The history of reactor technology development traces back to early twetieth-century discoveries of radioactive decay and the discovery of nuclear fission in laboratory experiments in 1938. Initially applied to military needs during World War II, the United States’ nuclear reactor technology has evolved through various phases, from the Manhattan Project to the present. This paper applies the multi-level perspective (MLP) framework to analyze the development of reactor technologies in the US over an 85-year period, from 1939 to 2024. By examining five key phases—ranging from initial invention and innovation to diffusion, stagnation, and decline—the study explores how different reactor technologies competed and how socio-political dynamics, including geopolitical tensions, economic factors, and public opposition, shaped the industry. The research highlights the challenges and constraints that nuclear power faced, including the path dependency created by the Manhattan Project and the economic risks taken by private industry. It concludes that despite political pushes for nuclear power, the industry’s stagnation resulted from technological misfit and external challenges.