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.

Figure 1 shows the fission cross-sections of 235U and 238U and the capture cross-section of 238U as functions of neutron energy. Because 238U is so abundant in natural uranium capture it dominates over fission in 235U except at energies below about 1 eV.

Neutrons generated in fission have average energies of about 2 MeV and at that energy cannot sustain a chain reaction in natural uranium. If a neutron survives many scattering interactions, however, its kinetic energy decreases until it is in thermal equilibrium with the atoms by which it is being scattered. It is then known as a “thermal” neutron and its most probable energy is about 0.025 eV.

If a chain reaction is to take place, therefore, either the fission neutrons have to be reduced in energy to near the thermal level, in which case natural uranium can be used, or the proportion of 235U has to be increased substantially. Both of these routes were followed in the early work on nuclear reactors. The first led to the development of “thermal” reactors and the second to “fast” reactors, so called because the neutrons causing fission are fast as opposed to thermal.