Numerical Study of Tritium Mitigation Strategies for Fluoride Salt-Cooled High-Temperature Reactors

Fluoride salt-cooled High-temperature Reactor (FHR) is one of the advanced non-Light Water Reactor (non-LWR) designs, which adopts a low-pressure fluoride salt as the primary coolant, high working temperatures, coated-particle fuel, and a passive safety system for decay heat removal. However, tritium management is perceived as a critical issue for FHRs since tritium is a radiation hazard when inhaled or ingested and its production rate in FHRs is expected to be significantly higher compared to that in LWRs. To reduce FHR tritium release rates into the ambient, two tritium mitigation options, such as using Double-Wall Fluted-Tube Heat eXchangers (DWFT-HXs) with a tritium carrier or Single-Wall Fluted-Tube HXs (SWFT-HXs) with a tritium barrier, are therefore proposed for key HXs in FHRs, which potentially provide major pathways for tritium release due to their elevated temperatures and large surface areas. Tritium carriers investigated include gases, such as helium, and liquids, such as FLiBe, FLiNaK, and KF-ZrF4, while the tritium barrier investigated in this paper is silicon carbide (SiC) due to its low permeability for tritium. These proposed HX designs are then optimized, using a Non-dominated Sorting in Generic Algorithms (NSGA) optimization approach, for the Advanced High-Temperature Reactor (AHTR), one of the FHR designs with a large power output. A system-level mass transfer model is developed to evaluate the tritium transport in the two proposed design options for tritium mitigation in FHRs and quantitively analyze the tritium release/leakage rate from the reactor primary system. Our study shows that both the DWFT-HX design with helium as the tritium carrier and SWFT-HX design with SiC coating as the tritium barrier are able to reduce the total tritium leakage rate in FHRs to the same order of magnitude of the typical average tritium leakage rate in LWRs (1.9 Ci/day).