During a boiling and drying accident, H2O and HNO3 vapors, aerosols, NOx, O2, and RuO4 are released into the gas phase and then to the environmental atmosphere. In a case when the cooling time of the reprocessed spent fuel is short, a large radiation dose might be given to the public due to 106Ru. Accordingly, it is important to know the leak path factor of RuO4. The H2O and HNO3 vapors will produce condensate in the leak path. When RuO4 is absorbed into the condensate containing HNO2 produced by absorbed NO2, it will change to nonvolatile Ru, which accelerates the RuO4 absorption rate.

We investigated the liquid-phase mass transfer coefficients of NO2 and RuO4 at around 100°C using the results from experiments with a glass flask and with analyses using the double film model. In the flask, the gas phase was stirred by a rotating blade to promote stirring, especially in the radial direction, and to decrease the gas-phase mass transfer resistance.

In the case of NO2 absorption, the gas-phase mass transfer resistance was negligible, and the liquid-phase mass transfer coefficient significantly depended on the temperature and nitric acid concentration in the condensate. In the case of RuO4, the gas-phase mass transfer resistance was not negligible and analysis considering this resistance showed that the liquid-phase mass transfer coefficient significantly depended on the NO2 concentration in the gas phase and the nitric acid concentration, but did not depend on the temperature.