Nuclear Power Generation
Nuclear power plants harness the energy released by controlled nuclear fission to generate electricity. As of 2024, approximately 440 commercial nuclear reactors operate in 32 countries, providing about 10% of the world's electricity and roughly 25% of its low-carbon electricity. Understanding reactor designs, fuel cycles, and safety systems is essential for evaluating nuclear energy's role in addressing climate change.
How a Nuclear Power Plant Works
All thermal nuclear power plants follow the same basic principle: fission heats a coolant, which (directly or indirectly) produces steam, which drives a turbine connected to an electrical generator. The nuclear-specific components are the reactor core (where fission occurs) and the containment structure (which prevents radioactive release). The turbine, generator, condenser, and cooling systems are essentially identical to those in fossil fuel plants.
A single uranium fuel pellet, about the size of a pencil eraser (8 mm diameter, 10 mm tall), contains as much energy as 480 cubic meters of natural gas, 810 kilograms of coal, or 560 liters of oil. A typical 1,000 MWe reactor consumes about 200 metric tons of uranium fuel over an 18-24 month operating cycle.
Pressurized Water Reactors (PWR)
The PWR is the most common reactor type worldwide, accounting for about 65% of all operating reactors. Its design uses two separate water loops:
- Primary loop: High-pressure water (about 155 bar, 315 degrees C) circulates through the reactor core, absorbing fission heat. The high pressure prevents boiling. This water also serves as the neutron moderator.
- Secondary loop: The hot primary water passes through a steam generator, where it transfers heat to a separate water loop that boils and drives the turbine. The two loops never mix, providing a barrier against radioactive contamination of the turbine system.
PWR advantages include inherent safety features (loss of coolant also removes the moderator, shutting down the chain reaction) and extensive operational experience. Major PWR designs include the Westinghouse AP1000, the French EPR, and the Korean APR1400.
Boiling Water Reactors (BWR)
The BWR, accounting for about 20% of reactors, simplifies the design by allowing the coolant water to boil directly in the reactor core. Steam from the top of the reactor vessel passes directly to the turbine, eliminating the need for separate steam generators. This makes BWRs simpler and cheaper to build, but it means slightly radioactive steam passes through the turbine, requiring additional shielding.
BWRs operate at lower pressure (about 75 bar) than PWRs. The General Electric BWR designs (from BWR/1 through the advanced ESBWR) have been the most widely deployed.
CANDU Reactors
Canada's CANDU (Canada Deuterium Uranium) reactor uses heavy water (D2O) as both moderator and coolant. Heavy water absorbs far fewer neutrons than ordinary water, allowing CANDU reactors to use natural uranium (0.7% U-235) without enrichment. This gives CANDU significant fuel flexibility -- it can also burn recycled uranium, thorium, or mixed oxide fuel.
CANDU reactors use horizontal pressure tubes rather than a single large pressure vessel, allowing online refueling without shutting down the reactor. About 30 CANDU reactors operate worldwide, primarily in Canada, India, South Korea, and Romania.
The Nuclear Fuel Cycle
The nuclear fuel cycle encompasses all activities from uranium mining to waste disposal:
- Mining and milling: Uranium ore (typically 0.1-0.5% U) is extracted and processed into yellowcake (U3O8).
- Conversion: Yellowcake is converted to uranium hexafluoride (UF6) gas for enrichment.
- Enrichment: The U-235 fraction is increased from 0.7% to 3-5% using gas centrifuges (replacing the older gaseous diffusion method).
- Fuel fabrication: Enriched UF6 is converted to ceramic UO2 powder, pressed into pellets, sintered at 1,750 degrees C, and loaded into zirconium alloy fuel rods assembled into fuel bundles.
- Irradiation: Fuel resides in the reactor for 3-6 years, producing energy through fission.
- Spent fuel storage: Discharged fuel is intensely radioactive and heat-generating. It is cooled in water pools for 5-10 years, then transferred to dry cask storage.
- Reprocessing (optional): Countries like France extract usable uranium and plutonium from spent fuel for recycling. The remaining high-level waste is vitrified (embedded in glass).
- Final disposal: Deep geological repositories are planned to isolate high-level waste for hundreds of thousands of years. Finland's Onkalo facility is the world's first, scheduled to begin operations in the 2020s.
Safety Systems and Defense in Depth
Nuclear safety is built on the principle of defense in depth -- multiple independent barriers preventing radioactive release:
- The ceramic fuel pellet itself retains most fission products.
- The zirconium fuel cladding provides a sealed metal barrier.
- The reactor pressure vessel (20+ cm thick steel) contains the entire core.
- The containment building (1-2 meter thick reinforced concrete with steel liner) is designed to withstand internal pressure, hydrogen explosions, earthquakes, and aircraft impact.
Modern reactor designs incorporate passive safety systems that function without operator action or electrical power, relying instead on gravity, natural circulation, and compressed gas. The AP1000, for example, can cool itself indefinitely after shutdown using only gravity-fed water tanks.