How is heat produced in a nuclear reactor during fission?

The production of heat in a nuclear reactor during the fission process primarily occurs through the kinetic energy of fission fragments. When a heavy nucleus, such as uranium-235 or plutonium-239, undergoes fission, it splits into two smaller nuclei, known as fission fragments. The mass of these products is slightly less than the original mass of the nucleus due to the conversion of some mass into energy (as described by Einstein's equation, E=mc²).

As these fission fragments are produced, they possess a significant amount of kinetic energy due to the intense repulsive forces that act on the positively charged fragments as they separate. This kinetic energy is transformed into thermal energy, or heat, as the fragments collide with surrounding atoms, increasing their motion and, consequently, the temperature of the reactor's coolant.

This process of heat production is fundamental in a nuclear reactor because it is the heat generated during fission that is used to produce steam, which drives turbines to generate electricity. Other mechanisms, such as absorption of neutrons, chemical reactions, or radiation release, do not directly contribute to the primary heat generation in the same way as the kinetic energy from fission fragments does.