Preliminary calculations in NIRR-1core conversion from high-enrichment to low-enrichment using WIMS and CITATION

The world over research reactors are being converted from highly enriched uranium (HEU) to low enriched uranium (LEU).The Nigerian Research Reactor-1(NIRR-1) falls into the category of HEUs,being 90% enriched in 235U.It is therefore desirable to convert it from this enrichment about 20% in order to conform to the global trend. The reason for this general trend is not farfetched. In this work, we have developed a computational scheme that would in principle easily achieve the conversion of MNSR type cores from high to low enrichment using a combination of the codes WIMS and CITATION by systematically changing the enrichment from any starting percentage to another one in any desired steps specified by the user computing important safety related parameters at each step. Two fuel types have been considered, namely UAI4 and UO2.Furthermore, variation of safety reactivity factor (SRF) with enrichment is also discussed and it is shown that within the range 90%--10%, SRF of 1.5946674 for UAI4 and 1.827 for UO2 at 20% enrichment are obtained, satisfying the International Atomic Energy Agency (IAEA) requirement that this parameter should remain greater than 1.5. The shutdown margin (SDM) at this enrichment is 2.1780mk for UAI4 and 2.9343mk for UO2.The excess reactivity at each step in the calculation is also obtained. This theoretical investigation indicates the need for a drastic review of the material composition of the NIRR-1 core to convert from an enrichment of 90% to about 20%. Comparison of the results obtained in converting from HEU (90%) to LEU (20%) in this work with that from MCNP is very encouraging.