In advanced fission and fusion reactors, high heat capacity fluoride molten salts allow operation at high temperature and atmospheric pressure, which could dramatically increase efficiency, reduce capital cost, and enable passive safety features. In these systems, tritium is of particular importance due to its ability to corrode structural materials as 3H+ in fluoride salts, and its potential to cause significant radioactive release as diffusive 3H0, which are cited as key barriers to technological deployment. Yet, the chemistry and transport behavior of the hydrogen species remain poorly understood due to the difficulties in handling and controlling experimental impurities in toxic and radioactive materials. To address these challenges, we used ab initio molecular dynamics to examine the coupled effects of tritium speciation and transport in prototypical salts 66.6%LiF-33.3%BeF2 (Flibe) and 46.5%LiF-11.5%NaF-42%KF (Flinak). Using extensively validated calculations on the local structure and dynamics, we find significant difference between 3H0 and 3H+ transport behaviors that are usually overlooked. We find that 3H0, which always exists as molecular H2 diffuses 3-5 times faster than 3H+ due to differences in their bonding and complexation in solution. Moreover, we examine the interactions between corrosion and atmospheric impurities and tritium atoms. We find that tritium is able to isotopically exchange with H2O (from moisture ingress) and bind to dissolved CrF complexes (corrosion product) reducing their transport mobility. This work explains contradicting experimental results and provides useful species data for predicting tritium transport and chemistry in fluoride salts.

Impact of tritium valence and impurities on chemical structure and transport in fluoride salts

ACS Spring Meeting 2021

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