How does nuclear fission release energy from uranium atoms?

Nuclear fission releases energy from uranium-235 atoms through a process involving the conversion of mass into energy. When a uranium-235 nucleus is struck by a slow neutron, it becomes unstable and splits into two smaller nuclei, releasing gamma rays and additional neutrons.1

The fundamental source of the released energy is the "mass defect." The total mass of the resulting fission products is slightly less than the mass of the original reactants (the uranium nucleus and the incident neutron). According to Albert Einstein’s mass-energy equivalence relation, $E=m{c}^2$, this difference in mass is converted into a significant amount of thermal energy.231

Additionally, this process is characterized by an increase in the nuclear binding energy per nucleon. As the uranium nucleus fissions into more stable, smaller fragments, the binding energy per nucleon increases, which is accompanied by the release of energy. In a nuclear reactor, this energy is primarily captured as heat to generate electricity.324