How does nuclear fission convert some mass into energy?

Nuclear fission releases energy because the total rest mass of the final products is less than the rest mass of the initial reactants. This difference in mass, often referred to as a mass defect, is converted into energy according to Albert Einstein’s mass-energy equivalence relation, $E=m{c}^2$.123

In this process, a heavy nucleus—such as uranium-235—splits into smaller, more stable fragments. Because these resulting fragments have a higher binding energy per nucleon than the original heavy nucleus, the system moves toward a more stable configuration. The "missing" mass is released as energy, primarily in the form of heat and the kinetic energy of the emitted particles.142

While the majority of this energy is released promptly during the fission event, additional energy is produced over time through the radioactive decay of the resulting fission products. This conversion of mass into energy is the fundamental principle that powers nuclear reactors and other nuclear applications.14