A reactor machine is a device used in nuclear reactors for the control and production of nuclear energy. Reactors produce energy through the splitting of atomic nuclei. The reactor machine controls the process of nuclear fission through the slowing and trapping of neutrons, allowing nuclear reactions to continue in a sustainable manner.

The reactor machine usually includes a core composed of various fuel elements. Fuel elements release neutrons needed for nuclear reactions to occur. These neutrons cause other nuclei to be captured and release more neutrons. This process is carried out in a controlled manner in nuclear reactors and ultimately produces heat. This heat helps an electricity-generating turbine spin and produce electrical energy.