Chain Reactions

Early in 1939 Meitner and Frisch suggested that the correct interpretation of the results observed when uranium is bombarded with neutrons is that the uranium nuclei undergo fission. Within a few months two very important things became clear: that fission releases a large amount of energy, and that fission of a nucleus by one neutron liberates usually two or three new neutrons. These discoveries immediately disclosed the possibility of a chain reaction that would produce power.

There was a difficulty, however, in making a chain reaction work. Natural uranium consists of two isotopes: 235U (with an abundance of 0.7%) and 238U (99.3%). Of the two only 235U is “fissile”, meaning that fission can be induced in it by neutrons of any energy. On the other hand 238U undergoes fission only if the neutrons have an energy greater than about 1.5 MeV, and even then they are more likely to be captured or scattered inelastically.