How Nuclear Power Plants Work: From Fission to Electricity

The Controlled Chain Reaction

A nuclear reactor maintains a controlled, self-sustaining fission chain reaction in which each fission event produces, on average, exactly one additional fission event (a condition called criticality). When a uranium-235 or plutonium-239 nucleus absorbs a neutron, it splits into two lighter nuclei (fission products) plus 2-3 free neutrons and about 200 MeV of energy. Those free neutrons can go on to cause additional fissions, creating a chain reaction. The reactor's control systems adjust the neutron population to keep the multiplication factor (k-effective) precisely at 1.0 during steady-state operation: above 1.0 for startup (power increasing), exactly 1.0 at desired power (steady state), and below 1.0 for shutdown (power decreasing).

Control rods made of neutron-absorbing materials (boron carbide, hafnium, or silver-indium-cadmium alloy) are inserted into or withdrawn from the reactor core to adjust the neutron population. Inserting control rods absorbs neutrons and reduces the multiplication factor. Withdrawing them allows more neutrons to cause fissions and increases power. In pressurized water reactors, dissolved boron (boric acid) in the coolant water provides additional neutron absorption that can be adjusted chemically over hours to days to compensate for fuel burnup and fission product buildup. Burnable poisons (gadolinium or erbium compounds mixed into fresh fuel pellets) provide self-adjusting neutron absorption that decreases as the fuel is consumed, extending the interval between refueling outages.

The moderator slows fast neutrons from fission (born with energies around 2 MeV) to thermal energies (about 0.025 eV at room temperature) where their probability of causing additional fissions in uranium-235 is roughly 500 times higher. In light water reactors (LWRs, comprising about 80% of global nuclear capacity), ordinary water serves as both moderator and coolant. Heavy water (deuterium oxide) moderates in CANDU reactors with greater efficiency (fewer neutron captures during moderation), allowing use of natural (unenriched) uranium fuel. Graphite served as moderator in the earliest reactors and continues in some gas-cooled designs. The choice of moderator profoundly affects fuel requirements, reactor design, safety characteristics, and economics.

Reactor Types

Pressurized water reactors (PWRs) keep primary coolant water at high pressure (about 155 atmospheres, 15.5 MPa) to prevent boiling despite temperatures around 320 degrees Celsius. This hot pressurized water flows through a steam generator (a large heat exchanger) where it heats a separate secondary water loop that boils at lower pressure to produce steam. The steam drives a turbine-generator before being condensed and recirculated. The two-loop design keeps radioactive primary coolant (containing dissolved and suspended radioactive materials from the core) physically separated from the non-radioactive steam system, simplifying maintenance and reducing contamination of turbine components.

Boiling water reactors (BWRs) allow coolant water to boil directly inside the reactor vessel, producing steam at about 7 MPa (70 atmospheres) that goes directly to the turbine. This single-loop design eliminates steam generators (expensive, large components requiring periodic replacement) but means the turbine and associated systems handle slightly radioactive steam containing short-lived nitrogen-16 activation products. BWRs use jet pumps and recirculation systems to maintain adequate coolant flow, and control core reactivity partly through adjustable recirculation flow rates (more flow means more moderation and higher power) in addition to control rods that enter from below the core.

Advanced reactor designs under development include small modular reactors (SMRs) with electrical output below 300 MW, designed for factory fabrication and modular deployment. High-temperature gas-cooled reactors use helium coolant and graphite moderator with ceramic-coated fuel particles (TRISO fuel) that can withstand temperatures above 1600 degrees without releasing fission products, providing inherent safety through material properties rather than active safety systems. Molten salt reactors dissolve fuel directly in a liquid fluoride or chloride salt that serves simultaneously as fuel, coolant, and moderator carrier. Sodium-cooled fast reactors use liquid metal coolant and fast (unmoderated) neutrons, enabling breeding of plutonium from uranium-238 and potential consumption of long-lived actinide waste.

