The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", with the detonation of each stage providing the energy to ignite the next stage. At a bare minimum, this implies a "primary" section that consists of an implosion-type fission bomb (a "trigger"), and a "secondary" section that consists of fusion fuel. The energy released by the "primary" compresses the "secondary" through a process called "radiation implosion", at which point it is heated and undergoes nuclear fusion. This process could be continued, with energy from the secondary igniting a third fusion stage; Russia's AN602 "Tsar Bomba" is thought to have been a three-stage fission-fusion-fusion device. Theoretically by continuing this process thermonuclear weapons with arbitrarily high yield could be constructed. This contrasts with fission weapons which are limited in yield because only so much fission fuel can be amassed in one place before the danger of its accidentally becoming supercritical becomes too great.