C. D. Sigwart 1999
A Pressurized Water Reactor (PWR) is characterized by having the core immersed in water in a large steel tank. The fuel rods and control rods make up a vertical array. Some of the control rod are movable, and are pulled up above the fixed control rods and fuel rods when the plant is in full operation. The purpose of the control rods is to absorb neutrons which trigger the splitting of atomic nuclei in the fissionable material in the fuel rods. With all the control rods inserted, there is negligible fission (and heating) in the fuel rods. When the control rods are pulled out, the fuel rods heat the water, which is circulated by pumps in the primary or, inner loop, to a heat exchanger.
A feature of this design is that only the water in the inner loop is in contact with the radioactive fuel rods. Thus only the inner loop has contamination from the inevitable small amount of rust and corrosion. There are filters in this inner loop to capture the small particles which are radioactively contaminated. Also note that there are additional pumps to circulate cooling water through the core, which form the Emergency Core Cooling System (ECCS). It is essential that circulation be maintained to carry heat away from the fuel rods to prevent them from melting in the event that the main primary circulation pumps should fail. The water in the tank and the primary circulation loop is never supposed to boil, i.e. it is always supposed to stay as water. Steam is a much poorer conductor of heat. The fuel rods are supposed to always stay under water. To prevent boiling, the tank and primary loop are maintained at very high pressure. Note that there is a pressure relief valve to prevent excess pressure from bursting the tank.
The secondary loop water is heated through a heat exchanger with the primary loop. Water in the secondary loop is allowed to boil in a steam generator tank (not shown). The steam is used to drive a turbine which turns an electrical generator. The residual steam is condensed back to water, which is pumped back through the heat exchanger again to make more steam. Also not shown is the circulation usually through a large cooling tower which is used to remove the waste heat. Also not indicated is the fact that the reactor and primary circulation plumbing are contained in a large reinforced containment dome. This special building is intended to prevent escape of radioactivity if the primary plumbing or the reactor vessel should leak.
C. D. Sigwart 1999