Analysis of Japanese Radionuclide Monitoring Data of Food Before and After the Fukushima Nuclear Accident

In an unprecedented food monitoring campaign for radionuclides, the Japanese government took action to secure food safety after the Fukushima nuclear accident (Mar. 11, 2011). In this work we analyze a part of the immense data set, in particular radiocesium contaminations in food from the first year after the accident. Activity concentrations in vegetables peaked immediately after the campaign had commenced, but they decreased quickly, so that by early summer 2011 only a few samples exceeded the regulatory limits. Later, accumulating mushrooms and dried produce led to several exceedances of the limits again. Monitoring of meat started with significant delay, especially outside Fukushima prefecture. After a buildup period, contamination levels of meat peaked by July 2011 (beef). Levels then decreased quickly, but peaked again in September 2011, which was primarily due to boar meat (a known accumulator of radiocesium). Tap water was less contaminated; any restrictions for tap water were canceled by April 1, 2011. Pre-Fukushima 137Cs and 90Sr levels (resulting from atmospheric nuclear explosions) in food were typically lower than 0.5 Bq/kg, whereby meat was typically higher in 137Cs and vegetarian produce was usually higher in 90Sr. The correlation of background radiostrontium and radiocesium indicated that the regulatory assumption after the Fukushima accident of a maximum activity of 90Sr being 10% of the respective 137Cs concentrations may soon be at risk, as the 90Sr/137Cs ratio increases with time. This should be taken into account for the current Japanese food policy as the current regulation will soon underestimate the 90Sr content of Japanese foods.2011年3月11日福岛核事故（Fukushima nuclear accident）发生后，日本政府启动了史无前例的放射性核素（radionuclides）食品监测行动以保障食品安全。本研究对该庞大监测数据集的部分内容展开分析，重点聚焦事故发生后首年内食品中的放射性铯（radiocesium）污染情况。蔬菜的活度浓度（activity concentrations）在监测行动启动后即刻达到峰值，但随后快速下降，至2011年初夏时仅有少量样品超出监管限值（regulatory limits）。后续因蘑菇和干燥农产品对放射性核素的富集作用，再度出现多起样品超标事件。肉类监测工作的启动存在显著滞后，在福岛县（Fukushima prefecture）以外地区尤为明显。经过一段累积期后，肉类（以牛肉为代表）的污染水平于2011年7月达到峰值，此后污染水平快速回落，但在2011年9月再度出现峰值，这主要源于野猪肉——已知野猪肉会富集放射性铯。自来水受污染程度较轻，所有针对自来水的管控措施均于2011年4月1日前解除。福岛事故前，食品中由大气核试验（atmospheric nuclear explosions）遗留的铯-137（137Cs）与锶-90（90Sr）活度浓度通常低于0.5贝可每千克（Bq/kg）；其中肉类的铯-137活度浓度普遍更高，而植物性农产品的锶-90活度浓度通常更高。对本底放射性锶（background radiostrontium）与放射性铯的相关性分析显示，福岛事故后日本监管机构提出的‘锶-90最大活度为对应铯-137活度的10%’这一假设或将很快不再适用，因为锶-90与铯-137的活度比值随时间不断升高。鉴于现行监管标准将很快低估日本食品中的锶-90含量，日本当前的食品政策亟需将这一因素纳入考量。