Nuclear reactor

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. When a fissile nucleus like uranium-235 or plutonium-239 absorbs a neutron, it splits into lighter nuclei, releasing energy, gamma radiation, and free neutrons, which can induce further fission in a self-sustaining chain reaction. The process is carefully controlled using control rods and neutron moderators to regulate the number of neutrons that continue the reaction, ensuring the reactor operates safely.

Heat from nuclear fission is passed to a working fluid (water or gas), which in turn runs through steam turbines. These either turn electrical generators' shafts or drive a ship's propellers. The efficiency of energy conversion in nuclear reactors is significantly higher compared to conventional fossil fuel plants; a kilo of uranium-235 can release millions of times more energy than a kilo of coal. Nuclear-generated steam in principle can be used for industrial process heat or district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium.

Reactor safety is maintained through various systems that control the rate of fission. The insertion of control rods, which absorb neutrons, can rapidly decrease the reactor's output, while other systems automatically shut down the reactor in the event of unsafe conditions. The buildup of neutron-absorbing fission products like xenon-135 can influence reactor behavior, requiring careful management to prevent issues such as the iodine pit, which can complicate reactor restarts.

As of 2022^[update], the International Atomic Energy Agency reported there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.^[1][2][3] Reactors come in various types, each using different methods of moderation and cooling. Common types include pressurized water reactors (PWRs), boiling water reactors (BWRs), and fast reactors, each with specific designs tailored to optimize efficiency, safety, and fuel usage. These reactors play a crucial role in generating large amounts of electricity with low carbon emissions, contributing significantly to the global energy mix.