Impacts of C-uptake by plants on the spatial distribution of 14C accumulated in vegetation around a nuclear facility-Application of a sophisticated land surface 14C model to the Rokkasho reprocessing plant, Japan

The impacts of carbon uptake by plants on the spatial distribution of radiocarbon (14C) accumulated in vegetation around a nuclear facility were investigated by numerical simulations using a sophisticated land surface 14C model (SOLVEG-II). In the simulation, SOLVEG-II was combined with a mesoscale meteorological model and an atmospheric dispersion model. The model combination was applied to simulate the transfer of 14CO2 and to assess the radiological impact of 14C accumulation in rice grains during test operations of the Rokkasho reprocessing plant (RRP), Japan, in 2007. The calculated 14C-specific activities in rice grains agreed with the observed activities in paddy fields around the RRP within a factor of four. The annual effective dose delivered from 14C in the rice grain was estimated to be less than 0.7 μSv, only 0.07% of the annual effective dose limit of 1 mSv for the public. Numerical experiments of hypothetical continuous atmospheric 14CO2 release from the RRP showed that the 14C-specific activities of rice plants at harvest differed from the annual mean activities in the air. The difference was attributed to seasonal variations in the atmospheric 14CO2 concentration and the growth of the rice plant. Accumulation of 14C in the rice plant significantly increased when 14CO2 releases were limited during daytime hours, compared with the results observed during the nighttime. These results indicated that plant growth stages and diurnal photosynthesis should be considered in predictions of the ingestion dose of 14C for long-term chronic releases and short-term diurnal releases of 14CO2, respectively.