|Fig. 1: Pressure Water Reactor. (Source: Wikimedia Commons)|
Most of the world's nuclear power plants are almost entirely made up of pressurized water reactors (PWR). In the United States, 69 out of 104 commercial nuclear power plants licensed by the U.S Nuclear Regulatory Commission are PWR's.  The PWR is one of three light water reactors and produces about 65,100 net megawatts (electric).  In an archetypal design of a PWR, as represented in Fig. 1, heat is created inside the core of the reactor. High-pressurized water is pumped to the core where it is further heated by the energy produced by the fission of atoms. The pressurized water in a primary coolant loop, then carries this thermal energy to the steam generator.  Heat carried from the primary coolant loop vaporizes water from a secondary loop, producing steam (within the steam generator). This stem is pushed to the main turbine generator, and powering it and further creating electricity. Any unused steam is condensed into water and pumped out of the condenser, reheated and then pumped back into the steam generator where the cycle begins again. 
Pressurized water reactors have advantages over the other light water reactors and earlier generation nuclear sites.  One major advantage of this reactor is that it is easy to operate because less power is being produced as the heat increases.  In addition, the core of the reactor contains less fissile material, decreasing the chances of additional fission events to occur, making the reactor safer and more controllable.  In other words, it contains "less fissile material than is required for them to go prompt critical".  Lastly, the most advantageous element of the PWR is the turbine cycle. Since the primary and secondary loops are separate, water can never be contaminated by radioactive material in the main system loop. Conclusively, the water from the primary and secondary loops will never touch or mix, so there is no chance for contamination.
Although the PWR makes up the majority of Western Nuclear plants and is the reactor choice for most Navy Nuclear Propulsion Systems, there are some notable disadvantages to using such a reactor.  For one thing, the reactor requires very strong piping and a heavy pressure vessel in order to ensure that the highly pressurized water remains at a liquid state when sustaining high temperatures, making the construction of the PWR costly.  Meanwhile, most reactors need to be refueled after about 18 months, and cannot be refueled while the reactor is running. Since the refueling process takes a few weeks, the reactors must go offline for this time. Lastly, although no water contamination in the main cycle exists, boric acid, which is corrosive to carbon steel, can get melted into the coolant causing radioactive products to circulate throughout the loop.  These radioactive yields are destructive to the reactor (i.e. potential for radiation exposure) ultimately limiting the reactor's operating life.
The pressurized water reactor is commonly used in nuclear power plants over the world.  The reactor converts heat (that is generated into fuel) into electrical power. This power can be used for industrial and residential purposes. The leftover water from the main condenser is recycled back to the stem generator.  Although the reactor has some disadvantages, it makes up for its deficits in the fact that it is a safe and reliable reactor.
© Sydney Shaw. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Zarubin, Introduction to Light Water Reactors, Physics 241, Stanford University, Winter 2015.
 "2012-2013 Information Digest," U.S. Nuclear Regulatory Commission, NUREG-1350, Vol 24, August 2012, p. 28.
 A. Crerend, "Nuclear Fission in the Context of Pressurized Water Reactors," Physics 241, Stanford University, Winter 2015.