detection and regulation limits of the radioisotope concentration in the air. Table 2 Comparison of detection and regulation limits of surface contamination by using the cart. Radiation  Device      Detection      Detection limit/           installed     limit (Bq/cm2)  legal limit       Alpha    Alpha/beta    0.05   0.013 Other   measuring system  0.04      0.001 134§9. Evaluating On-Site Monitoring Cart Conceptually Developed for Radiation WorkplaceKawano, T.

Evaluating On-Site Monitoring Cart Conceptually Developed for Radiation Workplace

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Evaluating On-Site Monitoring Cart Conceptually Developed for Radiation Workplace

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