Why most nuclear plants keep their water too pressurised to boil
Inside the most common kind of nuclear reactor, water runs at about 315 degrees Celsius without boiling. The trick is pressure, around 155 times that of the atmosphere, and the same water that carries heat away also slows the neutrons that keep the chain reaction going, which turns out to be a built-in brake.
Pressurised water reactors make up almost 70 percent of the world's commercial fleet. Water enters the bottom of the core at about 275 degrees and leaves the top near 315 degrees, kept liquid by a separate vessel called a pressurizer. Electric heaters hold the water there at 345 degrees, the boiling point at 155 bar, so it sits right at the edge of turning to steam and steadies the pressure of the whole loop.
The design uses two circuits that never mix. Pumps moving roughly 100,000 gallons a minute push the hot primary water through a steam generator packed with several thousand small tubes. Heat passes through the tube walls into a second, lower-pressure circuit, which boils into steam to spin the turbines. Keeping the fluids apart stops radioactivity reaching the turbine side, unlike a boiling water reactor, which has a single loop and roughly half the pressure.
Water also acts as the moderator, slowing fast neutrons through collisions with light hydrogen atoms until they can split more fuel. If the core overheats, the water expands, fewer collisions happen, and the reaction slows by itself. The Soviet RBMK design at Chernobyl, moderated by graphite and cooled by boiling water, behaved the other way, with reactivity climbing as temperatures rose.
The concept began in 1946 with the US Navy's submarine propulsion programme at Oak Ridge, and the first working submarine plant was built in Idaho. Admiral Hyman Rickover insisted that the Shippingport commercial station should use the same approach rather than the more exotic cycles others favoured. The 1979 partial meltdown at Three Mile Island unit 2 largely stalled new American construction for two decades.
Source: Pressurized water reactor
