Decay heat characterization for molten salt reactor concepts with power density, humidity, and air ingress as uncertain parameters

Accurately estimating post-shutdown decay heat is critical for molten-salt reactors (MSRs) yet remains uncertain because nuclear inventories evolve under coupled chemical and corrosion processes. We address this gap by linking high-fidelity depletion, phase-equilibria, and corrosion models in a Monte-Carlo workflow that samples power density, cover-gas humidity, and pre-shutdown air ingress. The framework is applied to 10,000 realizations each of two fast-spectrum concepts: a LiF-ThF4-UF4 fluoride salt and a NaCl-UCl3 chloride salt and decay heat is analyzed up to 10,000 years. Time-resolved decay heat, elemental inventories, Pearson complements, and total Sobol indices are extracted to identify which parameters and isotopes control the prompt pulse, intermediate shoulder, and millennial tail. Isotopic decomposition and variance apportionment highlight how external chemistry shifts the dominant heat carriers. Results show that humidity and oxygen explain up to 30% of the variance in select actinides and corrosion metals even though power density fixes most means. The fluoride tail is governed by the 233Pa  233U chain, whereas the chloride tail is set by plutonium–americium decay, implying different redox-control priorities. These insights translate directly into cover-gas specifications and spent-salt container designs for future MSR deployments. The workflow establishes a platform for computing the decay heat for optimizing every stage of the MSR fuel life-cycle, from in-core operation to final geologic isolation.