A nuclear power plant where heat is drawn from the reactor core via a closed cooling circuit containing water under high pressure (as the name suggests) in order to keep it in a liquid state at high temperatures. The heated primary coolant is itself in turn cooled by passing through a steam generator, exchanging heat with water in a secondary circuit which is then used to drive a steam turbine for power generation or marine motive power. It differs from the earlier water-cooled reactor design, the Boiling Water Reactor, where the coolant water (which, having passed through the core, is fairly radioactive) is used directly to drive the turbines, resulting in significantly worse danger of radioactive emissions. In use from the late 1950s on, the PWR is the most popular (in the broad sense of the word) type of nuclear plant; 57% of nuclear electricity generating plants operating today world wide are PWRs.
Although the PWRs are cleaner than the BWRs and other designs where power generation equipment is driven directly by coolant, they have a design weakness in the steam generator. In the American Westinghouse design (the earliest type of PWR used in the west) the steam generator is basically a big heat exchanger in a vertical cylindrical casing. The reactor cooling water enters at the bottom of the tank and passes up and back down through a bundle of narrow U-shaped tubes (to maximise the surface area for heat transfer) immersed in the secondary stage cooling water, and then the cooled water is returned to the reactor; steam from the heated secondary stage coolant is dried and drawn off at the top of the generator and run through the power-generation turbine and a condenser stage before being returned in liquid form to the steam generator.
Although this may seem a relatively simple process, the mechanical problems of vibration (metal fatigue and fretting of pipes against their mountings) as water is driven though the tube bundle, corrosion and the deposition of various impurities and turbine fragments (turbine blades are very vulnerable to impact damage from water droplets if the steam is not properly dried before exiting the steam generator) from the coolant water in both circuits mean that weaknesses appear and the chances that a tube will blow out increase significantly with age and use. A rupture of a steam tube would probably lead to emissions of radioactive steam and boiling water, but worse still would also depressurize the primary coolant circuit, allowing the core to overheat and melt down. An alternative design by Babcock & Wilcox uses more robust straight through pipes for the primary circuit in the steam generator; the drawback is that it reduces the quantity of secondary coolant held in the steam generator: this was the type of system involved in the Three Mile Island incident, in which the flow in the secondary circuit was accidentally interrupted leading to the core overheating.
The smaller PWRs used to power ships generally use a horizontally aligned steam generator which seems to be a more mechanically satisfactory arrangement.
Sources
- Stuff in my head from publicly available tender documents translated for a commercial client and related research.
- http://www.wowpage.com/tmi/ (Three Mile Island)
- http://www.icjt.org/an/tech/jesvet/jesvet.htm (ICJT nuclear training centre, Slovenia)
- http://www.nucleartourist.com
- http://www.mtn.org/pic/ The Prairie Island Coalition