What Are Fission Reactors?

Fission reactors are nuclear devices that sustain a controlled chain reaction of nuclear fission to generate heat, which is then converted to electricity or used directly for industrial and research purposes. In a fission reaction, a neutron strikes a heavy nucleus, typically uranium-235 or plutonium-239, causing it to split into two smaller nuclei and release additional neutrons along with a large quantity of energy. These released neutrons initiate further fissions, creating a self-sustaining chain reaction that is moderated and controlled to prevent runaway escalation. The International Atomic Energy Agency (IAEA) oversees safety standards and information sharing for the approximately 440 commercial power reactors currently in operation worldwide.

Fission reactors convert the kinetic energy of fission products and neutrons into heat through collisions within the reactor core. That heat is carried by a coolant to a steam generator or turbine, where it produces the mechanical work required for electricity generation. The choice of coolant, moderator, and fuel configuration defines the reactor type and its operational characteristics.

Water-Cooled Reactor Types

Water-cooled reactors account for more than 95 percent of the world's operating commercial fission reactors. The pressurized water reactor (PWR) is the most common design: water in the primary circuit is kept under high pressure (typically around 155 bar) to prevent boiling, and it transfers heat through steam generators to a secondary circuit where steam drives the turbines. The boiling water reactor (BWR) uses a single-circuit design in which water boils directly in the reactor core at lower pressure, with the steam passing directly to the turbines. The pressurized heavy water reactor (PHWR), developed in Canada as the CANDU design, uses heavy water as both coolant and moderator, which allows the reactor to operate on natural uranium fuel and be refueled while running at full power. According to the World Nuclear Association, PWRs number approximately 300 units globally, while BWRs account for roughly 65.

Pressure Vessels and Structural Components

The reactor pressure vessel (RPV) is the primary structural boundary that contains the reactor core, fuel assemblies, control rods, and coolant under operating pressure. In PWRs, the RPV operates at pressures above 150 bar and temperatures approaching 325°C, imposing demanding requirements on the low-alloy steel used for vessel fabrication. The vessel wall accumulates neutron irradiation damage over decades of operation, causing embrittlement that must be tracked through surveillance programs and periodic pressure-temperature limit revisions. Containment structures surrounding the pressure vessel provide a second barrier against the release of radioactive materials in the event of a loss-of-coolant accident. These structures are typically reinforced concrete or pre-stressed concrete domes designed to withstand internal pressure loadings far exceeding normal operating conditions. The ASME Boiler and Pressure Vessel Code Section III governs the design, fabrication, and inspection of nuclear pressure vessels and components.

Reactor Control and Radiation Protection

Reactor power is controlled by adjusting the neutron population within the core, primarily through control rods containing neutron-absorbing materials such as boron or hafnium. Inserting the rods increases absorption and reduces the chain reaction rate; withdrawing them allows power to rise. Chemical shim control, used in PWRs, dissolves boric acid in the primary coolant to provide coarse reactivity adjustment over longer timescales. Radiation protection for plant workers relies on shielding of activated components, controlled access zones, dose monitoring, and operating procedures that limit exposure time near high-dose areas. The IAEA safety standards series establishes the dose limits and radiation protection principles applied in nuclear facilities worldwide.

Applications

Fission reactors have applications in a range of energy and research contexts, including:

• Baseload electricity generation providing low-carbon power at large scale
• District heating systems using reactor waste heat in some European countries
• Research reactors producing neutron beams for materials science, medical isotope production, and fundamental physics experiments
• Naval propulsion for submarines and aircraft carriers requiring long-duration operation without refueling
• Desalination of seawater in water-scarce regions through co-generation with power production