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茨城周辺海域海底土中放射性核種濃度の経年変化

Temporal variations of radionuclides in seabed sediments off Ibaraki coast

永岡 美佳; 横山 裕也; 松原 菜摘; 藤田 博喜; 中野 政尚; 渡辺 均

Nagaoka, Mika; Yokoyama, Hiroya; Matsubara, Natsumi; Fujita, Hiroki; Nakano, Masanao; Watanabe, Hitoshi

東京電力福島第一原子力発電所事故により、様々な種類の放射性核種が大気や海洋に放出され、それらは核燃料サイクル工学研究所周辺にも到達した。そのため、海底土中の$$^{134}$$Cs, $$^{137}$$Cs及び$$^{90}$$Sr濃度の測定を行い、周辺環境における事故影響を調査した。2012年に茨城県北部の地点では、$$^{137}$$Csが最大濃度296.0Bq/kg・dryであったが、2014年には、39Bq/kg・dryと濃度が減少した。一方、2013年及び2014年に観測された$$^{137}$$Cs最大濃度においても、茨城県北部で観測された。$$^{90}$$Srにおいても、原子力発電所事故以前に茨城県沖で過去20年間に観測された値と比較して高い濃度が一部で観測され、原子力発電所事故の影響が認められた。

The radiological impact of the radionuclides released from the Fukushima Daiichi Nuclear Power Station to the sea offshore of Ibaraki has been studied in this research. Fifty-one seabed sediments were collected in 2012, 2013 and 2014, respectively. Then, all samples were measured for the activities of $$^{134}$$Cs and $$^{137}$$Cs and some samples were analyzed for the activities of $$^{90}$$Sr. The results indicated that the activities of $$^{134}$$Cs and $$^{137}$$Cs were elevated by the accident and showed a decreasing tendency between 2012 and 2014. The tendency of activity in Kuji Riverestuary area was different from Ibaraki coast area. The difference could be caused by geological feature and grain size of seabed sediment etc. On the other hand, $$^{90}$$Sr activities were ND-0.26 Bq/kg dry. It was no correlation between $$^{90}$$Sr and $$^{137}$$Cs activity. The $$^{90}$$Sr activity released by the accident was small influence on seabed sediment collected around sea offshore of Ibaraki.

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