The impact of neutron irradiation, graphite oxidation and fluorination on tritium uptake into and desorption from graphite in molten salt environments

Abstract

Tritium management is necessary in both fission and fusion nuclear reactors. In fusion reactors, tritium is a fuel that needs to be produced in breeding blankets. For fission reactors, and especially Fluoride Salt-Cooled High-Temperature Reactors (FHRs) and Molten Salt Reactors (MSRs), tritium is a contaminant to be separated and removed. The current literature on high-temperature hydrogen-graphite interactions is generated predominantly by the fusion research community and does not yet cover the low-pressure interval of relevance for FHRs and MSRs. Predictions of graphite uptake capacities and uptake rates at FHR conditions of few Pa partial pressure can only be performed using extrapolations. In order to make reliable extrapolations from the available data, the impact of phenomena that take place during operation and accident scenarios, such as neutron irradiation, air oxidation and reactions with molten fluoride salts must be accounted for. This article provides a summary of hydrogen-graphite thermodynamics and kinetics of interaction and discusses the effects of irradiation, air oxidation and fluorination on uptake capacities, uptake and desorption kinetics. We find that all three phenomena increase uptake capacities in graphite. Neutron irradiation and reactions with fluoride salts are expected to reduce tritium uptake rate, while oxidation is expected to increase it. In all three cases, the changes are more pronounced at low tritium partial pressures.