Practise explaining self-sustaining fission through neutron multiplication and determining whether a system is subcritical, critical or supercritical from losses and reproduction.
This question is about nuclear power production.
Outline the reason why fuel enrichment is necessary for the fuel used in a commercial nuclear reactor.
U-235 is (much) more fissionable that U-238 / U-238 is a good absorber of neutrons / U-238 removes neutrons from the reaction / U-238 cannot produce a chain/sustainable reaction;
(naturally occurring) uranium (ore) contains more U-238 (atoms) than U-235 (atoms) / very little U-235 in ore;
in fuel enrichment, U-238 is removed / the percentage of U-235 is increased / the ratio of U-235 to U-238 is increased / amount of U-235 increases;