Large-eddy simulations of recirculation zones in channel-type molten salt reactors

In an effort to curb carbon emissions and mitigate the effects of climate change, energy policymakers are considering advanced nuclear reactors as a potential source of clean base-load energy. Once such family of reactor designs is called the Molten Salt Reactor, which has been successfully demonstrated experimentally during the operation of the Molten Salt Reactor Experiment(MSRE) at the Oak-Ridge National Laboratory during the 1960s. Thermal-hydraulic simulations of this reactor are an important step towards validating and verifying simulation tools for other molten salt reactor designs and exploring the potential of such reactors for licensing. Modern CFD simulations of the MSRE reactor core often discount the effects of turbulence in this reactor due to the low Reynolds number inside the MSRE channels. They also neglect the pyramidal tip at the top of the moderator graphite blocks called stringers. However, recent research indicates turbulence can play a significant role in compact reactor cores at relatively low Reynolds numbers. Our main concern is the entrainment of fuel salt in recirculation zones and the subsequent creation of localized hotspots. Therefore, we investigated the presence of such recirculation zones and the effect of the tip-shape on turbulence and stationary vortices in the upper plenum. We analyzed the flow around an MSRE graphite stringer using large-eddy simulations performed in Nek5000. We also studied the effects of varying the size and shape of the stringer-tip using Nek5000’s mesh deformation capabilities. To our knowledge, this work is the first effort to apply large-eddy simulations to the MSRE and study the effects of geometry-induced turbulence and recirculation within the MSRE. We analyzed the output data and found that salt recirculation vortices do exist and geometry and turbulence affect the salt flow in the upper plenum. But fuel salt recirculation does not impact the temperature of the salt or graphite significantly. We also determined the ideal tip shape that minimizes salt recirculation and entrapment. We found that a pyramidal tip with an apex half-angle of 45◦ disrupts recirculation, encourages mixing, and improves heat transfer, keeping salt and graphite temperatures low, whereas angles that are as small as 30◦ or large enough to tend towards a flat-top lead to hotter temperatures. Implications for the design and simulation of similar channel-type molten salt reactors are discussed