RADIOCARBON CONCENTRATIONS IN ENVIRONMENTAL SAMPLES COLLECTED NEAR THE SPENT NUCLEAR FUEL REPROCESSING PLANT AT ROKKASHO, AOMORI, JAPAN, DURING TEST OPERATION USING SPENT NUCLEAR FUEL

The contribution of atmospheric discharged C-14 to local C-14 concentrations was investigated by analysis of C-14 in environmental samples collected around the spent nuclear fuel reprocessing plant in Rokkasho, Aomori, Japan. From June 2006 to October 2011, the range of monthly averaged specific activities in atmospheric CO2 collected 2.6 km from the plant was 0.226-0.279 Bq g C-1; at several sampling times, the observed concentrations were higher than the background value. Specific activities of atmospheric C-14 simulated with an atmospheric dispersion model coupled with a mesoscale weather model reproduced the measured values fairly well, supporting the idea that the higher measured specific activities were due to C-14 discharged from the reprocessing plant. For investigation of the C-14 distribution around the facility, samples of Cyperus microiria, wild annual sedge, were collected from nine locations. Plant samples collected east and west of the facility in 2008 showed slightly higher specific activities than samples collected at the other locations, reflecting the prevailing wind direction. The measured C-14 specific activities in vegetable, polished rice, pasture, and milk samples collected around the facility did not differ significantly from the background value ( except for one polished rice sample, which had a value slightly higher than background). The annual committed effective radiation dose to humans resulting from consumption of the polished rice was estimated as 3.6 x 10(-4) mSv, which is negligibly small compared with 1 mSv, an index of the dose limit for the general public.