Carbon-14 transfer into rice plants from a continuous atmospheric source; Observations and model predictions

大気中に連続的に放出された放射性炭素の稲への移行; 観測とモデル予測

小嵐 淳   ; Davis, P. A.*; Galeriu, D.*; Melintescu, A.*; 齊藤 眞弘*; Siclet, F.*; 内田 滋夫*

Koarashi, Jun; Davis, P. A.*; Galeriu, D.*; Melintescu, A.*; Saito, Masahiro*; Siclet, F.*; Uchida, Shigeo*

放射性炭素(C)は核燃料サイクルに起因して環境中に放出される主要な放射性核種であり、線量評価の観点から特に重要な核種である。本論文では、東海再処理施設から大気中へ放出されたCの放出率データと施設周辺における大気中及び米中C濃度データ(10年間)を解析し、施設運転に伴うCによる公衆への線量は極微小であったが、大気放出パターンの違いが収穫時の米中C濃度に影響を及ぼす可能性があることを示した。これらの実測データに基づいて、IAEA-EMRASプロジェクトにおいてC環境移行モデルの検証を行い、モデリングアプローチの異なる各種モデルによって予測された大気中,米中C濃度がいずれも観測結果とよく一致する結果を得た。

Carbon-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 C in airborne emissions, in atmospheric CO and in rice grain collected around the Tokai reprocessing plant (TRP) showed an insignificant radiological effect of the TRP-derived C on the public, but suggested a minor contribution of the TRP-derived C to atmospheric C concentrations, and an influence on 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 C concentrations in air and rice predicted with various models were compared with observed concentrations. The modelling results showed that Gaussian plume models with different assumptions predict monthly-averaged C concentrations in air well and also that specific activity and dynamic models were equally good for the prediction of inter-annual changes in C concentrations in rice grain.