Well, that's not entirely true. While the fusion phase does contribute to the output, the secondary fission, is the main source of energy. U235 is considered "weaponable", because it can be caused to spontaneously fission. This is because it produces enough relatively cold neutrons when a large enough mass is concentrated, to sustain its own fission. U238 is perfectly fissionable, but it does not produce enough neutrons to sustain its own fission (and requires hot neutrons as well). But . . . the fusion of a thermonuclear device produces a huge flux of neutrons, which is plenty to fission a large quantity of the U238, which is very abundant as tamper in nuclear devices. This is actually what produces the great jump in output, so the process is really fission-fusion-fission
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Well, that's not entirely true. While the fusion phase does contribute to the output, the secondary fission, is the main source of energy.
U235 is considered "weaponable", because it can be caused to spontaneously fission. This is because it produces enough relatively cold neutrons when a large enough mass is concentrated, to sustain its own fission.
U238 is perfectly fissionable, but it does not produce enough neutrons to sustain its own fission (and requires hot neutrons as well).
But . . . the fusion of a thermonuclear device produces a huge flux of neutrons, which is plenty to fission a large quantity of the U238, which is very abundant as tamper in nuclear devices.
This is actually what produces the great jump in output, so the process is really fission-fusion-fission