Radiocarbon Releases from the 2011 Fukushima Nuclear Accident

Radiocarbon activities were measured in annual tree rings for the years 2009 to 2015 from Japanese cedar trees (Cryptomeria japonica) collected at six sites ranging from 2.5-38 km northwest and north of the Fukushima Dai-ichi nuclear power plant. The C-14 specific activity varied from 280.4 Bq kg(-1) C in 2010 to 226.0 Bq kg(-1) C in 2015. The elevated C-14 activities in the 2009 and 2010 rings confirmed C-14 discharges during routine reactor operations, whereas those activities that were indistinguishable from background in 2012-2015 coincided with the permanent shutdown of the reactors after the accident in 2011. High-resolution C-14 analysis of the 2011 ring indicated C-14 releases during the Fukushima accident. The resulted C-14 activity decreased with increasing distance from the plant. The maximum C-14 activity released during the period of the accident was measured 42.4 Bq kg(-1) C above the natural ambient C-14 background. Our findings indicate that, unlike other Fukushima-derived radionuclides, the C-14 released during the accident is indistinguishable from ambient background beyond the local environment (similar to 30 km from the plant). Furthermore, the resulting dose to the local population from the excess C-14 activities is negligible compared to the dose from natural/nuclear weapons sources.