Improvement of the RuO vapor-liquid transfer model in the chemical behavior analysis code SCHERN for accident of evaporation to dryness by boiling of reprocessed high level liquid waste

Yoshida, Kazuo ; Hiyama, Mina*; Tamaki, Hitoshi 

An accident of evaporation to dryness by boiling of high-level radioactive liquid waste (HLLW) is postulated as one of the severe accidents caused by the loss of cooling function at a fuel reprocessing plant. In this case, volatile radioactive materials, such as ruthenium (RuO) are released from the tanks with water and nitric-acid mixed vapor into the atmosphere. Accurate quantitative estimation of released Ru is one of the important issues for risk assessment of those facilities. RuO is expected to be absorbed chemically into water dissolving nitrous acid (HNO). This behavior has been experimentally confirmed and plays an important role in the migration of Ru in the facility. A new model has been proposed as a chemical and physical absorption model based on the experimental results of the migration of RuO into nitric acid-water mixtures. In this study, to improve the analytical performance of SCHERN, these new analytical models have been incorporated and attempted to analyze the behavior of RuO in each phase. As a result, it has been observed a tendency that HNO in the liquid phase increases rapidly during the late boiling phase, when RuO release increases rapidly, and confirmed that this HNO concentration change significantly affects the subsequent migration behavior of RuO. These results indicate that it is essential to improve the analytical accuracy of the chemical behavior of HNO in each phase.