Safety handling characteristics of high-level tritiated water

高濃度トリチウム水の安全取扱特性

林 巧; 伊藤 剛士*; 小林 和容; 磯部 兼嗣; 西 正孝

Hayashi, Takumi; Ito, Takeshi*; Kobayashi, Kazuhiro; Isobe, Kanetsugu; Nishi, Masataka

核融合炉では高濃度のトリチウム水が発生し一次保管される。トリチウム水は、自己放射線分解により水素や酸素ガスを発生するうえに過酸化トリチウム水となると考えられる。しかし、トリチウム水を用いた系統的実験の報告例は少なく、発生量は線による放射線分解のG値(100eVのエネルギーを吸収した時の特定化学種分子の生成率)とは異なっている。本報告では、高濃度トリチウム水溶液を製造後、長期保管した結果を整理し、気相発生成分については、実効的なG値のトリチウム濃度及び温度依存性データから、防爆上必要な設計上の配慮(想定トリチウム水濃度/量とタンク容積及びその換気回数など)を議論する。また、液相発生成分については、トリチウム水の液性(水素イオン濃度や酸化還元電位など)に影響をあたえる(トリチウム濃度に依存する)ことを見いだし、その理由を考察するとともに、タンクの腐食防止に必要な設計上の配慮について議論する。

In a fusion reactor, high-level tritiated water of more than GBq/ml will be generated and stored temporally in the various areas. High level tritiated water decomposes by itself and generates hydrogen and oxygen, and becomes to tritiated hydrogen peroxide water, however, effective G-values from tritiated water are different from those obtained -ray experiments in our previous report. Furthermore, tritiated water of about 250GBq/ml has been stored for several years safely and checked its characteristics. Using the above experiences, this paper summarizes safety requirements for storage of high-level tritiated water and discusses design issues of the safety storage system. Concerning gaseous species, storage tank should be maintained at negative pressure and purged periodically or constantly to dedicated tritium removal system. Specially, it is important that the G-value of high-level tritiated water is increasing with decreasing the tritium concentration. The pH and ORP (Oxidation Reduction Potential) of tritiated water have been also changed depending on the tritium concentration and maintained for more than several years in glass vessel. High-level tritiated water of more than GBq/ml was acid and became to be corrosive depending on the dissolved species. Large amount of tritiated water will be stored in the various tanks of stainless steel, therefore, it should be monitored so that the liquid situation is maintained not to be corrosive.