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

Abstract

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.