Optimization of MOX enrichment distributions in typical LWR assemblies using a simplex method-based algorithm

The enrichment distributions within light water reactor (LWR) fuel assemblies are optimized using a modified linear programming (SIMPLEX Method) technique initiated from a flat enrichment distribution until a target, maximum local power peaking factor is achieved. The optimum rod enrichment distribution when each rod is allowed to have its own individual enrichment (for this case the target, maximum local power peaking factor is 1.0) is obtained at an intermediate point of the optimization procedure. Later, the optimal locations and values for a reduced number of rod enrichments (groups) are obtained for an input target maximum local power peaking factor by applying sensitivity to change techniques. After an initial set of enrichment groups have been defined, interchanges of rods among neighboring groups are carried out to obtain the final assembly enrichment distribution. The optimization procedure is demonstrated by presenting results for both boiling water reactor (BWR) and pressurized water reactor (PWR) fuel assembly designs. Reactor-grade plutonium (with a fissile Pu fraction of 59.6%) and weapons-grade plutonium (with a fissile Pu fraction of 94.0%) were assumed to be the feed Pu material for the MOX fuel rods in the BWR and PWR examples, respectively. Hot-full-power-temperature and beginning-of-life conditions were also assumed in the example problems.