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-
**Analysis**
The text appears to be discussing the principles and design of nuclear weapons, specifically the difference between fission and fusion reactions, and how they are used in atomic and thermonuclear weapons. The text also mentions the energy release and radioactivity associated with these reactions. Given the technical and scientific nature of the text, it is likely that the category is related to science and technology.

**Category**
Science and technology