TRUE

The operation of a thermonuclear weapon based on the Teller-Ulam design is widely defined by a three-step reaction sequence known as the fission-fusion-fission cycle (Britannica, 2026). In this process, a fission primary stage acts as a trigger to ignite a fusion secondary stage; the high-energy neutrons released by fusion then initiate a third stage of fission in the weapon's heavy uranium tamper or casing (World Nuclear Association, 2024). While most modern strategic warheads are physically constructed as two-stage devices (Primary and Secondary), they are described as having three reaction stages because the final fission step often accounts for more than half of the total explosive yield (Wikipedia, 2025). Additionally, ultra-high-yield weapons such as the Soviet Tsar Bomba and the American B-41 utilized three distinct physical stages—Primary, Secondary, and Tertiary—to reach megaton-level yields (Center for Arms Control and Non-Proliferation, 2022).