The Materials Management and Minimization (M3) Reactor Conversion Program of the National Nuclear Security Administration (NNSA) is supporting the conversion of the research reactor from Highly Enriched Uranium (HEU, 235U / U ≥ wt. 20%) fuel to Low Enriched Uranium (LEU, 235U / U < wt. 20% ) fuel. There are three research reactors in the world actively engaged in conversion that utilize involute shaped fuel elements: High Flux Isotope Reactor (HFIR) in USA; High Flux Reactor (RHF) in France and FRM II in Germany. These reactors share a similar configuration of coolant channel, which is of extremely thin thickness and involute shape. Better understanding of flow behavior and heat transfer mechanisms in these coolant channel is of great interest and importance for the design of LEU fuel elements. As is well known, many of the most-widely used RANS approaches has exposed the limitations on the prediction of these turbulent flows.

The proposed project consists of two parts. The first one focuses on using LES to reproduce the flow behavior and collect the turbulent statistics in an involute coolant channel for high Reynolds number. The Reynolds number in the research reactor can range from 50,000 to 100,000. Performing LES simulation with higher Reynolds number is of high importance to investigate the impact of the Reynolds number on the flow and heat transfer mechanism in involute plate research reactor. The second portion of the work focuses on using Large Eddy Simulation to provide benchmark for the highly simplified involute coolant channel. Hundreds of RANS simulations with all kinds of turbulence models have been performed with different commercial codes, such as STAR-CCM+, COMSOL, ANSYS-CFX under the cooperation of three institutions (ANL, ILL, TUM). Discrepancy are found between RANS simulations with different turbulence model, flow condition and codes. The LES results will be used for benchmarking and further assess the uncertainties of RANS models.