What type of reaction is nuclear fission primarily used in?

Nuclear fission is primarily utilized in nuclear power plants for energy production. In this process, atomic nuclei, typically those of heavy elements like uranium-235 or plutonium-239, are split into smaller fragments upon being bombarded by neutrons. This splitting releases a significant amount of energy, which is harnessed to heat water, produce steam, and drive turbines to generate electricity.

The energy produced in fission reactions is a result of the mass lost during the reaction, as described by Einstein's equation E=mc². This energy release is what makes fission a powerful source of energy, effectively utilized in power generation systems designed to convert this energy into usable electrical energy.

In contrast, other options such as fusion power generation involve the merging of light atomic nuclei rather than splitting heavy ones, which is not applicable here. SSR, or Safety Shutdown Reaction, refers to mechanisms designed to safely halt a reaction in case of an emergency, but it is not a type of reaction that generates power. Lastly, thermodynamic systems encompass a broader range of processes concerning heat and energy transfer but do not specifically pertain to nuclear fission. Hence, the correct answer highlights the central role of fission in the operation of nuclear power facilities.