Tritium Transport

The Fluoride-salt-cooled High-temperature Reactor (FHR), one type of molten salt reactors, is a promising Generation IV nuclear reactor. However, control of radioactive fission gases, i.e., tritium, is a critical issue in FHRs due to the significantly larger tritium production rate and potentially larger tritium leakage rate compared to those in Light Water Reactors (LWRs). Therefore, it is necessary to investigate tritium transport phenomenon and study mitigation strategies.

 

The tritium transport process typically consists of the following steps: (1) A fraction of tritium molecules (T2) transfer to the liquid-solid interface between the tube-side fluid and tube by convection and diffusion in the tube-side fluid; (2) Each tritium molecule undergoes a dissociative chemisorption and becomes two atoms (T) on the inner surface of the tube; (3) Tritium atom permeates through the tube; (4) Two tritium atoms combine to form a tritium molecule on the outer surface of the tube; (5) A fraction of tritium molecules transfer to the liquid-solid interface between the shell-side fluid and shell by convection and diffusion in the shell-side fluid​, while the remaining tritium molecules are taken to the exit; (6) Undergoing similar steps, namely, dissociative chemisorption on the inner surface of the shell, permeation through the shell, combination on the outer surface of the shell, tritium finally escapes to the environment.

 

Two tritium mitigation strategies, namely, Double-Wall Heat eXchanger with a Tritium Carrier (DWHX-TC) option and Single-Wall Heat eXchanger with a Tritium Barrier (SWHX-TB) option, are proposed. For the DWHX-TC option, two tube surface configurations, i.e., plain and spirally fluted tubes, and four tritium carriers, i.e., helium, FLiBe, FLiNaK, and KF-ZrF4, are investigated. For the SWHX-TB option, two cases, tritium barriers with and without cracks, are investigated. In addition, an in-house, system-level, coupled HEat and MAss Transfer (HEMAT) code is developed to evaluate the mitigation strategies that are proposed for tritium control in FHRs.