Numerical analysis of isotope production in molten salt reactors: A case study for Molybdenum-99 production

Abstract 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 been proposed for the production of Mo-99, and have the potential for safe, efficient operation at a variety of scales. However, 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 efficient mathematical model was used to analyze fission product accumulation in an MSR, with the specific goal of assessing Mo-99 production by continuous or near-continuous extraction from the reactor. The model includes generation and removal of isotopes in an MSR from fission, decay, activation and extraction. The extraction process was assumed to be mass-transfer limited, owing to the noble behavior and relatively low concentration of molybdenum in the reactor. It was found that the concentration at which Mo was present in the reactor had a profound effect on the specific activity 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 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.

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