Diverse Monitoring Approaches Reveal 14C Dispersion Pattern and Its Impact on the Environment around the Tokai Reprocessing Plant

Abstract

Carbon-14 (14C) is of great interest in the assessment of dose due to the nuclear industry because of the biological importance of carbon. We collected atmospheric CO2, rice grain, leafy vegetable, and Japanese mugwort samples at various locations at different time intervals, and measured their 14C concentrations to show the spatiotemporal distribution of 14C around the Tokai reprocessing plant (TRP). The results consistently gave the picture of a 14C dispersion pattern brought about by the prevailing wind in this area, which conveys the 14C plume, and as a result, caused elevated 14C concentrations in the area 2–3 km southwest of the main stack of the TRP. The increases in 14C concentration were, however, quite temporary, and the baseline concentration remained at the background level even in the elevated area. The 14C concentrations in vegetations generally followed those in atmospheric CO2, but seemed different in 14C uptake response between vegetation types, emphasizing the importance of this nature not only in its application as bioindicators in environmental monitoring but also in establishing a more realistic dose evaluation model for 14C. Through a series of investigations, we found the highest 14C concentrations of 374, 287, and 361 Bq kg-C−1 in atmospheric CO2, rice grains, and leafy vegetables, respectively. They corresponded to a maximum of ~110 Bq kg-C−1 excess above the background. The annual effective dose delivered from the concentrations was estimated to be around 0.7 μSv yr−1, only 0.07% of the annual effective dose limit for the public. Thus, we conclude that the 14C releases from the TRP have a very slight impact both on the environment and on the public.