Modeling and analyses of postulated UF6 release accidents in gaseous diffusion plant

Computer models have been developed to simulate the transient behavior of aerosols and vapors as a result of a postulated accident involving the release of uranium hexafluoride (UF6) into the process building of a gaseous diffusion plant. UF6 undergoes an exothermic chemical reaction with moisture (H2O) in the air to form hydrogen fluoride (HF) and radioactive uranyl fluoride (UO2F2). As part of a facility-wide safety evaluation, this study evaluated source terms consisting of UO2F2 as well as HF during a postulated UF6 release accident in a process building. In the postulated accident scenario, ∼7900 kg (17,500 lb) of hot UF6 vapor is released over a 5 min period from the process piping into the atmosphere of a large process building. UO2F2 mainly remains as airborne-solid particles (aerosols), and HF is in a vapor form. Some UO2F2 aerosols are removed from the air flow due to gravitational settling. The HF and the remaining UO2F2 are mixed with air and exhausted through the building ventilation system. The MELCOR computer code was selected for simulating aerosols and vapor transport in the process building. MELCOR model was first used to develop a single volume representation of a process building and its results were compared with those from past lumped parameter models specifically developed for studying UF6 release accidents. Preliminary results indicate that MELCOR predicted results (using a lumped formulation) are comparable with those from previously developed models