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 14 C specific activity varied from 280.4 Bq kg −1 C in 2010 to 226.0 Bq kg −1 C in 2015. The elevated 14 C activities in the 2009 and 2010 rings confirmed 14 C 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 14 C analysis of the 2011 ring indicated 14 C releases during the Fukushima accident. The resulted 14 C activity decreased with increasing distance from the plant. The maximum 14 C activity released during the period of the accident was measured 42.4 Bq kg −1 C above the natural ambient 14 C background. Our findings indicate that, unlike other Fukushima-derived radionuclides, the 14 C released during the accident is indistinguishable from ambient background beyond the local environment (~30 km from the plant). Furthermore, the resulting dose to the local population from the excess 14 C activities is negligible compared to the dose from natural/nuclear weapons sources.