Analysis on the influence of forest soil characteristics on radioactive Cs infiltration and evaluation of residual radioactive Cs on surfaces
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We investigated the depth profiles of radioactive Cs, ignition loss, and cation exchange capacity (CEC) in five types of forest soils sampled using scraper plates. We then simulated the monitored depth profiles in a compartment model, taking ignition loss as a parameter based on experimental results showing a positive correlation between ignition loss and the CEC. The calculated values were comparable with the monitored values, though some discrepancy was observed in the middle of the soil layer. Based on decontamination data on the surface dose rate and surface contamination concentration, we newly defined a surface residual index (SRI) to evaluate the residual radioactive Cs on surfaces. The SRI value tended to gradually decrease in forests and unpaved roads and was much smaller in forests and on unpaved roads than on paved roads. The radioactive Cs was assumed to have already infiltrated underground 18 months after the nuclear power plant accident, and the sinking was assumed to be ongoing. The SRI values measured on paved roads suggested that radioactive Cs remained on the surfaces, though a gradual infiltration was observed towards the end of the monitoring term. The SRI value is thought to be effective in grasping the rough condition of residual radioactive Cs quickly at sites of decontamination activity in the field. The SRI value may be serviceable for actual contamination works after further research is done to elucidate points such as the relation between the SRI and the infiltration of radioactive Cs in various types of objects.
KeywordsEffect of forest decontamination Infiltration of radioactive Cs Ignition loss Compartment model Residual radioactive substances Types of radiation
Mr. Sei Kato, director of the Fukushima Office for Environmental Restoration, Ministry of the Environment, kindly provided the data and information on the current state of decontamination works. Mr. Keiichi Kawase, Mr. Masanori Watanabe, and Mr. Yuto Terakawa of the Japan Atomic Energy Agency and Mr. Koichi Mitsuo of the Tokyo Electric Power Company provided the technical support for the field studies. Mr. Hitoshi Kondo, Mr. Tatsunari Kometani, Mr. Hendra Adhi Pramata, Mr. Yongqiang Cui, and Mr. Adriraga Pramata from my laboratory helped me collect samples in the field.
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