Capacitance sensors for the detection of uranium dioxide precipitation in fluoride-based molten salt reactors

Molten salt reactors (MSRs) fueled with fluoride-based salts possess an inherent risk of developing uranium dioxide precipitation, either from atmospheric contamination during salt processing or the activation and decay of fluorine, which produces oxygen. Concentrations of UO₂ reduce fuel inventory and pose a criticality risk. Limited MSR operations history suggests that additional methods of monitoring for UO₂ precipitation are required. This thesis investigates the potential for capacitive sensors to monitor for UO₂ precipitation. Adapting methods used for monitoring two-phase flow in water and gas systems, two electrode designs were simulated on a Hastelloy-N pipe containing ⁷LiF-BeF₂ fueled with UF₄. Capacitance values were determined against increasing amounts of UO₂ precipitation. The ring electrodes produced a sinusoidal response on the order of 10⁻¹⁰ farads and the concave plate electrodes produced an exponential response on the order of 10⁻¹² farads. This suggested that capacitive sensors are a viable non-destructive testing method for determining precipitation and reducing the risk of criticality accidents in MSRs. A stronger response may be possible by using impedance methods due to significantly elevated values for the dielectric constant of UO₂ at low frequencies and further research into capacitive sensors is encouraged.