Tritium Control and Mitigation in Fluoride-Salt-Cooled High-Temperature Reactors

Abstract

Fluoride-salt-cooled high-temperature reactors (FHRs) receive wide attention because of their advantageous passive safety features and potential high thermal efficiency in power generation when compared with current commercial nuclear reactors. However, tritium generation and release are potentially significant challenges of FHRs, and tritium in the reactor need to be carefully handled and managed. This dissertation proposes a potential solution for tritium management in FHRs.

In this study, meteorological models were built to perform tritium-dispersion analyses and to evaluate the potential health impact of chronic airborne tritium release on the public and the contributions of meteorological parameters to the dose assessment, which are rarely investigated. Key meteorological and geometric parameters in favor of reducing the impact on the public were then identified. Uncertainty analysis and sensitivity study were performed for the dose assessment of three potential FHR construction sites, and daily tritium release limits were calculated.

The tritium-dispersion analyses indicated that the release of tritium to the environment must be controlled and limited in FHRs. A tritium control and mitigation system was designed both to reduce the tritium release rate from FHRs and potentially to eliminate an intermediate loop in the pre-conceptual design of the advanced high-temperature reactor (AHTR) as well as other FHR designs. The proposed system consists of four major components: redox control, a cross-flow tritium removal facility, an optional double-wall intermediate heat exchanger (IHX), and tritium-permeation-barrier coating on structural materials as necessary. Comparisons of different tritium control strategies from an economic assessment show that the proposed two-loop FHR design with the tritium control and mitigation system exhibits economic advantages over the original three-loop AHTR design.

To verify the performance of the proposed tritium control and mitigation system, a tritium transport-analysis model was developed. A novel method involving the logarithmic mean square root of partial pressure difference (LMSPD) was developed to calculate mass transfer in complex geometries and flow configurations. The method was implemented in MATLAB and validated using data from a hydrogen-permeation experiment available in the literature. The obtained calculation results agree reasonably well with the experimental data—especially in the temperature range of interest to this study.

To further validate the LMSPD code and the cross-flow configuration of the tritium removal facility, laboratory-scale experiments were designed using hydrogen as a surrogate for molecular tritium (T2). A hydrogen-removal experiment was carried out using the reactor off-gas krypton as the carrier gas for H2. The obtained experimental data confirm the effectiveness of the cross-flow tritium-removal design, and the code calculation results validate the LMSPD method.

In summary, the results and conclusions of this study provide significant insights for tritium management in FHRs. The atmospheric-dispersion modeling of airborne tritium identifies the major influencing factors on public dose assessment and the corresponding measures needed to help reduce health impacts. This study also contributes to future research on tritium control strategies. A cross-flow tritium removal facility was designed as the key component of a tritium-mitigation strategy. A novel LMSPD model was developed and validated for tritium extraction in the cross-flow configuration. The model can be used for future mass transfer calculations in complex geometries and flow configurations. In addition, hydrogen-removal experiments were designed to evaluate the efficiency of the cross-flow tritium removal facility, which, if carried out in a molten salt environment, will serve as valuable data for the FHR community.