Favorable passive reactivity shutdown performance in response to unprotected accident initiators has been shown to be achievable when several measurable, integral reactivity parameters related to the power, flow, and inlet temperature coefficients of reactivity satisfy certain simple constraints among their dimensionless ratios. The trends in these dimensionless ratios with reactor size for both oxide- and metal-fueled cores have been developed, based on a data base of ∼24 reactor designs in the range from 400 to 3600 MW(thermal). Based on the trends, it is possible to conclude that the favorable passive reactivity shutdown features that accrue to the metallic-fueled reactors in the mod-ular-size range can be achieved as well in the larger commercial sizes.