低圧環境下での塩化物添加熱処理による土壌除染

Soil decontamination by heat treatments with chlorides under low-pressure conditions

下山 巖   ; 小暮 敏博*; 奥村 大河*; 馬場 祐治  

Shimoyama, Iwao; Kogure, Toshihiro*; Okumura, Taiga*; Baba, Yuji

放射性Cs汚染土壌は熱処理により99%以上の除染が可能だが、1000C以上の高温を要するため処理コストの大きさが課題である。我々はより低温での効率的な除染のため、4種類の塩化物反応剤による低圧環境下での熱処理を試みた。約1万Bq/kgの汚染土壌に対し、アルカリ塩を重量比1/1で添加し20Pa程度の低圧もしくは大気中で2時間の熱処理を行った。処理後の土壌に対し、放射能変化と構造変化を調べた。CaClの場合は圧力条件による除染率の差は小さく、790Cで約95%の除染率が得られた。一方、MgCl, NaCl, KClの場合は大気中よりも低圧条件の方が高い除染率を示した。MgCl添加時は低圧下で黒雲母の分解と相変態が促進され、695Cの熱処理に対し大気条件では除染率40%だったのに対し、低圧下では90%に達した。NaClとKClの場合、黒雲母は分解せずイオン交換によりCsが除去され、NaCl添加時は790Cでの除染率は大気圧下15%に対して低圧下94%であった。我々はこれらの知見に基づいて、海水塩を反応剤に用いた低圧熱処理について提案する。

More than 99% of radioactive Cs can be removed from soil by heat treatment at more than 1000C, however, its high cost inhibits application of this method. In this work, we attempted heat treatment with four kinds of chloride reagents under low-pressure condition to decrease processing temperature. Each reagent was added to contaminated soil and heat treatment was applied in air and under low-pressure conditions for two hours. In the case of CaCl, the difference in decontamination ratio was small for different pressure conditions and decontamination ratio reached to 95% at 790C. On the other hand, MgCl, NaCl, and KCl had higher decontamination ratios under low-pressure conditions than in air. When MgCl was added, decomposition and phase transformation of biotite in soil was facilitated under low-pressure conditions, and decontamination ratios at 695C were 40 and 90% in air and under low-pressure conditions, respectively. In the cases of NaCl and KCl, biotite remained after the heat treatments and Cs was removed through ion exchange. When NaCl was added, decontamination ratios at 790C were 15 and 94% in air and under low-pressure conditions, respectively. Based on these results, we propose heat treatment with sea water salt as a reactant.