HTO transport and OBT formation after nighttime wet deposition of atmospheric HTO onto land surface

夜間降水による陸面への大気中HTO湿性沈着時のHTO輸送及びOBT生成

太田 雅和  ; 永井 晴康  

Ota, Masakazu; Nagai, Haruyasu

大気中トリチウム水(HTO)の土壌への湿性沈着及びその後のトリチウム(T)移行過程が、線量評価上重要となる有機結合型トリチウム(OBT)の生成に及ぼす影響の定量評価を目的とし、大気-植生-土壌複合系内T移行詳細計算モデルを開発して数値実験を行った。上記複合系内熱,水輸送及びCO交換モデル(SOLVEG-II)に導入した大気中HTO拡散,土壌中HTOの移流及び拡散,植生葉中T交換及びOBT生成過程に加え、新たに大気中HTOと降水間のHTO蒸発及び凝縮過程をモデル化してHTOの湿性沈着を計算可能な体系とした。降水の在る夜間におけるHTOの大気放出事象を仮定して数値実験を行った。夜間のHTOの湿性沈着及び日中の土壌から大気へのHTO再放出が組織自由水T(TFWT)変動及びOBT生成に及ぼす影響を定量評価した。

A simulation study was carried out, to evaluate formation of organically-bound tritium (OBT) after wet deposition of atmospheric HTO onto soil, a process which would be important for an accidental HTO release. To evaluate tritium transfers in atmosphere-vegetation-soil system, we adopted a sophisticated numerical-model SOLVEG-II which calculates diffusion of HTO in atmosphere, diffusion and advection of HTO in soil, and, tritium exchange and OBT assimilation in leaves, in conjunction with transport of heat and water, and CO exchange. Evaporation and condensation between atmospheric HTO and rainwater are newly modeled, to calculate wet deposition of atmospheric HTO. Then a simulation was carried out, assuming an acute release of atmospheric HTO in nighttime at which a rainfall event exists. Successive TFWT variation and OBT formation were calculated, and, importance of wet deposition and re-emission of HTO for the OBT formation was discussed.