The basic function of a pressurized water reactor (PWR) nuclear power plant is to convert the energy of nuclear fission into electricity. A PWR power plant uses many separate systems to make the conversion of nuclear fission into electricity. These systems are divided up into three main categories: primary systems, secondary systems, and control systems.
The primary and the secondary systems make the energy conversion a two stage process. First, the primary systems convert the fission energy into steam. Second, the secondary systems convert the steam into electricity, which is delivered to the consumer. The control systems control the plant parameters in both the primary and the secondary systems, such as pressure, temperature, flow rate, and level. The control systems also allow operators to monitor the operation of the plant and the control systems can be shifted to manual if necessary.
Figure 1: A typical PWR plant
As stated earlier, the purpose of the primary systems is to convert fission energy into steam for the secondary systems. To complete this conversion, the primary systems use the Reactor Coolant System (RCS) and several support systems.
The RCS is made up of a pressurized water reactor vessel and a number of reactor coolant loops connected in parallel to the reactor vessel. Each reactor coolant loop contains a reactor coolant pump and a steam generator.
Figure 2: A Westinghouse 4 loop RCS
In the secondary systems for a PWR power plant, steam is converted from the steam generators in the primary systems to electrical power. From the steam generator, steam is supplied to the turbines. The steam turbine drives a generator connected to the electric grid for distribution. From the turbines, the steam water mixture is cooled and condensed in a condenser before being fed into the steam generator again. steam into water called condensate, which completes the secondary systems.
As stated before, the control systems monitor and control key plant parameters. Some key control systems that are vital to the PWR plants operation are the fuel control, rod control system, and pressurizer control system.
World PWR Use
As can be seen from the chart below the PWR is the predominant nuclear power plant in the world today. With 264 of the 439 commercial plants, PWRs account for about 60% of the commercial nuclear reactors. That 60% produces 250.5 Giga-Watts of electric power which accounts for over 65% of commercial nuclear power generation.
Table 1: Operating Commercial Nuclear Power Plants
|Reactor Type||Main Countries||Number||GWe|
|Pressurized Water Reactor (PWR)||US, France, Japan, Russia||264||250.5|
|Boiling Water Reactor (BWR)||US, Japan, Sweden||94||86.4|
|Pressurized Heavy Water Reactor 'CANDU' (PHWR)||Canada||43||23.6|
|Gas-cooled Reactor (AGR & Magnox)||UK||18||10.8|
|Light Water Graphite Reactor (RBMK)||Russia||12||12.3|
|Fast Neutron Reactor (FBR)||Japan, France, Russia||4||1.0|
U.S. PWR Use
There are currently 104 commercial nuclear power plants fully licensed by the U.S. Nuclear Regulatory Commission (NRC) to operate in the United States. Out of the 104, 69 plants are categorized as Pressurized Water Reactors (PWRs), which produce a total of 65,100 net megawatts (electric), and 35 plants are Boiling Water Reactors (BWR), which produce a total of 32,300 net megawatts (electric). In the year 2006, the 104 nuclear facilities operating in the United States generated 787,219 Million Kilowatt-hours of electricity. That accounted for 19.4% of all U.S. electricity generated. The table below shows all the companies currently operating nuclear PWR plants in the U.S. and also lists the number of plants they operate.
Table 2: U.S. Commercial Nuclear Power Plant Operators
|Amergen Energy Co.||1|
|Arizona Public Service Company||3|
|Constellation Nuclear Group||3|
|Carolina Power & Light Co.||2|
|Duke Power Co.||7|
|Exelon Nuclear Co.||4|
|First Energy Nuclear Operating Co.||3|
|Florida Power & Light Co.||5|
|Florida Power & Light Co.||2|
|Nuclear Management Co.||6|
|Omaha Public Power District||1|
|Pacific Gas & Electric Co.||2|
|Progress Energy Corp.||1|
|Public Service Electric & Gas Co.||2|
|Southern California Edison||3|
|Southern Nuclear Operating Co.||4|
|STP Nuclear Operating Co.||2|
|Tennessee Valley Authority||3|
|TXU Electric Co.||2|
|Wolf Creek Nuclear Operation Corp.||1|
Additional Use: The Nuclear Navy
Along with providing eletrical power for the U.S. and Worldwide, the PWR is the reactor of choice for Navy Nuclear Propulsion systems. The PWR Nuclear Propulsion system is displayed below. The PWR navy reactor has a much higher level of enrichment and is permanently sealed, meaning unlike commerical PWR reactors, it doesn't need refueled. The reactor core remains inaccessible throughout the long life of the fuel. Most commercials reactors are refueled every 18 months.
Figure 3: Navy Nuclear Propulsion PWR system
Pressurized water reactors have many advantages over earlier generation nuclear sites. As the temperature increases, they tend to produce less power. This makes the reactor easier to operate from a stability standpoint. They can also be operated with a core containing less fissile material than is required for them to go prompt critical, a term used during nuclear fission where an additional fission event sparks a rapid exponential increase in fission events. This drastically reduces the chances that the PWR will go out of control, making them relatively safe from critical accidents. The final advantage is that the turbine cycle is completely separate from the primary loop. The water is in the secondary loop, and will never be contaminated by radioactive materials in the main system loop.
However, there are a few disadvantages to using a pressurized water reactor. The coolant water is needed to be highly pressurized to remain at a liquid state for high temperatures. This will require high strength piping, and a heavy pressure vessel, implying increased construction costs. In addition, most water reactors cannot be refueled while the system is operating. This means that the reactor will have to go offline for long periods at a time, usually about a few weeks. The most significant downside is the possibility and risk of radiation exposure. Boric acid is dissolved into the high temperature water coolant, which is corrosive to carbon steel. This can cause radioactive corrosion products to circulate in the main coolant loop. This not only limits the lifetime operating life of the reactor, but the addition of a system to filter out the corrosion products and adjust the concentration of boric acid will add significantly to the overall cost the system, and the possibility for radiation exposure.