¹⁴C levels in the vicinity of the Fukushima Dai-ichi Nuclear Power Plant prior to the 2011 accident

A 50-year-old Japanese cedar (Cryptomeria japonica) from Okuma, ∼1 km southwest of the Fukushima Dai-ichi Nuclear Power Plant, was cored and each annual ring was analysed for ¹⁴C. The ¹⁴C specific activity values varied from 330.4 Bq kg⁻¹ C in the tree ring formed in 1971 to 231.2 Bq kg⁻¹ C in the 2014 ring. During the periods 1971–1976 and 2011–2014, the ¹⁴C specific activities are indistinguishable from the ambient background values. However, compared with the ambient atmospheric levels, the 14C specific activities between 1977 and 2010 are significantly elevated, clearly indicating ¹⁴C discharges from the reactors during their normal operations. In addition, the specific activities are positively correlated with the annual electricity generation values. The excess ¹⁴C specific activities were <36 Bq kg⁻¹ C, corresponding to an additional annual effective dose of <2 μSv via the food ingestion pathway in the study location. The primary wind direction is east-southeast/southeast with a frequency of ∼30%, in comparison to ∼20% frequency for the direction of the site under study (north-northeast/northeast). This would tend to indicate a similar magnitude of additional effective dose and consequently no significant radiological impact of atmospheric ¹⁴C discharges from the FDNPP during the entire period of normal operations. Additionally, no ¹⁴C pulse in activity can be observed in the year 2011 ring. This might be caused by a limited ¹⁴C release from the damaged reactors during the accident or that the prevailing wind during the short period of release (11th–25th March 2011) was not in the direction of Okuma.