Study of a Passive Decay Heat Removal System and Tritium Mitigation for Fluoride-salt-cooled High-temperature Reactors

Abstract

Molten Salt Reactors (MSRs) are a class of Generation IV nuclear reactors using molten salts as Heat Transfer Fluids (HTFs). MSRs have two major variants, namely, solid fuel reactor (salt-cooled reactor) and liquid fuel reactor (salt-fueled reactor). This study primarily focuses on the solid fuel MSRs, i.e., the Fluoride-salt-cooled High-temperature Reactors (FHRs).

FHRs adopt TRistructural-ISOtropic (TRISO) fuel particles, low-pressure liquid fluoride salts, and passive decay heat removal systems, such as the Direct Reactor Auxiliary Cooling System (DRACS). Although FHRs bring these benefits, there are a number of key technology gaps/issues that need to be addressed for FHR development. In this study, two key technology gaps/issues have been addressed for FHRs: (1) molten salt natural circulation in a passive decay heat removal system and (2) management/control of tritium, whose production rate in FHRs is expected to be significantly higher than that in Light Water Reactors (LWRs).

In this study, both the heat transfer and friction characteristics of molten salts in straight circular pipes were first numerically investigated by a Computational Fluid Dynamics (CFD) tool, STAR-CCM+. The CFD results were then validated by comparing with experimental data in the literature and correlation predictions. Our analysis showed that molten salts in the transitional and turbulent flow regimes acted as ordinary HTFs, for which conventional forced convective heat transfer correlations could be applied with ±20% uncertainties in general. As for molten salts in the laminar flow regime, however, both the correlation results and CFD predictions without considering the effects of buoyancy and radiative heat transfer in molten salts significantly underestimated the salt Nusselt number, which could be as large as 40% and 30% lower, respectively. These significant discrepancies were attributed to the fact that the buoyancy effect and radiative heat transfer effect were generally not negligible for laminar flows of molten salts, which was demonstrated by good agreement (within ±14% uncertainties) between experimental data and CFD predictions considering those two effects.

The molten salt natural circulation in a single loop and multiple coupled loops, similar to the passive decay heat removal system DRACS, was then investigated by a one-dimensional (1D) NAtural Circulation COde, NACCO, developed in this research. The above finding, i.e., the non-negligible effects of buoyancy and radiative heat transfer in laminar flows of molten salts, was further confirmed by comparing the code results with two high-temperature natural circulation experiments using NaNO3-KNO3 and FLiBe as coolants, respectively. This validated 1D code NACCO was then applied to a FLUoride Salt Test FAcility, FLUSTFA, which was designed based on a scaling analysis and constructed in this research to experimentally investigate the DRACS performance under steady-state and various transient scenarios.

In addition, another technology gap/issue, tritium management, was studied for FHRs. Two strategies were proposed for tritium mitigation in FHRs, namely, Double-Wall Fluted-Tube Heat eXchanger (DW-FTHX) with a tritium carrier and Single-Wall FTHX (SW-FTHX) with a tritium barrier options. These two options were then evaluated by a 1D coupled HEat and MAss Transfer code, HEMAT, developed and benchmarked in this study. It was found that both the optimum designs of the two options, DW-FTHX with a tritium carrier (helium) and SW-FTHX with a tritium barrier (silicon carbide), could effectively reduce the total tritium leakage rate to several curies per day, the same order of magnitude in a typical LWR.