The 2-component model was applied for the estimation of the removal of exchangeable 90Sr in paddy-field soil. The monitoring data of exchangeable 90Sr in paddy-field soil and global fallout 90Sr data from 1963 to 1995 at 11 sites in Japan were used for this analysis. From the results of nonlinear regression analyses, it is concluded that the 2-component model is more suitable for the estimation of the time-dependent concentration of exchangeable 90Sr in paddy-field soil than the 1-component model. The values of the environmental half-lives of rapid component and slow component, which are parameters of the 2-component model, were estimated as 2y and 10y, respectively. It was estimated that the ratio of rapid component changes from 0.81 to 0.98.
Based on the track-etching technique, a convenient, low-cost, time-integrating monitor was developed for measuring the deposition rate of attached thoron (220Rn) progeny indoors. Measurements of the deposition rates and concentrations of 220Rn progeny were simultaneously carried out in 13 dwellings throughout a year. Quite different deposition rates and concentrations of 220Rn progeny were found among those dwellings. However, seasonal variations of the deposition rates and concentrations were not significant in each dwelling. An average of (0.53±0.11)×10-2cm·s-1 was estimated for the deposition velocity of attached 220Rn progeny indoors, in fair agreement with most of the values for attached 222Rn progeny in dwellings in the literature. Significant correlation (r=0.981) was found between the deposition rates and concentrations of 220Rn progeny, and the deposition velocity for attached 220Rn progeny was relatively constant among the typical dwellings. It suggests that it is possible to estimate indoor 220Rn progeny concentrations using their deposition rates with the proposed simple measurement technique.
We evaluated the environment around Ningyotoge in the northern part of Okayama Prefecture. (I) Measurement with TLD. 1) Environmental gamma ray measurement results were evaluated by TLD at the time of confirming an investigation using a continuous gamma ray monitor. Doses during that time span for passing persons and individuals in the area were lower than 89nGy/h. 2) We observed Fading for TLD. The result was that the Fading revision coefficient for the Tenno district in Ningyotoge between October 10, 1997 to December 18, 1997 was 1.02. That for the premises of the Okayama Prefectural Institute for Environmental Science and Public Health was 1.05. The Tenno district in Ningyotoge during the period between June 29, 1998 to October 6, 1998 was 1.04. 3) A relationship between the height from the ground and gamma ray dose was found. A straight line was shown by both logarithm graphs. (II) Wastewater processing. We evaluated radioactive wastewater. When considering the processing of wastewater that flows into rivers, a discharge of zero release is desirable. We evaluated the BOD, COD, etc, as well as the radioactive materials that are processed by microbes such as Bacillus and Arthrobacter.
Three workers were heavily exposed to radiations in the Tokaimura nuclear accident, and one of them died due to the acute effects of radiations. Doses for the heavily exposed persons were estimated to be 2.5, 10 and 18Sv, according to the Science and Technology Agency. Workers who tried to stop the chain reaction by breaking the water pipe were estimated to have been exposed up to 120mSv. Possible doses for other workers and residents in the neighborhoods were estimated to be less than 10mSv, with a few workers with slightly higher film badge records. After the accident, many reports in mass-media warned that the exposed persons may develop cancers and leukemias in future and follow-up healthcare should be needed. Judging from our knowledge of the extensive epidemiological survery of the atomic bomb survivors in Hiroshima and Nagasaki, these reports are very misleading. There would be absolutely no or extremely small possibility of developing any health hazard among the workers and the residents except for the three unfortunate heavily exposed workers. If so-called follow-up health checks would involve x-ray diagnosis for cancers, the radiation doses by the diagnosis would exceed the exposure by the accident. Also, the test for the DNA damage applied to some workers and residents is not reliable at all, and could cause unnecessary fear among the persons who were mistakingly said to be of high-risk.