How does a nuclear reactor work

Overview

Nuclear reactors are sophisticated machines that harness energy from atomic fission—the splitting of heavy atomic nuclei. When an atom of uranium-235 or plutonium-239 splits, it releases extraordinary amounts of energy in the form of heat and radiation. This heat is carefully controlled and converted into electricity through a turbine system.

Nuclear Fission Process

The chain reaction begins when a neutron strikes a uranium-235 nucleus, causing it to split into two lighter elements and release energy plus 2-3 additional neutrons. These new neutrons can strike other uranium atoms, creating a cascade of reactions. Without control, this would become explosive; reactors use engineered systems to maintain a steady, controlled rate of about one fission per released neutron.

Control Systems

Control rods made of materials like boron or cadmium absorb excess neutrons, slowing or stopping the chain reaction. Operators move these rods in and out of the reactor core to maintain the desired power level. Moderators such as water or graphite slow down fast neutrons to thermal speeds where they're more likely to cause fission. The combination of control rods and moderators allows precise management of the nuclear reaction.

Heat Generation and Steam Production

Fission releases heat—lots of it. Water or liquid metal circulating through the reactor core absorbs this heat. In pressurized water reactors (PWR), primary coolant remains under high pressure, preventing boiling. This heated primary coolant transfers heat to secondary water through a steam generator. The secondary water boils into steam, which flows to turbines at controlled pressure and temperature.

Electricity Generation

High-pressure steam drives large turbines connected to electrical generators. As steam expands through turbine blades, it spins the rotor at thousands of RPM. The generator converts mechanical rotation into electrical current. Exhaust steam cools and condenses back to water in a cooling tower (which releases waste heat to the atmosphere) or through cooling with water from rivers or oceans. The condensed water returns to the steam generator, completing the cycle.

Safety Containment

Reactors employ multiple safety layers: (1) The reactor vessel contains the core and primary coolant, (2) A primary containment structure surrounds the vessel and cooling systems, (3) A massive secondary containment building of reinforced concrete houses everything. Emergency cooling systems activate automatically if normal cooling fails, and operators can insert control rods to halt the reaction within seconds.