In a nuclear fission reactor, each fission of a uranium nucleus
is accompanied by the emission of one or more highspeed neutrons,
which travel through the surrounding material. If one of these
neutrons is captured in another uranium nucleus, it can trigger
fission, which produces more fast neutrons, which could make
possible a chain reaction. However, fast neutrons have low
probability of capture and usually scatter off uranium nuclei
without triggering fission. In order to sustain a chain reaction,
the...