Fast Fission - Fissionable But Not Fissile

Fissionable But Not Fissile

Some atoms, notably uranium-238, do not usually undergo fission when struck by slow neutrons, but do split when struck with neutrons of high enough energy. The fast neutrons produced in a hydrogen bomb by fusion of deuterium and tritium have still higher energy than the fast neutrons produced in a nuclear reactor. This makes it very easy to increase the yield of any given fusion weapon almost indefinitely by the simple expedient of adding layers of cheap natural uranium. In fact, fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.

Fast fission of uranium-238 does also occur in fission bombs.