ANALYSIS OF LOSS-OF-FLOW ACCIDENTS FOR THE NIST RESEARCH REACTOR WITH FUEL CONVERSION FROM HEU TO LEU

O-moderated researchreactor (NBSR) at the National Institute of Standardsand Technology. A RELAP5 model has been developed toanalyze postulated accidents in the NBSR with the presentHEU fuel and a proposed LEU fuel. The model includesthe reactor vessel, primary pumps, shutdown pumps,various valves, heat exchangers, and average and hottestfuel elements and flow channels in the region where flowenters through an inner plenum (6 fuel elements) and aregion where flow enters through an outer plenum(24 elements). The equilibrium cycle power distributionsin the fuel elements were determined based onthree-dimensional Monte Carlo neutron transport calcu-lations performed with the MCNPX code.In this paperwediscuss safety analysesconducted forthe loss-of-flow accidents resulting from either loss ofelectrical power or inadvertent throttling of flow controlvalves at the inlets to the inner and outer plena. Theanalysis shows that the fuel conversion will not lead tosignificant changes in the safety analysis and that thecalculated maximum clad temperatures, minimum criticalheat flux ratios, and minimum onset of flow instabilityratios assure that there is adequate margin to fuel failure.