The Behavior of Radiocaesium Deposited in an Upland Reservoir After the Fukushima Nuclear Power Plant Accident
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The Behavior of Radiocaesium Deposited in an Upland Reservoir After the Fukushima Nuclear Power Plant Accident Hironori Funaki, Hiroki Hagiwara, and Tadahiko Tsuruta Fukushima Environmental Safety Center, Headquarters of Fukushima Partnership Operations, Japan Atomic Energy Agency, Sahei Building, 1-29, Okitama-cho, Fukushima-shi, Fukushima, 960-8034, Japan ABSTRACT In the autumn of 2012, the Japan Atomic Energy Agency (JAEA) launched a new research project named F-TRACE (Long-Term Assessment of Transport of Radioactive Contaminant in the Environment of Fukushima). The aims of this project are to develop a system for prediction of radiation exposure, taking into consideration the transport, deposition, and remobilization behavior of radiocaesium (RCs) from the highest contaminated mountain forests, down through the biosphere, before deposition in a number of different aquatic systems. Especially, it is important to understand balances of suspended and deposited particles and RCs inventory in inflow water, discharge water and bottom sediments of an upland reservoir. In this paper, we describe current research activities performed by JAEA at the Ogi Reservoir, Fukushima prefecture, Japan. According to our analyses the specific sediment yield and the average rate of storage capacity loss at the Ogi Reservoir are 210 m3 km-2 year-1 and 0.15 % year-1, respectively. The vertical distribution of RCs exhibits clear peaks at several sites in the reservoir formed by deposition of eroded soil particles from the catchment that were contaminated by accident fallout. Above the depth of each of the RCs peaks, the distribution of RCs was found to be variable with depth, with concentrations ranging over five orders of magnitude for a single core. The peaks in the sedimentation profiles are probably formed from eroded soil particles entering the reservoir from the surrounding contaminated watershed (most probably during storm events). Results from grain size analyses suggest that contaminated fine sediment tends to be deposited thickly within deeper parts of the reservoir. In addition, above the depth of RCs peaks at these deeper sites, the concentration of RCs approximately increased or decreased as the proportion of fine sediments increased or decreased. However, some fine particles are possibly discharged downstream during operational releases from the dam. INTRODUCTION The Fukushima Daiichi nuclear power plant (NPP) accident occurred as a consequence of the massive earthquake and associated tsunami on March 11, 2011. Large quantities of volatile radionuclides were released to the atmosphere from the NPP, and consequently caused widespread contamination of residential, agricultural, forested and aquatic areas of northeast Japan. The total activities of 131I and 137Cs discharged into the atmosphere from 10 Japanese Standard Time (JST) on March 12 to 0 JST on April 6 are estimated to be approximately 1.5u1017 and 1.3u1016 Bq, respectively [1]. After decay of shorter-lived nuclides, the major
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radionuclides of concern from the
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