The type of force responsible for nuclear fission is the strong nuclear force. This fundamental force holds the protons and neutrons together in an atomic nucleus. When a nucleus absorbs a neutron, it becomes unstable and can split into smaller nuclei, releasing a significant amount of energy. This energy release occurs due to the conversion of mass into energy as described by Einstein's equation E=mc². The fission process is critical in nuclear reactors and atomic bombs, showcasing the power of nuclear energy.