CORE LOADING REQUIREMENTS IN CONVERTING HEU MNSR CORE TO LEU USING WIMS AND CITATION CODES

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Date
2010-10
Authors
SYLVANUS, AZANDE TERUNGWA
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Abstract
The world over research reactors are being converted from highly enriched uranium (HEU) to low enriched uranium (LEU). The NIRR-1 falls in the category of HEUs, being 90% enriched in the fissile U-235. It is therefore desirable to convert it from this enrichment to about 20% in conformity with this global trend. The reason for this general trend is to make research reactor fuel as unattractive as possible to groups that may be interested in using such highly enriched reactor cores for nonpeaceful purposes. In this work, we have developed a computational scheme that would theoretically achieve this objective as easily as possible. The scheme systematically reduces the enrichment from 90% (or any other initial values) to 20% and even below, in steps of 5% or any desired percentage variation. At each step, important neutronics parameters, especially safety related parameters are computed. Two fuel types are considered – UAl4 and UO2 – and the variation of safety reactivity factor (SRF) with enrichment is also discussed for each fuel type. It is shown that between 90% and 10% inclusive, the value of this parameter remains higher than 1.5. At 20% enrichment, SRF values of 1.595 and 1.827 are obtained for UAl4 and UO2 respectively, satisfying the IAEA requirement that this parameter should be greater than 1.5. The shutdown margin (SDM) at this enrichment is found to be 2.1780mk for UAl4 and 2.9343mk for UO2. The control rod curves give the characteristic s-shapes, with excess reactivity values in good agreement with the design specifications. Comparison of some of the results obtained in this work (e.g. mass loading) with those of MCNP calculations reported in the literature is very encouraging. It should be noted however that this theoretical investigation indicates a drastic review of the fuel loading of NIRR -1 core to achieve low enrichment.
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A RESEARCH WORK SUBMITTED TO THE POSTGRADUATE SCHOOL, AHMADU BELLO UNIVERSITY ZARIA, IN PARTIAL FULFILLMENT FOR THE AWARD OF MASTER OF SCIENCE DEGREE (M.SC. PHYSICS) PHYSICS DEPARTMENT, AHMADU BELLO UNIVERSITY ZARIA October, 2010
Keywords
CORE,, LOADING,, REQUIREMENTS,, CONVERTING,, HEU,, MNSR,, CORE,, LEU,, USING,, WIMS,, CITATION,, CODES,
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