Most modern nuclear weapons utilize 238U as a " tamper " material ( see nuclear weapon design ) . A tamper which surrounds a fissile core works to reflect neutrons and to add inertia to the compression of the Pu-239 charge . As such , it increases the efficiency of the weapon and reduces the critical mass required . In the case of a thermonuclear weapon 238U can be used to encase the fusion fuel , the high flux of very energetic neutrons from the resulting fusion reaction causes 238U nuclei to split and adds more energy to the " yield " of the weapon . Such weapons are referred to as '' fission-fusion-fission '' weapons after the order in which each reaction takes place . An example of such a weapon is Castle Bravo
Castle Bravo
Castle Bravo
Uranium-238