aton, In the fission bomb, ttre process involved. 1s the spli.tting of uraniun or plutonium nucl"ei into lighter fragments, fission Products. fn a thermonucl-ear or hydrogeD bomb, nuclei of heavy hydTogen isotopes - deuterium and tritiun - *re irr".d together at the very high terrperatures triggered through the fission I)rocess. 13. Th€ speed of the uuclear reactions is enormous. Both in a fission and a fusion explosive, the eotire nuclear ener8-y is released in about one millionth of a second. with todayts technique, it is tbus possible to release by one weapon nol'e energy in one microsecond than that fron all conventional veapons in al-l wars of history. Ifl:i1::' 5-nnez a' . i_L l-4. In order to sustain the chain reaction in a fission explosion" it i.s necessarJ- lo have more than a certain minimurn anount of fissile naterial, thecritical nass. This nass depends ulon the purity and d.ensity of the material,its leor.etrical- shape: the possible presence of neutron reflectin5 nateriols a.r:jother factors. The fissile material has to be brought together very quickly ifthe l,'eapon is to explode rrith great force. Conwentional expfosives are used fot:this prrrpose and the fissile rrraterial put together, with or vithout compression,to a size vhich, fo} a plutonium bomb, needs to be no larger in volume than a marl sfist. At this tine the chain reaction is inj.tiated. The 1968 United liations stud.r on nuclearweapons (see para. 4 above) set 8 hg. of plutonium containing !0 to 95 per cent of llutonj.um-239 and 25 kg. of highly enriched uranium-235 a: thc amcunts necessary to achieve an explosion with a yield correspcnding to 20 kt. of high explosive. Depending on the design sophistication and r,rith hinh quality t-alerial , this nass can range from It to 25 kg. for uranium 235 and fxon )r to C lS-for plutonium.-239. 1/