Atmospheric discharge of 14C from the Tokai reprocessing plant: comprehensive chronology and environmental impact assessment

Abstract

Quantitative evaluation of the atmospheric discharge of radiocarbon (14C) is of primary importance for accurately assessing the radioecological impact of the operation of the Tokai reprocessing plant (TRP). However, monitoring the atmospheric 14C discharge was not conducted at the TRP prior to October 1991. The main purpose of this study was to establish the chronology of atmospheric 14C discharges for the entire operation period (1977–2014). We found a strong correlation between the 14C discharge and spent fuel reprocessing data obtained after October 1991; we used this correlation to estimate the monthly 14C discharges in the 1977–1991 period. The total amount of atmospheric 14C discharge was estimated at 7741 ± 217 GBq for the entire period, with the largest annual discharge of 978 GBq in 1992. Tree-ring 14C analysis and model calculations using the established discharges were conducted to assess the excess 14C concentrations around the TRP in the past. The results suggested that the excess 14C concentrations were <45 Bq kg−1 C and the annual effective dose via the atmosphere–rice ingestion pathway was as low as 1.2 μSv y−1, indicating no significant radioecological impact of atmospheric 14C discharges from the TRP.