Deflections of Plates in Research Reactor Fuel by Disparities in Thicknesses of Flow Channels

Low-enriched uranium (LEU) fuel element designs for the U.S. high performance research reactors LEU conversion cores have been optimized by each reactor facility to allow the reactors to meet mission, operational, and safety basis requirements using monolithic uranium-molybdenum fuel. As a part of work supporting the Preliminary Safety Analysis Report (PSAR) submitted to the NRC by the University of Missouri Research Reactor (MURR), the impact of thinner 1.12 mm LEU curved fuel plates, compared to the currently used highly-enriched uranium (HEU) curved plate thickness of 1.27 mm, has been assessed for hydro-mechanical performance. Plate deflection can be induced by the hydrodynamic pressure differential caused by differences in the thicknesses of surrounding coolant flow channels. An experimental study was conducted on relevant Materials Test Reactor-type (MTR-type) reactor plate geometries in a water flow test loop to validate computational models simulating flow-induced plate deflection. Three-dimensional fluid-structure interaction (FSI) simulations of the experiments were performed using several commercially available multi-physics simulation codes. Inclusion of as-built geometry of the plates and channels in the simulations was key to achieving good agreement with measured deflections. The validated computational methodology was applied to a model of the prototypic MURR LEU plate. For the nominal flow conditions, a small deflection of the plate on the order of 5 micrometers was predicted. That deflection is significantly less than the allowances in the PSAR for change in coolant channel thickness. The experimental model validation of plate deflection is important since conventional figures of merit for the robustness of MTR-type fuel plates under flow, such as the Miller critical velocity, often show a weak correlation with the prediction of stability. Subsequent to this work, irradiation qualification of the MURR LEU fuel element design has begun and will conclude with a full-size demonstration element test.