Prototype Miniature Neutron source Reactor (PMNSR) is a pool-tank type research reactor,applying high enrichment Uranium as fuel, light water as moderator and coolant, beryllium as reflector. Recently, in order to prevent nuclear proliferation, PMNSR carried out low enrichment uranium (LEU) core conversion, and the enrichment of U-235 decreased from 90% to under 20%. Research on PMNSR with LEU core mainly includes theory design, zero power experiment, core replacement. The physical design of PMNSR with LEU is the main part of theory design, which plays a great role in LEU conversion. At the first stage of LEU conversion, it performs preliminary physical calculation and analysis concerning U-235 fuel enrichment, and the number of critical fuel elements, the reactivity worth of control rod, the reactivity worth of top beryllium reflector, the neutron flux of inter-irradiation tube are calculated, which provides important data for the fuel elements design, fabrication, zero power test safety analysis and experiment for LEU conversion. In the second phase, it conducts the result verification on zero power test and preliminary physical design and a preliminary error analysis resulted from it thereof. More over, it modifies input file of LEU conversion, optimizes core element loading deployment, the reactivity worth of central control rod, the neutron flux rate of inner radiation site, offering statistics for the replacement and start-up experiments. In the last period, grounded on the counting abnormal analysis in loading, it explains the reasons with calculation results, completing PMNSR LEU conversion. PMNSR physical design takes the leading position in LEU conversion. It supplies reference data to ensure completion of PMNSR conversion and lays a theoretical foundation for Ghana and Nigeria MNSR LEU core conversion.

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