UC Berkeley

With the growing demand for clean, carbon free electricity around the globe due to industrialization and increasing populations, nuclear reactors will become a necessity to supplement power from intermittent renewable sources. Advanced reactor designs such as the Fluoride Salt-cooled High Temperature Reactor, or FHR, are especially desirable due to their small, modular design, their passive safety systems, and their smaller capital costs. The reactor’s small size, high temperatures, and single-phase molten salt coolant meant that conventional heat exchanger designs could not be used as the primary form of heat removal. This lead to the development of the Coiled Tube Gas Heater, or CTGH, so that the reactor could be coupled with an air Brayton reheat cycle. The CTGH is a shell-and-tube heat exchanger that uses an annular tube geometry to reduce the overall volume of the heat exchanger. In the FHR design, the air flows up the center of the annular bundle and flows out radially through the coiled tubes. The molten salt coolant is distributed vertically to multiple tubes within multiple sub-bundles in the CTGH. Starting at the outer radius of the bundle, the salt tubes coil around the bundle multiple times before reaching the inner manifolds and flowing out the bottom of the heat exchanger. This creates a heat exchanger with high effectiveness due to a large heat transfer surface area density and with a design that is essentially a counterflow heat exchanger. By using seamless tubes that are in compression rather than tension, the design minimizes the points of stress concentration and the risk of tubes bursting. Due to its high effectiveness, relatively low pressure drops, compact design, structural integrity, and resistance to damage from thermal shocks, the CTGH is an optimal choice for the primary heat exchanger in the FHR design.

In order to design the CTGH specifically for the FHR, it was necessary to simulate the conditions in the FHR. Conventional modeling codes would take too long to model the complex geometry of the CTGH to be an effective design tool, so this dissertation developed an effectiveness modeling code specifically for CTGHs called Transverse Heat Exchange Effectiveness Model, or THEEM. THEEM is an unconventional finite volume method code that uses empirical heat transfer and pressure drop correlations instead of governing equations for its calculations. It was used to model the temperature and pressure distributions across the CTGH bundle and, consequently, to calculate the effectiveness, fluid outlet temperatures, and fluid pressure drops across the bundle. This program was then used to model the CTGH for the FHR.

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It found that this design, compact enough to fit on a rail car, would transfer the desired heat between the salt and the air, and it would have relatively low pressure drops for the salt and air.

In order to use THEEM as a design tool, it was necessary to validate the code experimentally. Two different experiments that both used water and air as heat transfer fluids were performed for this purpose. The first experiment primarily served as a proof of concept for fabrication of a CTGH tube bundle, but due to poor construction, it did not provide useful experimental data. The second experiment was constructed specifically for comparison with the THEEM code. Its measurements provided useful data to validate THEEM for larger CTGH designs. The second experimental setup also provided opportunities for other experiments. First, airflow measurements were taken around the bundle in order to measure the airflow distribution around the bundle, which can be used to model flow distribution in CTGH bundles. Next, using the Wilson plot method, the setup was used to derive empirical Nusselt number correlations for both the tube-side and shell-side of the heat exchanger. These can be used in system modeling codes to calculate the heat transfer in the CTGH for different reactors. Finally, the setup was used to perform binary impulse measurements for the CTGH. This measured the response of the heat exchanger to an immediate loss of heating power. With further additions to the setup, these tests could be modified to simulate the CTGH’s response to various reactor accidents and transients in both the FHR and other reactors.

Once THEEM had gone through initial experimental validation, it could be expanded as a design tool for other applications besides the FHR. First, a parametric study was performed to measure the effect of changing each aspect of the CTGH geometry on the outlet parameters of the bundle. These results were then used to develop an optimization tool that could design CTGHs for various applications outside of the FHR. This optimization tool used a Monte Carlo method algorithm as well as physical constraints set by the user to design the optimal CTGH for a given application. This tool was then used to design CTGHs for different applications coupling a single-phase coolant with a Brayton cycle. These examples included a CTGH coupling a sodium fast reactor with a supercritical carbon dioxide Brayton cycle, a CTGH coupling a different molten salt reactor with an air cooling system, and CTGHs used in different electrically-heated salt loops that thermally modeled nuclear reactors. This showed that the CTGH could be used in multiple nuclear applications and that THEEM would be an effective design tool for those CTGHs.

The work performed in this dissertation will be essential to deploying large scale CTGHs for the FHR and other reactors. The THEEM code and the results of the experiments in this dissertation can be used for performing a structural analysis of the heat exchanger, studying flow-induced vibration through the tube bundle, studying the effects of tube fouling over the lifetime of the heat exchanger, and performing a cost analysis on fabrication of the heat exchanger. With these future studies and the work presented in this dissertation, the CTGH design will become a more efficient and cost-effective heat exchanger for the FHR and other nuclear Brayton cycles.