Safety Systems and Defense in Depth

Nuclear safety relies on defense in depth: multiple independent barriers and safety systems, each capable of preventing radioactive release independently. The fuel pellet's ceramic structure retains most fission products below about 1200 degrees. The zirconium alloy fuel cladding provides a sealed metallic barrier around each fuel rod. The reactor pressure vessel (20-25 cm thick carbon steel) contains the entire primary coolant system. The containment building (1-2 meter thick reinforced concrete, often with steel liner) surrounds the entire reactor system and is designed to withstand internal pressurization from a complete loss-of-coolant accident without releasing radioactivity to the environment.

Emergency core cooling systems (ECCS) provide backup water injection to keep the fuel covered and cooled even if the primary coolant system fails completely (a loss-of-coolant accident, or LOCA). Multiple redundant trains of high-pressure injection, low-pressure injection, and core spray systems are powered by emergency diesel generators if offsite electrical power is lost. Modern reactor designs incorporate passive safety systems that rely on natural forces (gravity, natural circulation, compressed gas) rather than pumps and electrical power, providing cooling for at least 72 hours after an accident without any operator action or external power supply.

Reactor shutdown systems can terminate the chain reaction within seconds through rapid insertion of control rods (SCRAM). The shutdown margin (the degree of subcriticality achieved with all rods inserted) is sufficient to keep the reactor shut down even under the most reactive credible conditions. After shutdown, the fuel continues generating decay heat from radioactive fission products at about 7% of full power initially, decreasing to about 1% after one hour and 0.2% after one day. This decay heat requires continued cooling for days to weeks after shutdown and is the reason nuclear accidents like Fukushima can develop even after successful reactor shutdown: the cooling systems must continue operating long after the chain reaction has stopped.

The Nuclear Fuel Cycle

Nuclear fuel begins as natural uranium ore mined from deposits in Kazakhstan, Canada, Australia, and other countries. After mining and milling to produce uranium oxide concentrate (yellowcake, U3O8), the uranium is converted to uranium hexafluoride (UF6) gas for enrichment. Gas centrifuge plants increase the concentration of fissile U-235 from the natural 0.7% to 3-5% needed for LWR fuel. The enriched UF6 is then converted to uranium dioxide (UO2) powder, pressed into dense ceramic pellets, sintered at high temperature, ground to precise dimensions, and loaded into zirconium alloy tubes (fuel rods) assembled into fuel assemblies containing 200-300 rods each. A typical 1000 MWe reactor core contains 150-200 fuel assemblies totaling about 100 tonnes of uranium dioxide.

Fresh fuel assemblies are loaded into the reactor during refueling outages (every 12-24 months for most commercial reactors). Approximately one-third of the core is replaced each outage, with assemblies typically remaining in the reactor for 4-6 years. Spent fuel discharged from the reactor is stored in water-filled pools at the reactor site for initial cooling, then may be transferred to dry cask storage or sent for reprocessing (in countries that reprocess). The back end of the fuel cycle, managing spent fuel through interim storage and eventual permanent disposal in geological repositories, remains the most politically challenging aspect of nuclear power.

Next Generation Reactor Designs

The nuclear industry is developing advanced reactor designs that aim to improve on current technology in safety, efficiency, and flexibility. Small modular reactors (SMRs) generate between 50 and 300 megawatts of electrical output, compared to the 1,000+ megawatts typical of conventional plants. Their smaller size allows factory fabrication and delivery by truck or rail, potentially reducing construction costs and timelines. Several SMR designs are in advanced licensing stages, with NuScale Power receiving the first SMR design certification from the U.S. Nuclear Regulatory Commission. Generation IV reactor concepts include high-temperature gas reactors that can produce industrial process heat above 700 degrees Celsius, molten salt reactors with fuel dissolved in liquid fluoride or chloride salts operating at atmospheric pressure, and sodium-cooled fast reactors that can convert fertile uranium-238 into fissile plutonium-239, effectively multiplying the available fuel supply by a factor of 60 or more. These advanced designs also address the waste challenge: fast-spectrum reactors can fission the long-lived transuranic elements (plutonium, americium, curium) that dominate spent fuel radioactivity after a few centuries, potentially reducing the required isolation period for geological repositories from hundreds of thousands of years to a few hundred years.