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, similar to 1 km southwest of the Fukushima Dai-ichi Nuclear Power Plant, was cored and each annual ring was analysed for C-14. The C-14 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 C-14 specific activities are indistinguishable from the ambient background values. However, compared with the ambient atmospheric levels, the C-14 specific activities between 1977 and 2010 are significantly elevated, clearly indicating C-14 discharges from the reactors during their normal operations. In addition, the specific activities are positively correlated with the annual electricity generation values. The excess C-14 specific activities were <36 Bq kg(-1) C, corresponding to an additional annual effective dose of <2 mu Sv via the food ingestion pathway in the study location. The primary wind direction is east-southeast/southeast with a frequency of similar to 30%, in comparison to similar 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 C-14 discharges from the FDNPP during the entire period of normal operations. Additionally, no C-14 pulse in activity can be observed in the year 2011 ring. This might be caused by a limited C-14 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. (C) 2016 Elsevier Ltd. All rights reserved.