Time trends in radiocaesium in the Japanese diet following Nuclear Weapons Testing and Chernobyl: implications for long term contamination post-Fukushima

Estimation of time changes in radiocaesium in foodstuffs is key to predicting the long term impact of the Fukushima accident on the Japanese diet. We have modelled >4000 measurements, spanning 50 years, of 137Cs in foodstuffs and whole diet in Japan after nuclear weapons testing (NWT) and the Chernobyl accident. Broadly consistent long term trends in 137Cs activity concentrations are seen between different agricultural foodstuffs; whole diet follows this general trend with remarkably little variation between averages for different regions of Japan. Model blind tests against post-NWT data for the Fukushima Prefecture showed good predictions for radiocaesium in whole diet, spinach and Japanese radish (for which good long term test data were available). For the post-Fukushima period to 2015, radiocaesium in the average diet followed a declining time trend consistent with that seen after NWT and Chernobyl. Data for different regions post-Fukushima show a high degree of mixing of dietary foodstuffs between regions: predictions which assumed that only regionally produced food was consumed significantly over-estimated empirical data. Predictions of average committed effective internal doses from dietary 137Cs (2011 to 2061) in nonevacuated parts of the Fukushima Prefecture show that average internal dose is relatively low. This study focuses on average regional ingestion dose rates and does not attempt to make site specific predictions. However, the temporal trends identified could form a basis for site specific predictions of long term activity concentrations in agricultural products and diet both outside and (to assess potential re-use) inside currently evacuated areas