QUANTITATIVE EVALUATION OF CONSERVATISM INCLUDED IN PARAMETER SETTINGS FOR CLEARANCE LEVEL CALCULATIONS: A CASE STUDY IN JAPAN.

In the evaluation of radiation safety standards, representative, average or conservative parameter values are set for dose calculation, and it is expected that excessive conservatism may be occasionally included in the standards already established. In this study, we attempted to quantitatively evaluate the 'factor of conservatism' for parameters used in the dose calculations for an actual case of clearance level calculations in Japan. The 'factor of conservatism' was calculated using the selected value for the parameter actually applied in the clearance level calculation and the expected values evaluated from the probability distribution. As a result, it was estimated that the 'factor of conservatism' for the parameters used in the clearance level calculation for the main nuclear facilities in Japan is approximately 1 to 7.

Estimation of the mortality rate of workers in Japan.

BACKGROUND: Risk-based decision-making is used to identify risk factors for which threshold points have not been identified. The occupational mortality rate was referred to as a reference risk. This study aimed to analyze recent trends in worker mortality using three data sources. METHODS: The Reports of Worker Casualties (RWC), the Annual Statistics Report of the Industrial Accident Compensation Insurance Council (ASR), and the Annual Business Report of the Industrial Accident Compensation Insurance Council (ABR) were used. Mortality rates were estimated by age group and industry category (overall, and manufacturing and construction industries) from 1991 to 2019. The mortality rates were compared with those estimated in Western countries. RESULTS: The mortality rates for the three industry categories in the RWC and ASR decreased, whereas those for the manufacturing and construction industries in the ABR did not. In 2019, the mortality rates in the ABR were 3.1, 4.8, and 3.1 times higher than those in the RWC overall and in the manufacturing and construction industries, respectively. The differences decreased when deaths after long-term medical treatment were considered. The mortality rate trends in Japan were similar to those in Western countries. The upper mortality rate among Japan and Western countries was at least five to 15 times higher than the lowest. CONCLUSION: When occupational mortality rates are used as a reference risk, it is important to consider any changes with time, the data sources used, and the differences between countries.

Quantitative evaluation of conservatism in the concept of committed dose from internal exposure for radiation workers.

For compliance with dose limits, the International Commission on Radiological Protection (ICRP) recommends that the committed dose be assigned to the year in which radionuclide intake occurred in the case of internal exposure. For radiation workers, the committed dose is evaluated over the 50 year period following the intake, which is a rounded value for the working-life expectancy of a young person entering the workforce. In this study, we develop an approach to the quantitative evaluation of the conservatism in the concept of the committed dose from internal exposure for radiation workers from the viewpoint of radiological risk. Actual annual doses due to an intake of radionuclides for strontium-90 (90Sr), caesium-137 (137Cs), and plutonium-239 (239Pu) were simulated. Risks of fatal cancer, i.e. unconditional death probability rates, were calculated in accordance with the risk estimation method in ICRP Publication 60. It was found that the conservatism ranged from 1.1 to 1.6 for90Sr, 1.0 to 1.6 for137Cs, and 1.6 to 2.2 for239Pu. The importance of understanding the extent of this conservatism and the uncertainty for practical radiological protection are also discussed.

Case Study of the Minimum Provable Risk Considering the Variation in Background Risk: Effect of Residual Risk on Epidemiological Studies and a Comparative Assessment of Fatal Disease Risk Due to Radiation Exposure.

In order to prove a small increment in a risk of concern in an epidemiological study, a large sample of a population is generally required. Since the background risk of an end point of interest, such as cancer mortality, is affected by various factors, such as lifestyle (diet, smoking, etc.), adjustment for such factors is necessary. However, it is impossible to inclusively and completely adjust for such factors; therefore, uncertainty in the background risk remains for control and exposed populations, indicating that there is a minimum limit to the lower bound for the provable risk regardless of the sample size. In this case study, we developed and discussed the minimum provable risk considering the uncertainty in background risk for hypothetical populations by referring to recent Japanese statistical information to grasp the extent of the minimum provable risk. Risk of fatal diseases due to radiation exposure, which has recently been the focus of radiological protection, was also examined by comparative assessment of the minimum provable risk for cancer and circulatory diseases. It was estimated that the minimum provable risk for circulatory disease mortality was much greater than that for cancer mortality, approximately five to seven times larger; circulatory disease mortality is more difficult to prove as a radiation risk than cancer mortality under the conditions used in this case study.

Operational level for unconditional release of contaminated property from affected areas around Fukushima Daiichi nuclear power plant.

