A pressurized water reactor (PWR) generates electricity by using nuclear fission to heat water that never boils, then transferring that heat to a separate water supply that does boil into steam, which spins a turbine. The key design principle is keeping two water loops physically separated so that radioactive water from the reactor core never reaches the turbine or the outside environment. PWRs are the most common type of nuclear power plant in the world, making up roughly two-thirds of all commercial reactors.
Two Loops, One Purpose
The entire system revolves around two closed loops of water. In the primary loop, water flows directly through the reactor core, absorbing intense heat from nuclear fuel. This water is kept under enormous pressure, about 155 bar (roughly 2,235 psi), to prevent it from boiling despite temperatures well above water’s normal boiling point. That pressurized hot water then travels to a component called the steam generator.
Inside the steam generator, the primary loop water flows through thousands of thin metal tubes. Secondary loop water surrounds those tubes on the outside. Heat passes through the tube walls, and because the secondary loop operates at much lower pressure, that water boils into steam. The two water supplies never physically mix. The steam then flows to a turbine, spins it, and the turbine drives a generator that produces electricity. After passing through the turbine, the steam is cooled back into liquid water in a condenser and pumped back to the steam generator to repeat the cycle.
What Happens Inside the Reactor Core
The reactor core contains fuel assemblies made of ceramic pellets of uranium dioxide. These pellets are stacked inside long, thin metal tubes called fuel rods, and bundles of these rods form each fuel assembly. The uranium is enriched so that about 3 to 5 percent of it is the fissile isotope uranium-235, compared to less than 1 percent in natural uranium. Current regulations cap enrichment at 5 percent by weight for commercial power reactors, though the industry is exploring levels up to 10 percent for advanced fuel designs.
When a uranium-235 atom absorbs a neutron, it splits into two smaller atoms, releasing energy as heat along with two or three additional neutrons. Those neutrons go on to split other uranium atoms, sustaining a chain reaction. The water in the primary loop serves double duty: it carries heat away from the fuel and it slows down (or “moderates”) the neutrons to the right speed for efficiently splitting more uranium atoms. This moderating role is critical. If the water were lost, the chain reaction would slow dramatically on its own, which is an inherent safety feature of the design.
Keeping the Water From Boiling
Maintaining precise pressure in the primary loop is essential. If pressure drops too low, the superheated water could flash into steam, disrupting cooling and neutron moderation. If pressure climbs too high, it risks damaging pipes and components. A specialized component called the pressurizer handles this balance.
The pressurizer is a tall, heavy-walled tank connected to the primary loop. Its lower section holds water and its upper section holds a cushion of steam. When pressure starts to drop, electric heaters at the bottom of the tank switch on, boiling some of the water into steam and pushing pressure back up. If the drop is severe, banks of backup heaters activate as well. When pressure rises too high, spray nozzles at the top of the tank release a mist of cooler water from the primary loop into the steam space. This condenses some of the steam and brings pressure back down. The entire system constantly adjusts, keeping pressure locked near its 2,235 psi setpoint.
Controlling the Chain Reaction
Operators control the reactor’s power output primarily through control rods. These are long rods made of materials that absorb neutrons, typically filled with boron carbide powder inside stainless steel tubes. Some designs also incorporate hafnium. Inserting the rods deeper into the core absorbs more neutrons, slowing the chain reaction and reducing heat output. Withdrawing them allows more neutrons to sustain fission, increasing power.
For finer, slower adjustments, operators can also dissolve boric acid directly into the primary loop water. Boron is an excellent neutron absorber, so increasing its concentration in the coolant gently dials down reactor power, while diluting it lets power rise. This chemical approach is useful for gradual changes over hours or days, while control rods handle faster adjustments.
In an emergency, all control rods drop into the core simultaneously under gravity. This is called a reactor trip or “scram,” and it shuts down the chain reaction within seconds.
The Steam Generator Up Close
The steam generator is the bridge between the radioactive primary side and the clean secondary side. In the most common design, the primary water enters at the top, flows down through thousands of U-shaped tubes, and exits at the bottom. Secondary water surrounds these tubes, absorbing heat and progressively turning to steam. Some designs use straight tubes instead, with the primary water entering at the top and the secondary water entering at the bottom in a counterflow arrangement. A large steam generator can contain around 16,000 individual tubes.
As the secondary water moves through the tube bundle, it first reaches a boiling point, then continues absorbing heat until it becomes fully dry steam. In some designs, the steam picks up enough additional energy to become slightly superheated, meaning its temperature rises above the boiling point at that pressure. This improves the efficiency of the turbine downstream.
Why PWR Efficiency Is Modest
Despite the immense energy released by nuclear fission, PWRs convert only about 30 to 33 percent of their thermal energy into electricity. A standard large PWR like the AP1000 operates at roughly 30 percent thermal efficiency. That’s noticeably lower than modern natural gas plants, which can exceed 60 percent.
The reason comes down to steam temperature. Because the primary loop water must stay below its boiling point, the heat it delivers to the steam generator is limited. The secondary steam is produced at relatively modest temperatures compared to what a coal or gas boiler can achieve, and lower steam temperatures mean less energy can be extracted by the turbine. It’s a fundamental thermodynamic tradeoff: the pressure constraint that keeps the reactor safe also caps its efficiency.
Layers of Containment
PWRs are surrounded by multiple physical barriers designed to keep radioactive material contained even in worst-case scenarios. The fuel pellets themselves are ceramic, which locks most fission products inside the pellet structure. Those pellets sit inside sealed metal fuel rod cladding. The entire primary loop is enclosed within a massive containment building.
The containment vessel is a freestanding steel cylinder with walls about 1.75 inches thick. Surrounding that is a shield building, a concrete-and-steel structure with walls 36 inches (3 feet) thick, built to withstand earthquakes and external impacts. Internal structural walls around the reactor vessel, steam generators, and other critical components are at least 2 feet 6 inches of reinforced concrete. These nested barriers mean that even if the fuel were damaged and the primary loop breached, radioactive material would still be contained within the building.
Passive Safety in Modern Designs
Newer PWR designs, like the AP1000, incorporate passive safety systems that work without electricity, pumps, or operator intervention. If something goes wrong, these systems rely on natural forces: gravity, compressed gas, and natural water circulation driven by temperature differences.
Core makeup tanks, pressurized to match the full reactor coolant system pressure, can inject water directly into the reactor vessel through gravity and natural circulation alone. A passive residual heat removal system uses a heat exchanger connected to the primary loop that can remove 100 percent of the decay heat (the heat that continues after the chain reaction stops) through natural water circulation. For the containment building itself, air naturally circulates up the outside of the steel containment vessel, and water from a tank on the roof can evaporate on the vessel’s surface, cooling it without any pumps. Together, these passive systems can maintain safe cooling indefinitely without power or human action.