Analyzing thermal characteristics of projected molten salt compositions in EBR-II used nuclear fuel processing

This study evaluates the evolution of the composition and thermal behavior of the Mark-IV electrorefiner salt at Idaho National Laboratory during continued processing of sodium-bonded Experimental Breeder Reactor II driver fuel. MASTERS flowsheet simulations were used to project salt composition changes during the remaining campaign, and thermodynamic equilibrium calculations were coupled to those compositions to estimate phase behavior, liquidus temperature, and operating margin. Thermal measurements were performed on actual Mark-IV electrorefiner salt: differential scanning calorimetry used a December 2022 dip sample, and bulk-cooling experiments used an approximately 100 g salt sample recovered directly from the Mark-IV salt cell in March 2026. The measured liquidus range of 408 to 437 ^∘C brackets the model-projected initial liquidus of approximately 422 ^∘C, providing quantitative experimental support for the thermodynamic calculations. Under current operating assumptions, the liquidus temperature is projected to remain essentially constant at approximately 425 ^∘C throughout the present campaign, rising by less than 3 ^∘C from the initial value of 422 ^∘C to 425 ^∘C at end of campaign, while remaining well below the nominal 500 ^∘C electrorefiner operating temperature, corresponding to a thermal margin of approximately 75 ^∘C. Alternative processing scenarios further show that operating choices can materially affect salt inventory, composition, and liquidus behavior. These results demonstrate the value of combining process simulation, thermal measurements, and thermodynamic interpretation to support electrorefiner campaign planning and salt management.