Carbon-14 transfer into rice plants from a continuous atmospheric source: observations and model predictions

摘要

Carbon-14 (~(14)C) is one of the most important radionuclides from the perspective of dose estimation due to the nuclear fuel cycle. Ten years of monitoring data on ~(14)C in airborne emissions, in atmospheric CO_2 and in rice grain collected around the Tokai reprocessing plant (TRP) showed an insignificant radiological effect of the TRP-derived ~(14)C on the public, but suggested a minor contribution of the TRP-derived ~(14)C to atmospheric ~(14)C concentrations, and an influence on ~(14)C concentrations in rice grain at harvest. This paper also summarizes a modelling exercise (the so-called rice scenario of the IAEA's EMRAS program) in which ~(14)C concentrations in air and rice predicted with various models using information on ~(14)C discharge rates, meteorological conditions and so on were compared with observed concentrations. The modelling results showed that simple Gaussian plume models with different assumptions predict monthly averaged ~(14)C concentrations in air well, even for near-field receptors, and also that specific activity and dynamic models were equally good for the prediction of inter-annual changes in ~(14)C concentrations in rice grain. The scenario, however, offered little opportunity for comparing the predictive capabilities of these two types of models because the scenario involved a near-chronic release to the atmosphere. A scenario based on an episodic release and short-term, time-dependent observations is needed to establish the overall confidence in the predictions of environmental ~(14)C models.