Why heavy elements other than Uranium and Thorium cannot be used as nuclear fuels?
Fission occurs only with certain nuclei of high atomic and mass number and large value of square of atomic number since the repulsive force within the nucleus is an important factor. Only three nuclides having sufficient stability to permit storage for a long time namely U233, U235 and PU239 are fissionable by neutrons of all energies, from thermal values or less to millions of electron volts. Of these nuclides, Uranium-235 is the only one which occurs in nature, the other two are produced artificially from Uranium-238 and Thorium-232 respectively by neutron capture followed by two stages of radioactive decay. Thorium cannot be used straight away as nuclear fuel until it is converted into U233 Several other species are known to be capable of undergoing fission by neutrons of all energies, but they are highly radioactive and decay so rapidly that they have no radical value for the release of nuclear energy.
