NEW MAXIMUM Keff SEARCH OTIONS FOR "SMORES"

New semi-automatic search options for maximum keff of systems having a given number of fixed composition fuel elements were successfully developed for SMORES – a new prototypic analysis sequence of SCALE-5.0. They are useful for identifying that distance between the fuel elements that will result in maximum keff. The new optimization algorithms were tested by applying them to identify the maximum keff thermal systems could have when made of a given number of fixed composition fuel elements, a moderator and a reflector. It was found that the optimal systems generally feature non-uniform spacing of the fuel elements – the optimal spacing tends to increase from the center towards the reflector. The spacing non-uniformity depends on the combination of moderator and reflector materials, on the number of fuel elements in the system and on the reflector thickness. The general trend is increasing spacing width with reduction in the number of fuel slabs and with reduction in reflector thickness. Particularly large can be the spacing of the outermost to the next-to-outermost fuel elements when using very low absorbing moderator, such as graphite, along with a relatively absorbing reflector such as water. When using a thick low absorbing reflector, such as graphite, the spacing of the fuel elements is nearly uniform and only slightly larger than the optimal spacing in an infinite lattice. Otherwise the optimal spacing can be significantly larger than the optimal infinite lattice spacing.