During 1956–1962, reactor science shifted toward a physics-based framework for predicting and controlling reactor behavior, anchored in neutron multiplication, moderation, and feedback mechanisms. Key results included a matured understanding of criticality and kinetics that underpinned engineering design, safety analyses, and operational strategies, while measurement of fission neutron multiplicities supplied essential inputs for models. This period also broadened fuel concepts with molten fluoride salts and explored thermonuclear reactor ideas, signaling a move toward diverse, high-performance power systems beyond traditional solid fuels.