Feasibility and challenges of 22Na based radioactive tracer dilution for mass
measurement of molten salts

Molten salts play a crucial role as electrolytes in the pyroprocessing of spent nuclear fuels, as well as being used as fuel salt in molten salt reactors (MSRs). These salts contain special nuclear materials that are dissolved within them, and accurate knowledge and monitoring of their inventory is necessary for nuclear materials accounting and safeguards purposes. However, determining the total mass of molten salts is a difficult task due to the complicated shapes of containers in pyroprocessing, and density variation caused by fission, on-line refueling, temperature differential, and changes in composition over time during an MSR operation. In recent years, an innovative technique called radioactive tracer dilution (RTD) coupled with gamma spectroscopy has been proposed and evaluated as a potential means of accurately measuring the total mass of molten salt for pyroprocessing of spent nuclear fuels. In this study, 22Na was used as a radioactive tracer, and LiCl-KCl based tracer salt that bears 22NaCl was prepared and spiked into the molten LiCl-KClUCl3 salt for uranium electrorefining, a critical step for pyroprocessing. To evaluate the potential application of the 22Na based RTD for mass monitoring of the molten salt system in MSRs, a small amount of MgCl2-KCl-UCl3 (depleted uranium) fuel salt was irradiated in-core in a research reactor at The Ohio State University and then analyzed for the fission products’ gamma spectrum. The primary objective of this irradiation was to evaluate the potential gamma spectrum interference at 1274.5 keV of 22Na peak as well as the low energy peaks from 154Eu. In this paper, the feasibility and challenges of 22Na-based radioactive tracer dilution for mass measurement of molten salt systems were summarized, and some approaches for addressing the challenges were proposed for future research.