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Activity of $$^{90}$$Sr in fallout particles collected in the difficult-to-return zone around the Fukushima Daiichi Nuclear Power Plant

福島第一原子力発電所周辺の帰還困難区域で採取したフォールアウトパーティクル中に含まれるストロンチウム90

Zhang, Z.*; 五十嵐 淳哉*; 佐藤 志彦 ; 二宮 和彦*; 末木 啓介*; 篠原 厚*

Zhang, Z.*; Igarashi, Junya*; Satou, Yukihiko; Ninomiya, Kazuhiko*; Sueki, Keisuke*; Shinohara, Atsushi*

福島第一原子力発電所周辺で採取したフォールアウト粒子に含まれるストロンチウム90($$^{90}$$Sr)の分析を行なった。採取した放射性粒子にはセシウム137($$^{137}$$Cs)も含まれており、最大4100Bq含まれていた。一方、ストロンチウム90は1.4Bqで$$^{137}$$Cs/$$^{90}$$Srは1000倍以上であった。これらの比は環境中で観測された結果と良くあっており、そして1号機内でのちに観測されたストロンチウム比よりも低くかった。

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released abundant radioactive particles into the surrounding environment. Herein, we analyzed the activity of $$^{90}$$Sr in these particles to estimate the contribution of this radionuclide to the overall radiation exposure and shed light on the processes that occurred during the accident. Seven radioactive particles were isolated from the dust and soil samples collected from areas surrounding the FDNPP, and the minimum/maximum $$^{137}$$Cs activities were determined as 224/4,100 Bq. Based on the size, specific activity, and $$^{134}$$Cs/$$^{137}$$Cs activity ratios, we concluded that six of the seven radioactive particles were released from the Unit 1 reactor, while one particle was released from the Unit 3 reactor by a hydrogen explosion. Strontium-90 was detected in all radioactive particles, and the minimal/maximal $$^{90}$$Sr activities were determined as 0.046/1.4 Bq. $$^{137}$$Cs/$$^{90}$$Sr activity ratios above 1000 were observed for all seven particles, that is, compared to $$^{137}$$Cs, $$^{90}$$Sr had negligible contribution to the overall radiation exposure. The $$^{137}$$Cs/$$^{90}$$Sr activity ratios of the radioactive particles were similar to those of terrestrial environmental samples and were higher for particles released from the Unit 1 reactor than for samples collected from the Unit 1 reactor building, which indicates possibility of additional $$^{90}$$Sr -rich contamination after release of the particles.

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