Question

What is the cause of release of unusually large energies in nuclear fission reactions? How is the energy per fission calculated?
OR
What is a thermal neutron? Draw a schematic diagram depicting fission of a U-235 nucleus on absorption of a thermal neutron.

Answer 1

When fission occurs, the nuclei of the fissile atoms (usually uranium or plutonium) split into smaller nuclei. The sum of the masses of the smaller nuclei is less than the mass of the original uranium or plutonium nuclei - not by much, but enough to matter. The difference in masses is converted into energy per Einstein's equation E=mc^2.