Terrapower’s Molten Chloride Fast Reactor (MCFR): Advancements in Electrochemical Monitoring and Control
Authors: Unger Aaron , Chatterjee Sayandev , Byrne Nicole 
Published: December 2024

TerraPower’s Molten Chloride Fast Reactor (MCFR) project is pioneering a nuclear reactor technology that leverages fast spectrum chloride as a fuel salt and chloride salts as coolant salts, offering vast potential for decarbonizing various economic sectors. The fast spectrum enhances the neutron economy, mitigating fission product poisoning. Utilizing a NaCl-UCl3 binary eutectic as the fuel salt, the reactor undergoes significant chemical transformations during burn-up. The resulting species from the fission process, including Kr(0), Xe(0), Rb(I), Cs(I), Sr(II), Ba(II), Zr(II), Nb(0), Mo(0), Te(0), I(0), Pd(II), Re(I), Rh(II), Ag(I), Cd(II), rare earths(III), are in oxidation states equal to or lower than the initial U(III). This transformation leads to an oxidizing melt, potentially converting U(III) to U(IV), which can trigger oxidative corrosion of the structural materials.

To address these challenges, TerraPower is employing diverse electrochemical and electroanalytical approaches for process monitoring and control, helping bridge existing knowledge gaps in fuel and coolant salts. These methods provide key insights into species thermodynamics and kinetics, enabling direct measurement of redox information crucial for the operation of a molten salt nuclear reactor. This presentation will discuss TerraPower’s efforts in applying diverse electrochemical and electroanalytical approaches towards process monitoring and process control, addressing the knowledge gaps that exist for both fuel and coolant salts.

Basic electrochemical experiments on initial fuel salt provide data for models that predict and prevent corrosion, identify species affecting reactor physics, and offer real-time monitoring of the need to remove fission product poisons. Spectroelectrochemical methods are adapted to examine how redox transformations impact the coordination and speciation of uranium complexes in the fuel, crucial for predicting precipitation and volatilization of U(IV) species. This presentation will discuss TeraPower’s progress in electrochemical and spectroelectrochemical experiments.

TerraPower has designed two novel electrochemical and spectroelectrochemical cells that serve as testbeds for understanding fundamental redox chemistry and developing probes for process monitoring. For instance, measuring salt potential relative to the ratio of uranium oxidation states enables monitoring the U(III):U(IV) ratio, crucial for maintaining an optimal reactor environment. The temperature dependency of this value will also be characterized, providing a reference for system perturbations.

The presentation will also touch upon the influence of fission and corrosion products on the equilibrium behavior of uranium throughout the fuel's operational life. Fission products may reach substantial concentrations in the salt, imposing varied redox conditions that drive different behaviors in uranium. The dependence of uranium oxidation state ratio on burnup will be evaluated, and method development will explore the relationship between chlorine potential and species solubility.

Through this comprehensive approach, TerraPower aims to advance the MCFR technology, ensuring safe and efficient nuclear energy generation with significant benefits for decarbonizing the economy.