U-235 is the fissionable isotope
that enables uranium to be used as fuel for commercial nuclear power reactors.
Natural uranium occurs in several isotopes, including a fissionable isotope
, symbolized as U-235, which we could in principle use to make nuclear "devices," nuclear power plants, etc.
Uranium enrichment for nuclear fuel increases the concentration of U235, the fissionable isotope
that makes bombs and reactors go boom.
Caesium-137, or radiocaesium, is a radioactive isotope of Caesium formed as one of the more common fission products by the nuclear fission of Uranium-235 and other fissionable isotopes
in nuclear reactors and nuclear weapons.
Uranium, in its natural form, contains too low a concentration of fissionable isotopes
to be used as fuel for reactors or weapons material and must be put through an enrichment process.