Pre- and post-accident C activities in tree rings near the Fukushima Dai-ichi Nuclear Power Plant

Matsunaka, Tetsuya*; Sasa, Kimikazu*; Takahashi, Tsutomu*; Matsumura, Masumi*; Satou, Yukihiko   ; Shen, H.*; Sueki, Keisuke*; Matsuzaki, Hiroyuki*

Areas affected by routine radiocarbon (C) discharges from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) and accidental releases in March 2011 were investigated by analysis of cores from Japanese cypress and cedar trees growing at sites 9 and 24 km northwest of the plant. C concentrations in tree rings from 2008-2014 (before and after the accident) were determined by accelerator mass spectrometry, with C activities in the range 231-256 Bq kg C. Activities during the period 2012-2014, after FDNPP shutdown, represent background levels, while the significantly higher levels recorded during 2008-2010, before the accident, indicate uptake of C from routine FDNPP operations. The mean excess C activity for the pre-accident period at the sites 9 and 24 km northwest of the plant were 21 and 12 Bq kg C, respectively, indicating that the area of influence during routine FDNPP operations extended at least 24 km northwest. The mean excess tree-ring C activities in 2011 were 10 and 5.8 Bq kg C at 9 and 24 km northwest, respectively, documenting possible impact of the FDNPP accident on C levels in trees.