Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR)

Power Plant Reactors

With the exception of solar, wind, and hydroelectric plants, most power plants are steam generating plants using different systems to create steam.  A nuclear power plant uses uranium fuel to produce nuclear fission which heats water into steam to drive the turbine that ultimately produces electricity. 

There are many different reactor types used in nuclear power plants world-wide to create nuclear energy.  Two of the most common reactors are Pressurized Water Reactors and Boiling Water Reactors, both of which are light water reactors (LWR).  Light water reactors use ordinary water to cool and heat the nuclear fuel.  LWRs are generally the most economical and common type of reactors.

Pressurized water reactor (PRW)

Nuclear fission produces heat inside the reactor.  That heat is transferred to water circulating around the uranium fuel in the first of three separate water systems.   The water is heated to extremely high temperatures, but doesn’t boil because the water is under pressure.  The water within the primary system passes over the reactor core to act as a moderator and coolant but does not flow to the turbine.   It is contained in a pressurized piping loop. The hot, pressurized water passes through a series of tubes inside the steam generator. 

These tubes are surrounded by another water system called the secondary or steam generating system.  The heat, but not the water, from the primary coolant is transferred to the secondary, system which then, turns into steam. 

The primary and secondary systems are closed systems.  This means the water flowing through the reactor remains separate and does not mix with water from the other systems. 

The steam is pumped from the containment building into the turbine building to push the giant blades of the turbine.  The turbine is connected to an electrical generator. 

After turning the turbines, the steam is cooled by passing it over tubes carrying a third water system called the condenser coolant.  As the steam is cooled, it condenses back into water and is returned to the steam generator to be used again and again. 


 

Boiling Water Reactors (BWR)

Unlike the PWR, inside the boiling water reactor, the primary water system absorbs enough heat from the fission process to boil its water.  In contrast to the PWR, the BWR uses only two separate water systems as it has no separate steam generator system.   This steam and water mixture rises to the top of the reactor and passes through two stages of moisture separation.  Water droplets are then removed and steam is allowed to enter the steam line.  The steam is directed to the turbine.  The turbine begins to turn within the generator and electricity is produced. 

Once the turbines have turned, the remaining steam is cooled in the condenser coolant system. This is a closed water system.  Heat from the steam is absorbed by the cool water through heat transference.  The water within the two systems does not mix.  Once through the condenser system, the water is recycled back into the reactor to begin the process again.

 

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