SLOWPOKE-2 alternative core loading configurations analysis for highly improved reactor performance

Abstract

A thorough analysis was carried out and is presented here exploring the possibilities for improving the performance of the LEU-fueled SLOWPOKE-2 reactor design, mainly with regard to the fuel pins distribution throughout the core. The chosen performance metric to maximize was the thermal neutron flux at the inner irradiation sites, while the essential constraint was to maintain the same Onset of Nucleate Boiling Ratio (ONBR) at nominal power, as safety criterion. MCNP5 and PLTEMP/ANL codes were used for the neutronics and thermal hydraulics simulations respectively, to analyze the reactor behavior under almost 300 different core configurations or fuel loading patterns. The main proposed configuration, comprising 270 fuel pins and minor modifications to the fuel cage, is predicted to allow a performance improvement of about 75% while maintaining the same safety margins. Additional or alternative, more complex modifications of the reactor for further performance increases, involving changes to the fuel, the annular reflector and the control rod, are also suggested and analyzed in some detail. The basis to reduce or recover safety margin to reach higher power and flux levels, with the potential to at least double the current performance when combined with the suggested modifications, is also briefly examined. Possible effects of the higher reactor power on the reactor operation are considered as well, providing potential modifications to extend the maximum continuous operation time and the period between Be shim adjustments. Overall, this work provides a detailed basis for substantial potential performance improvements for SLOWPOKE-2 reactors that are pending conversion to LEU and those that may need refueling in the future. The work also points out potential performance improvements on MNSR reactors.