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 even higher energy than the fast neutrons produced in a nuclear reactor. This makes it possible to increase the yield of any given fusion weapon by the simple expedient of adding layers of cheap natural (or even depleted) uranium. Fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.