Tritium control is potentially a critical issue for Fluoride salt-cooled High-temperature Reactors (FHRs) and Molten Salt Reactors (MSRs). Tritium production rate in these reactors can be significantly higher compared to that in Light Water Reactors (LWRs). Tritium is highly permeable at high temperatures through reactor structures, especially. Therefore, heat exchangers with large heat transfer areas in FHRs and MSRs provide practical paths for the tritium generated in the primary salt migrating into the surroundings, such as Natural Draft Heat Exchangers (NDHXs) in the direct reactor auxiliary cooling system (DRACS), which are proposed as a passive decay heat removal system for these reactors.

A double-wall heat exchanger design was proposed in the literature to significantly minimize the tritium release rate to the environment in FHRs. This unique shell and tube heat exchanger design adopts a three-fluid design concept and each of the heat exchanger tube consists of an inner tube and an outer tube. Each of these tube units forms three flow passages, i.e., the inner channel, annular channel, and outer channel. While this type of heat exchangers was proposed, few such heat exchangers have been designed in the literature, taking into account both heat and tritium mass transfer performance.

In this study, a one-dimensional heat and mass transfer model was developed to assist the design of a double-wall NDHX for FHRs. In this model, the molten salt and air flow through the inner and outer channels, respectively. A selected sweep gas acting as a tritium removal medium flows in the annular channel and takes tritium away to minimize tritium leakage to the air flowing in the outer channel. The heat transfer model was benchmarked against a Computational Fluid Dynamics (CFD) code, i.e., ANSYS Fluent. Good agreement was obtained between the model simulation and Fluent analysis. In addition, the heat and mass transfer models combined with non-dominated sorting in generic algorithms (NSGA) were applied to investigate a potential NDHX design in Advanced High-Temperature Reactor (AHTR), a pre-conceptual FHR design developed by the Oak Ridge National Laboratory. A double-wall NDHX design using inner and outer fluted tubes was therefore optimized and compared with a single-wall design in terms of performance and economics.

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