Processes affecting land-surface dynamics of 129I impacted by atmospheric 129I releases from a spent nuclear fuel reprocessing plant

Terrestrial environments impacted by atmospheric releases of I-129 from nuclear plants become contaminated with I-129; however, the relative importance of each land-surface I-129-transfer pathway in the process of the contamination is not well understood. In this study, transfers of I-129 in an atmospherevegetation-soil system are modeled and incorporated into an existing land-surface model (SOLVEG-II). The model was also applied to the observed transfer of I-129 at a vegetated field impacted by atmospheric releases of I-129 (as gaseous I-2 and CH3I) from the Rokkasho reprocessing plant, Japan, during 2007. Results from the model calculation and inter-comparison of the results with the measured environmental samples provide insights into the relative importance of each I-129-transfer pathway in the processes of I-129 contamination of leaves and soil. The model calculation revealed that contamination of leaves of wild bamboo grasses was mostly caused by foliar adsorption of inorganic I-129 (81%) following wet deposition of I-129. In contrast, accumulation of I-129 in the leaf due to foliar uptake of atmospheric I-129(2) (2%) was lesser. Root uptake of soil I-129 was low, accounted for 17% of the I-129 of the leaf. The low root-uptake of I-129 in spite of the I-129 contained in the soil was ascribed to the fact that the most fraction (over 90%) of the soil I-129 existed in soil-fixed (not plant-available) form. Regarding the I-129-transfer to the soil, wet deposition of I-129 was ten-fold more effective than dry deposition of atmospheric I-129(2); however, the deposition of I-129 during the year represented only 2% of the model-assumed I-129 that pre-existed in the soil; indicating the importance of long-term accumulation of I-129 in terrestrial environments. The model calculation also revealed that root uptake of inorganic I-129 can be more influential than volatilization by methylation in exportation of I-129 from soil. (C) 2019 The Authors. Published by Elsevier B.V.