l-5, If a fission device i.s accom-panied by the heavy isotopes of hydrogen, the high tenperatule and pressute triggered by the explosion can cause the fusion of these isotapes into heavier ones, thereby releasing vast amount s of enerfiy" Even thougb one fusion reaction releases less energy than one fission reaction, the amount of energy released per liilogram- of nuclear explosive materiaf can be nore than four times as large in a fusion device as in a fission device. lb. The energy is usually expressecl in units of kiloton (tt) or negaton (l'lt) corresponding to the energy refease in a thousand or a million metTic tons of TliT ( trinitrotol-uene ) . The atonic bonb dropped on lliroshima on 6 Aucust 19115 derived its energy from a chain reaction fissioning the nucl,ei of uraniun 235 atomE anLl had a yield of t3 k-t. The critical size was achieved using a "gun" to shoot one piece of uranilux into another, In contrast, the ivagasaki bomb of 9 August 1l\5 utitjzed plutonir-ur-239 and had a yiefd of 2? kt" The plutonium was arranged as a spherical shell vhich was crushed together by a surrounding she1l of chemical explosive, This is referred to as a t'nuclear i.nplosion weapont' . U, The design of a thermonuclear veapon is publ-icJ.y tess veIl knom in all its details. The energy relessed comes both from the fission 'trigger" and the fusion materia.ls. Theie nay also be add.ed a considerabfe amount of fission energy by surrounding the fusion weapon with a shell of uranium- 238. The fission reactions give rise to much larger anounts of radioactivity than the fusion reactions. For this reason, thefmonuclear ueapons are sometines spoken of as "clean" or "dirt.'1" . depending on what fracti.on of their total energy release derives frorn fission-