Why do light water fission reactors need enriched uranium to work?

A nuclear power plant uses uranium as fuel. Natural
uranium primarily consists of U-238 atoms with a small percentage of U-235 atoms. It is
the U-235 atoms that undergo fission. When one atom of U-235 is bombarded with a
neutron, the neutron is absorbed and the uranium atom is broken into an atom of barium,
an atom of krypton with the release of 3 nucleons in the process. As the mass of the
resultant particles is less than that of the uranium atom and the neutron there is a
large amount of energy released.

For a nuclear reactor to
function the number of neutrons being released when the U-235 is split should be
sufficient to sustain a chain reaction; when this condition is achieved it is called
criticality. Light water fission reactors use normal water to transfer heat from the
fission reaction to produce electricity. As the mass of a nucleon is very close to that
of a water molecule, the water absorbs many of the nucleons. This prevents the chain
reaction from continuing unless the percentage of U-235 in the uranium fuel is high
enough.

As natural uranium has only 0.72% of U-235, it has
to be processed or enriched to increase the percentage of U-235. The enrichment process
is necessary for the functioning of light water fission
reactors.