Monitoring of Radioactive Substances in Foods Distributed in Kyoto, Japan (1991-2011): Comparison of Detection Rates and Concentrations before and after the Fukushima Daiichi Nuclear Power Plant Accident

摘要

Since the Chernobyl Nuclear Power Plant accident, radioactive substances have been continually monitored in foods collected in the city of Kyoto, Japan. The importance of the monitoring was increased by the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Here, the detection rates and concentrations of radioactive substances were compared among food samples collected before and after the accident in Fukushima prefecture. Before the accident, ~(137)Cs was the only radioactive substance detected in foods. The detection rate was 70％ for fish and shellfish samples and the highest concentration was 1.7 Bq/kg. It was also 83％ for fresh mushroom samples and the highest concentration was 7.5 Bq/kg. In contrast, the detection rate was low for vegetables and the concentrations were also lower than those of the above samples. On the other hand, after the accident, ~(131)I was detected in food produced in the Tohoku and Kanto areas. Actually, ~(131)I (3,400 Bq/kg), ~(134)Cs (280 Bq/kg), and ~(137)Cs (280 Bq/kg) were detected in mizuna, a leaf vegetable, on March 23, 2011. These radioactive substances were detected in all leaf vegetable samples examined in March and April 2011, but they were not detected in samples examined in November 2011. ~(131)I was not detected in any food sample examined after May 2011. However, ~(137)Cs (average=7.9 Bq/kg) was consistently detected in fish and shellfish samples until November, although the concentrations were less than the regulatory limits. It appears unlikely that foods containing radioactive substances over the regulatory limits are currently being distributed in Kyoto.%チェルノブイリ原子力発電所事故以後，京都市内に流通する食品中の放射性ヨウ素(~(131)I)および放射性セシウム(~(137)Csおよび~(134)Cs)のモニタリングを実施してきたが,2011年3月の福島原子力発電所の事故は.調查の重要性を改めて認識させることとなった.福島原子力発電所事故前後において検出した核種と検出率，および濃度について検討した.検查にはゲルマニウム半導体検出器を用いた.福島原発事故以前は，輸入品，囯産品をモニタリングの対象とした.核種としては~(137)Csのみが検出された.魚介類からの検出頻度は約70％であり.濃度は最卨でも1.7Bq/kgであった.乾燥キノコを除くキノコ類からの検出頻度は.83％と高く,濃度の^高値は7.5Bq/kgであった.菜類は，207検体のうち2件のみ(根菜を除く)で検出したが濃度も明らかに低かった.福島原発事故以降は，東北•関東地方産の流通食品を検企した.3月23日に中央卸壳市場から採取したミズ菜から3，400Bq/kgの~(131)I,280Bq/kgの~(134)Cs,および280Bq/kgの~(137)Csを検出したのをはじめ.3月と4月に検查したすべての葉菜類でこれらの放射性物質を検出した.しかし.11月以降はすべてが不検出となった.魚介類から検出された~(137)Csは，平均で7.9Bq/kgだった.肉煩では，トレーサビリティーによって汚染稲わらを与えられたことが判明したゥシの肉からのみ暫定規制値を超える~(137)Csが検出された.また.甲状腺に対するリスクが懸念されるは，5月以降すベての試料で小検出となつた.基準值を超える食品が"都市内を流通する恐れは，すでにほとんどないと考えられた.