Thermodynamic models of the solution chemistry in a homogeneous reactor used for molybdenum-99 (sub 99Mo) production are being developed at Argonne National Laboratory (Argonne) to aid in the optimization of molybdenum generation and recovery processes. The models calculate concentrations of aqueous species and saturation states of possible uranium and fission product precipitates (e.g., uranyl oxide hydrates, peroxides, molybdates) over a relevant range of pH values, oxidization/reduction potentials, and component concentrations. Predicting conditions under which precipitation may occur is particularly important because solids formed in the reactor solution could complicate 99Mo recovery and have deleterious effects on reactor operation. Results presented in this paper indicate that, for uranyl nitrate based reactors, the radiolytic decomposition of nitrate (loss of nitrogen to off-gas) could cause the precipitation of uranyl and/or fission products due to an increase in pH. This precipitation process is readily counteracted by the addition of nitric acid. The modeling results presented help quantify the optimal pH envelope for a uranyl nitrate solution reactor used for 99Mo production and highlight the need for future experimental studies to address uncertainties in the thermodynamic models.
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