Presentation 福島第一原発事故後の堆積物中の粒径別の放射性セシウム存在量分布とその挙動

福田, 美保  ,  山崎, 慎之介  ,  青野, 辰雄  ,  吉田, 聡  ,  長沼, 翔  ,  久保, 篤史  ,  伊藤, 友加里  ,  石丸, 隆  ,  神田, 穣太

The Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident in March 2011 led to releases of large amounts of artificialradionuclides including 134Cs and 137Cs (radiocaesium) into the environment (ocean, atmosphere and land). As of February2015, based on TEPCO’s monitoring data, 137Cs activities in seawater had exponentially decreased more than 10 times comparedto the activities before the FDNPS accident. However, the activities in sediments have decreased more slowly than thosein seawater and they have had large fluctuations. Some controversy remains about the radiocaesium transition process fromseawater to sediments and its behavior after accumulation. In soil and sediments, the radiocaesium activities for finer grain sizestend to be higher because specific surface areas of smaller grains are larger (e.g. He and Walling, 1996). Therefore, it is necessaryto consider the differences of grain size composition in sediments in order to compare distributions of lateral and verticalradiocaesium activities and inventories in coastal sediments, which are commonly of several grain sizes. This study was aimedat elucidating spatial variation and the behavior of radiocaesium activities and inventories for different grain sizes in sedimentscollected in the Fukushima coastal area in May 2014.The sediments were divided into four classes based on grain sizes using several mesh sizes: granules (grain size larger than2 mm); very coarse to coarse sand particles (1 to 2 mm); coarse to very fine sand particles (0.063 to 1 mm); and silt particles(smaller than 0.063 mm). Radionuclides were measured for each grain size class using high-purity gamma ray spectrometry andthen corrected to the sampling date.In collected sediments, the only artificial radionuclides detected were radiocaesium. In the surface layer of sediments (0-5cm), the percentage ranges were: granules, 0 to 23 %; very coarse to coarse sand particles, 0 to 39 %; coarse to very fine sandparticles, 38 to 98 %; and silt particles, 0 to 46 %. The137Cs activities for coarse to very fine sand particles and silt particlesranged from 8.5 to 609 Bq kg−1-dry and 18 to 1487 Bq kg−1-dry, respectively and the latter particle activities were higher thanthose for the former particles in most layers. The 137Cs inventories for coarse to very fine sand particles ranged from 972 to 3285Bq m−2 and those in the water depth range of 100 to 150 m were highest. The 137Cs inventories for silt particles ranged from1387 to 31321 Bq m−2 and they decreased with increasing water depth.The fractions of 137Cs inventories in the uppermost layerof sediments (0-3 cm) to those in the surface layer of sediment (F’0−3) for coarse to very fine sand particles and silt particleswere 0.33 to 0.88 and 0.24 to 0.77, respectively. The F’0−3 values for silt particles were lower than those for coarse to very finesand particles. It appeared that silt particles more easily adsorbed radiocaesium that had been transported to a deep sedimentlayer compared to coarse to very fine sand particles.He, Q., and Walling, D., E. (1996) Journal of Environ. Radioact, 20 (2), 117-137.Keywords: radiocaesium, Fukushima coastal area, marine sediment, grain size, FDNPS.
日本地球惑星科学連合 連合大会 2015年大会

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