Ch. 5 Chain reactions The power of repeated doubling Neutrons initiate fission, and fission creates more neutrons Runaway chain reaction is a bomb Critical mass is required (15 kg for U-235, 5 kg for Pu-239) to ensure enough neutrons are absorbed Make critical mass with “gun” or with implosion

How to make a bomb Enrich uranium to increase U-235  Gas centrifuges  Modern, efficient, but need strong material for their construction  Calutrons  Slow but sure; Saddam tried this method Use plutonium  Implosion must be used; hard to do Use fission bomb to start a fusion reaction (H-bomb)

Bomb damage 20 kiloton about a mile away Light/x-ray pulse Thermal pulse Blast wave Radioactive fallout New York City example The only winning move is not to play the game

A reactor hodge-podge Fast vs. slow neutrons  Bombs need fast neutrons to get enough doublings in a short time  Reactors generally use slow neutrons, since they are absorbed more readily by the smaller fraction of U-235 Moderators  Used to slow the neutrons down - “thermal”  Water, heavy water, graphite

A reactor hodge-podge, p.2 Control rods  Absorb neutrons to control the reaction Neutron absorbed by U-238 gives Pu “breeder” reactor  Sometimes uses fast neutrons. Less safe but more efficient. Depleted uranium “bullets” Radioactive waste (see p. 5-29)