Abstract: Critical experiment studies were performed, varying the parameters U235, B10 and metal to water ratio, in the SM-2 7 x 7 core configuration with 38 stationary elements and seven control rods of the SM-1 (APPR-1) type. An experimental mock-up of the SM-1 was assembled using the basic SM-2 fuel plates. Excellent agreement between the SM-1 boron loading, determined by chemical analysis, and the SM-1 mock-up boron loading, for equivalent bank positions, was noted. Several SM-2 mock-ups, cold clean and midlife, were assembled and studied with regard to reflector effects, flow divider effects, relative control rod array worths, critical rod configurations, and relative power distributions. The results of these experiments indicate as satisfactory a U235 loading of 36.4 Kg and a B10 loading of 63.4 grams for the SM-2. Attention is drawn to numerous power peaks present in the active core. The open seven control rod array has a slight reactivity advantage over the closed seven array and consequent minor disadvantage with respect to "stuck rod" criteria.