Environmental Tritium in the Vicinity of Tokai Reprocessing Plant

Abstract

The distribution of 3H in the atmosphere, vegetations and soil water was observed in the vicinity of Tokai reprocessing plant (TRP) over the period 1990–2004. The annual means of the atmospheric HTO and HT concentrations were in the range of 12–40 mBqm−3 with a significant seasonal variation and 14–51 mBqm−3 with no seasonal variation, respectively. Long-term atmospheric dilution factors, defined as the annual mean of the atmospheric HTO concentration divided by the discharge rate of HTO from the TRP, were estimated to be 10−8–10−6 sm−3. The atmospheric HTO concentrations decreased with distance from the TRP, falling to the current background level in Japan at 5 km off-site. The HTO concentrations observed were compared with those calculated by a simple mathematical model with input data of the monthly 3H discharge rates and actual meteorological conditions. The calculations were correlated well with the observations even for only a little HTO-elevated situation, considering the naturally occurring 3H level in atmospheric vapor around the TRP. Tritium concentrations in vegetation and soil water samples were roughly the same as the atmospheric HTO concentrations, suggesting the rapid equilibrium of 3H concentrations in the atmosphere-soil-vegetation system around the TRP.