Modeling the behavior of tritiated water vapor in a research reactor containment building

Mathematical models were developed to predict the changes in tritiated water (HTO) concentrations in water pools and of HTO vapor in the Kyoto University Reactor (KUR) containment building in which approximately 3.4 x 10(2) GBq of HTO vapor had leaked from a heavy water facility for more than 1 y. Models reveal that the mechanism of HTO vapor transfer between air and water is controlled by two key parameters: the kinetic constant for HTO exchange and the evaporation rate constant from water to air. A model was constructed based on laboratory experiments using small glass dishes containing various volumes of HTO and was validated by comparing estimates to actual measurements for HTO concentration in water pools of various depths in the containment building. After the leakage from the heavy water facility had been stopped, the decrease in HTO concentration in the sub-pool could be described by this model with a half-life of 15 wk. A mathematical model was also developed to estimate the average HTO vapor concentration in air, which is strongly dependent on the ventilation system's operation even after the removal of the HTO sources. This is due to the continued release of HTO from the concrete material and is analogous to the dynamics of radon emanation. The amount of HTO that has emerged from the concrete was estimated using a model developed for HTO concentration changes in the containment building air, based on long-term monitoring.