This paper focuses on the surface contamination control of slightly contaminated property after the Fukushima nuclear accident. The operational level for the unconditional release of contaminated properties is calculated in counts per minute (cpm) to enable the use of a typical Geiger-Muller (GM) survey meter with a 50-mm bore, on the basis of the surficial clearance level of 10 Bq cm(-2) for (134)Cs and (137)Cs derived in the previous studies of the authors. By applying a factor for the conversion of the unit surface contamination to the count rate of a survey meter widely used after the Fukushima accident, the operational level for the unconditional release of contaminated properties was calculated to be 2300 cpm on average and 23 000 cpm at the highest-contamination part. The calculated numerical values of the operational levels are effective as long as the typical GM survey meter is used in the radiation measurement.

Radiological protection from radioactive waste management in existing exposure situations resulting from a nuclear accident.

In environmental remediation after nuclear accidents, radioactive wastes have to be appropriately managed in existing exposure situations with contamination resulting from the emission of radionuclides by such accidents. In this paper, a framework of radiation protection from radioactive waste management in existing exposure situations for application to the practical and reasonable waste management in contaminated areas, referring to related ICRP recommendations was proposed. In the proposed concept, intermediate reference levels for waste management are adopted gradually according to the progress of the reduction in the existing ambient dose in the environment on the basis of the principles of justification and optimisation by taking into account the practicability of the management of radioactive waste and environmental remediation. It is essential to include the participation of relevant stakeholders living in existing exposure situations in the selection of reference levels for the existing ambient dose and waste management.

Verification of screening level for decontamination implemented after Fukushima nuclear accident.

The screening level for decontamination that has been applied for the surface of the human body and contaminated handled objects after the Fukushima nuclear accident was verified by assessing the doses that arise from external irradiation, ingestion, inhalation and skin contamination. The result shows that the annual effective dose that arises from handled objects contaminated with the screening level for decontamination (i.e. 100 000 counts per minute) is <1 mSv y(-1), which can be considered as the intervention exemption level in accordance with the International Commission on Radiological Protection recommendations. Furthermore, the screening level is also found to protect the skin from the incidence of a deterministic effect because the absorbed dose of the skin that arises from direct deposition on the surface of the human body is calculated to be lower than the threshold of the deterministic effect assuming a practical exposure duration.

Discussion on optimisation below dose limit for public using background-cancer-risk-based approach.

To improve a radiological protection system, the lower bound of optimisation was discussed using a newly proposed background-cancer-risk-based approach. This approach provides a new finding that the standard deviation of the background cancer risk in Japan is 7.4 × 10(-5) y(-1). If this risk level corresponding to 1σ can be regarded as a non-concern level, it would be possible that the lower bound of optimisation of radiological protection could be set at 1 mSv y(-1), since the assumed risk of 1 mSv y(-1) is equivalent to ~5 × 10(-5) y(-1) obtained using the overall fatal risk coefficient of 5 % Sv(-1), which is lower than the non-concern level.

Validity of generic scenarios used in derivation of exemption levels for surface contamination considering transport-specific aspects.

The exemption levels for surface contamination in units of Bq cm(-2) were derived by developing a new universal dose assessment model that consists of three generic scenarios assessed by considering manually, closely and remotely handled objects. In this paper, as part of the process of verifying the validity of these generic scenarios, annual doses that arise from transport-specific aspects are calculated. The maximum annual doses are found to be lower than 10 µSv, which is the bottom line of the exemption dose criterion. The result verifies the validity of the generic scenarios used in the previous derivation of exemption levels for surface contamination.

Calculation of isotope-specific exemption levels for surface contamination.

Isotope-specific exemption levels for surface contamination are calculated for representative radionuclides in general nuclear power plants by developing a deterministic dose assessment model for surface contamination that can be applied to radiation, transport and waste safety, and a practical idea of judging exemption for gross surface contamination by measuring gross gamma-ray emission has been proposed. In the dose assessment model, the objects with surface contamination are classified into three types: manually handled, closely handled and remotely handled objects, and the exemption criteria are chosen to be 0.01mSv/yr in the case of using realistic exposure parameters and 1mSv/yr in the case of using low-probability exposure parameters in accordance with the IAEA Safety Standards Series No. RS-G-1.7. Taking into account the distribution area of surface contamination assumed in the dose assessment model, instead of using the evaluation area of 100cm(2) without variation, the exemption levels for gross surface contamination are found to be higher than those obtained by the conventional method for some radionuclides such as Mn-54, Co-60, Zn-65, Nb-94, Cs-134, Cs-137, Eu-152 and Eu-154.

Reconsideration of the minimum dose constraint for public exposures in radiological protection.

By using a probabilistic approach, the effects of the dose distribution of radiation due to man-made radioactive nuclides when added to those of natural background radiation have been studied. These results show that additional exposure to man-made radiation of up to 0.5 mSv y(-1) (as a dose constraint) would not significantly change the distribution of total public doses. Taking into consideration such probabilistic analysis and rationales of derivations of exemption and clearance levels, it can be concluded that the minimum dose constraint that requires optimisation in radiation protection, should be set to 0.1 mSv y(-1), which is one-order magnitude higher than 0.01 mSv y(-1), the current dose criterion for exemption and clearance.