A simplified calculation method of dose rates in the environment when radionuclides are deposited on a finite surface soil is proposed. Based on the concept of the point-kernel method, the dose rate from a sufficiently small volume source is calculated by D = KBφ. K is the dose conversion factor. The dose buildup factor B has been examined for major substances and summarized as a function of mean free paths. The formulas expressing the primary photon fluence rate φ have already been derived. In this study, we propose the calculation method of the mean free path until photons emitted from a radiation source reach the detector. When a plane source exists in a single medium, calculation results by the simplified calculation method agree well with those calculated using the detailed Monte Carlo method. In stratified layers of soil and air, the results were greatly influenced by the accuracy of the double-layered dose buildup factor employed. When the radius of the plane source is 100 m or more, the differences from the detailed calculation results were less than 7%.
We have developed a tritium screening method focused on seafood. Time saving and simplicity of tritium analysis were achieved by performing analysis as total tritium that simultaneously obtains the recovered water of TFWT and OBT. We carried out the experiment using HTO and L-leucine [4, 5 – 3H], and confirmed that total tritium was sufficiently recovered. The developed method has a low detection limit, and it gives us a tool to judge smoothly the annual radiation dose caused by consumption of seafood is below the guidelines authorized by international organizations, the safety level for public. The detectable radiation dose by tritium is 9.2 × 10–6 mSv/y for average consumption of seafood in Japan. This method can evaluate 2–3 orders lower dose level authorized by international organizations for public.
When discussing dose-rate effects, comparing distinct epidemiological studies is important. Excess relative risks calculated under various analysis conditions in distinct epidemiological studies have often been directly compared without considering the analysis conditions. To address these issues, we first quantitatively evaluated the effects of analysis conditions on risk estimation for radiation-induced solid cancers in comparative epidemiological studies. “Apparent risks” dependent on age and follow-up periods were estimated in a virtual Japanese population with “a net risk” using the Life Span Study data. The estimated cancer risks were affected by the exposed and attained ages, length of follow-up period, age at the start of follow-up, and estimation methodologies for excess relative risks. These apparent risks differed by a maximum of approximately 5 times; thus, it greatly impacts the discussion of the dose-rate effectiveness factor. We estimated the excess relative risks under three basic scenarios corresponding to accidental release, occupational, and lifelong exposure for this virtual population. Considering that the initial age at exposure remained unchanged, the estimated risks of the 10-year exponential decline exposure and acute exposure were almost the same. The estimated risk of chronic exposure was lower than that of acute exposure, without considering the biological effects of dose and dose-rate.