To estimate the effect of corrosion products on alloys for clearance inspection, the relationships between increasing corrosion weights and decreasing radiation count rates of test samples were investigated using carbon steel (SS400) and stainless steel (SUS304) contaminated by a standardized solution with the radioisotopes 241Am as an alpha emitter and 60Co as a beta emitter. The test results showed that the experimental data were nearly equal to the theoretical values. 238U is an important nuclide for uranium clearance inspection. The radiation count rate decrease caused by corrosion of 4.2 MeV alpha particles emitted by 238U was estimated taking into account the energy difference between 241Am (5.4 MeV) and 238U (4.2 MeV).
The monetary value of unit collective dose, which represents the benefit of dose reduction due to the protective measures, is a key element in implementing the optimization principle of the radiation protection. The objectives of this paper are to evaluate the country-specific monetary value of unit collective dose reflecting the economic and population characteristics in Japan, as well as to analyze the balance of the cost and benefit of the radiological protective measures. When we calculated the monetary values of unit collective dose in Japan using human capital method and willingness to pay approach, the values were around several million yen/person-Sv and 20 million yen/person-Sv, respectively. In addition, we surveyed the data on costs of radiological protective measures per the avoided person-Sv. As the results of comparing the cost and benefit, the costs of radiological protective measures to reduce the concentration of radon in homes and to protect workers in nuclear facilities were balanced with the benefit. On the other hand, the costs of radionuclide emission control measures at industrial facilities tend to be higher than the benefit of those measures.
Since the reprocessing contracts with electric power companies were completed in March 2006, the horizontal distribution of 129I in soil within 10 km radius from Tokai Reprocessing Plant (TRP) as well as the vertical distribution at 2 km southwest from TRP were measured using the Accelerator Mass Spectrometry (AMS) method. As a result, the small amounts of 129I were observed due to the atmospheric discharge from TRP. Moreover, it was good agreement with the predicted concentrations in safety assessment. The validity of the diffusion calculation was confirmed in the 10 km radius. Furthermore, it was suggested that 129I adsorbing by soil was difficult to move down for several decades from the vertical profile of 129I concentration.
In the case of clearance inspection for materials used at nuclear facilities, the applicants of clearance licenses have to show that the radioactivity concentration is lower than the clearance level, and it is necessary for the regulatory body to confirm this claim. Corrosion products might be generated on alloys in the period between the first measurement in the facility and the confirmation measurement by the regulatory body. Corrosion products shield radiation and cause a drop in radiation count rates. In order to investigate the growth of corrosion products on alloys in actual warehouse environments, carbon steel and stainless steel coupon specimens were set in warehouses of five facilities. The weight increases of the coupon specimens caused by the generation of corrosion products were measured over one year at each warehouse. The decrease in the radiation count rate as a result of shielding by the corrosion products on coupon specimens at each warehouse was estimated, referring to a previous report on the relationship between the amount of corrosion products and the decrease in the radiation count rate.