2007 International RERTR Meeting

Abstracts and Available Papers Presented at the Meeting

MNSR Flux Performance and Core Lifetime Analysis with HEU and LEU Fuels

J. R. Liaw and J. E. Matos
RERTR Program
Nuclear Engineering Division
Argonne National Laboratory
Argonne, Illinois 60439-4815 USA

ABSTRACT

This paper discusses the results of flux performance and the core lifetime analyses for conversion of an MNSR core using LEU-UO₂ fuel containing uranium with 12.5% enrichment. Detailed MCNP5 models were used to evaluate the flux performance. Diffusion theory REBUS-3 models were used to estimate fuel depletion rates under various operational schemes and to simulate the long-term fuel depletion history. Core lifetimes were estimated under realistic operational constraints. This study found a reduction of 7- 10% in thermal neutron flux in the irradiation channels for the LEU-UO₂ fuel in comparison with the HEU fuel. Therefore, for an LEU fueled core, the reactor power would have to be increased by ~10% from the current level of 30 kW in order to match the nominal flux level. MNSRs have a very small window of excess reactivity for operation and are designed for a very short operational cycle to avoid excessive xenon buildup. Consequently, fuel depletion calculations need to be performed using realistic operational constraints. As fuel depletes, top Be shim plates are added periodically to restore the excess reactivity for continuing operation. The maximum allowed excess reactivity at the beginning of each operating cycle is 4 mk. This study concludes that an MNSR core with LEU UO₂-12.5% fuel and a power level of 33 kW can be operated ~25% longer than the current HEU core operated at 30 kW. Both cores will have the same thermal neutron flux in the experiment positions.

 

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