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

摘要

Terrestrial environments impacted by atmospheric releases of ~(129)I from nuclear plants become contaminated with ~(129)I; however, the relative importance of each land-surface ~(129)I-transfer pathway in the process of the contamination is not well understood. In this study, transfers of ~(129)I in an atmosphere-vegetation-soil system are modeled and incorporated into an existing land-surface model (SOLVEG-Ⅱ). The model was also applied to the observed transfer of ~(129)I at a vegetated field impacted by atmospheric releases of ~(129)I (as gaseous I_2 and CH_3I) 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 ~(129)I-transfer pathway in the processes of ~(129)1 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 ~(129)I (81%) following wet deposition of ~(129)I. In contrast, accumulation of ~(129)I in the leaf due to foliar uptake of atmospheric ~(129)I_2 (2%) was lesser. Root uptake of soil ~(129)I was low, accounted for 17% of the ~(129)I of the leaf. The low root-uptake of ~(129)I in spite of the ~(129)I contained in the soil was ascribed to the fact that the most fraction (over 90%) of the soil ~(129)I existed in "soil-fixed" (not plant-available) form. Regarding the ~(129)I-transfer to the soil, wet deposition of ~(129)I was ten-fold more effective than dry deposition of atmospheric ~(129）I_2; however, the deposition of ~(129)I during the year represented only 2% of the model-assumed ~(129)I that pre-existed in the soil; indicating the importance of long-term accumulation of ~(129)I in terrestrial environments. The model calculation also revealed that root uptake of inorganic ~(129)I can be more influential than volatilization by methylation in exportation of ~(129)I from soil.