The specific mass (or mass per unit power) is a fundamental performance metric in space power systems. For surface power, a low specific mass reduces launch costs and lander size. For nuclear electric propulsion, a low specific mass enables fast transit within the solar system. Studies on specific mass have typically focused on point designs and have not adequately explored the design space and scaling of specific mass. This research explores the design space for radiatively cooled closed nuclear Brayton systems. Specifically, the key innovation in this work is to determine the scaling according to the maximum temperature capability and total power system power. When these two factors are analyzed together, the resulting analyses show a clear scaling for specific mass.