Boiling and Drying Accident of High-Level Liquid Waste in a Reprocessing Plant: Measurement of the Liquid-Phase Mass Transfer Coefficients of NO₂ and RuO₄ to the Condensate

During a boiling and drying accident, H₂O and HNO₃ vapors, aerosols, NO_x, 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.