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

Abstract

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.