Transport, chemistry, and thermodynamics of uranium hexafluoride in the atmosphere—evaluation of models using field data

Accidental releases of uranium hexafluoride (UF6) can occur from nuclear fuel cycle facilities. Upon release to the atmosphere, UF6 enters into exothermic chemical reactions with atmospheric water vapor producing hydrogen fluoride (HF) and uranyl fluoride (UO2F2); HF undergoes further polymerization, depolymerization, and hydrolysis. A number of models have been used in the past to simulate the transport of UF6 and its reaction products in the atmosphere. Performances of two models, HGSYSTEM/UF6 and SAIC were assessed by comparing model predictions with field measurements. Field data for UF6 were available from three experimental releases made at Bordeaux in France between 1986 and 1989; an accidental release at Gore, Oklahoma; and an accidental release at the Comurhex Plant in France. The Gore and Comurhex data are of questionable quality since they were from a real accident. The Bordeaux data are of better quality since they were from a research-grade study. Predictions from the HGSYSTEM/UF6 model were within an order of magnitude of the observations, with most within a factor of two of the observations. Most predictions from the SAIC model were within an order of magnitude of the observations, but the model also over-predicted beyond an order of magnitude for a few observations. Detailed sensitivity analyses were also conducted on all modules of the HGSYSTEM/UF6 and SAIC models. At large distances from the source, the output concentration of total uranium is most sensitive to meteorological parameters; and at distances close to the source, it is most sensitive to certain source-specific and meteorological parameters.