(351e) Viability of Molten Salt Reactors for the Production of Molybdenum-99

This study examines the feasibility of producing molybdenum-99 in molten salt reactors (MSR). With the aging of current isotope-producing reactors and the rising demand for medical isotopes, future shortages are predicted to greatly increase the cost of Mo-99. Molten salt reactors have the potential for safe, efficient operation at a variety of scales. The liquid nature of the fuel makes processing the coolant salt without shutting down the reactor possible, a possibility which we will refer to as “in situ processing.” Such processing could be used to reduce the waste vector of the fuel, including long-lived radioisotopes. It can also be used to remove valuable fission products, including medical isotopes such as Mo-99. However, the viability of the MSR for Mo-99 production has not been rigorously analyzed. While Mo-99 is a major fission product, it is unclear how accessible it will be for direct extraction from a fuel salt given its relatively short half-life.

In this study, a computationally inexpensive mathematical model is used to analyze fission product accumulation in an MSR, with the specific goal of assessing the feasibility of in situ Mo-99 extraction. The model includes generation and removal of all isotopes in an MSR from fission, decay, activation and in situ processing or separation. The nuclear interaction data used in the model are taken from the ENDF/B-VII.1 database. The reactor model is based on a homogenous (liquid-fueled) mono-energetic neutron spectrum core design. The influence of several important MSR operating parameters were examined including uranium concentration, neutron energy, and power density, in addition to parameters that describe the separation process. This separation was assumed to be mass-transfer limited, owing to the relatively low concentrations of molybdenum in the reactor. Because the model is much less computationally expensive than large depletion codes, it was possible to perform the large number of simulations needed to determine the effect of reactor design parameters on molybdenum extraction. It was found that the concentration at which Mo was present in the reactor had a profound effect on the isotopic composition and the fraction of the Mo-99 that could be successfully extracted. If molybdenum is allowed to buildup in the reactor, the majority of Mo-99 is lost by decay or activation, leaving only a small fraction of Mo-99 in the extracted molybdenum. Therefore, separation processes that require a high relative molybdenum concentration for extraction are not desirable. On the other hand, separation processes become more difficult at low concentrations. Thus, a trade-off exists between the difficulty of the separation and effectiveness of the extraction as measured by the recovery fraction and product quality. It was also found that increasing the power density enabled improved Mo-99 recovery for separation processes at increased concentrations. These results demonstrate the use of a model to explore the viability of Mo-99 extraction from an MSR, the influence of operating parameters on that extraction, and the conditions under which Mo-99 extraction can be optimized.