Pressurized water reactors (PWR) are generation II nuclear power reactors that use ordinary water under high pressure as coolant and neutron moderator. The primary coolant loop is kept under high pressure to prevent the water from reaching film boiling, hence the name. PWRs are the most common type of power producing reactor and are widely used all over the world. More than 230 of them are in use to generate electric power, and several hundred more for naval propulsion. They were originally designed at the Oak Ridge National Laboratory for use as a nuclear submarine power plant. Follow-on work was conducted by Bettis Atomic Power Laboratory.
In a typical commercial pressurized light-water reactor the core inside the reactor vessel creates heat, pressurized water in the primary coolant loop carries the heat to the steam generator, inside the steam generator, heat from the steam, and the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted in to the condenser where it condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need power. Pressurized-water reactors contain between 150-200 fuel assemblies.
The Pressurized Water Reactor has 3 separate cooling systems. The reactor coolant system, shown inside the containment, consists of 2, 3, or 4 cooling "loops" connected to the reactor, each containing a reactor coolant pump, and steam generator. The reactor heats the water that passes upward past the fuel assemblies from a temperature of about 530°F to a temperature of about 590°F. Boiling, other than minor bubbles called nucleate boiling, is not allowed to occur. Pressure is maintained by a pressurizer connected to the reactor coolant system. Pressure is maintained at approximately 2250 pounds per square inch through a heater and spray system in the pressurizer. The water from the reactor is pumped to the steam generator and passes through tubes. The reactor cooling system is expected to be the only one with radioactive materials in it.
In a secondary cooling system, which include the main steam system and the condensate-feedwater systems, cooler water is pumped from the feedwater system and passes on the outside of those steam generator tubes, is heated and converted to steam. The steam then passes through the main steam line to the turbine, which is connected to and turns the generator. The steam from the turbine condenses in a condenser. The condensed water is then pumped by condensate pumps through low pressure feedwater heaters, then to the feedwater pumps, then to high pressure feedwater heaters, and then to the steam generators. The diagram above simplifies the process by only showing the condenser, a pump, and the steam generator.
The condenser is maintained at a vacuum using either vacuum pumps or air ejectors. Cooling of the steam is provided by condenser cooling water pumped through the condenser by circulating water pumps, which take a suction from water supplied from the ocean, sea, lake, river, or cooling tower.
Nuclear Power Plants
The United States currently has 104 nuclear power plants. Of those, only 69 are pressurized water reactors and the remaining are boiling water reactors (BWR). Combined, they produce 19.3% of its power from nuclear power. These power plants also have a high level of performance. There are extension of reactor lifetimes from 40 to 60 years taking place with is enhancing the economic competitiveness of plants. In the United States, the industry envisages substantial new nuclear capacity by 2020 and several regulatory initiatives are preparing the way for new orders.
There are currently 439 nuclear power plants. Of those, 57% are pressurized water reactors, 22% are boiling water reactors, 8% are gas cooled reactors, and 13% are other reactors. 15% of the world energy production is nuclear. 30 countries are occupying the 439 nuclear power plants. As of August 2008 there are 39 power plants under construction. The International Atomic Energy Agency has significantly increased its projection of world nuclear generating capacity. It now anticipates at least 60 new plants in the next 15 years, making 450 to 690 GWe in place in 2030 - very much more than projected in 2000 and 21% to 85% more than actually operating in 2008. The change is based on specific plans and actions in a number of countries, including China, India, Russia, Finland and France, coupled with the changed outlook due to the Kyoto Protocol. This would give nuclear power a 17% share in electricity production in 2020. The fastest growth is in Asia.
Significance of PWRs
Pressurized water reactors have been around for over 50 years. The first nuclear power plant that was built in the United States was the Shippingport Atomic Power Station. It was designed as a PWR. It went critical on December 2, 1957. The Shippingport Power Station was operated for Duquense Light for 25 years. PWRs were a generation II design that evolved into a generation III design that include better safe guards and new design technology. Many of the generation II and generation III designs, which are still in operation today, have paved the way for future designs.
With nearly 1/6 of the world power and 1/5 of the United States power coming from nuclear power, the current infostructure for electrical power depends on nuclear power for much of its power supply. Over 1/2 of all nuclear power is created from nuclear reactors that are designed as PWRs. These designs have a strong hold on the present nuclear power industry and look to grow in numbers in the near future. Many of the nuclear power plants that are currently being constructed by Westinghouse and Areva are PWR designs. These designs are hopefully going to be implemented in many places in the future.
Currenty, Westinghouse is working on a generation III+ design, the AP 1000. Its design, based on the prior AP 600, will use new, up to date technology, and also use a completely new interface in the control room. This generation III+ design has great importance as we wait to welcome the generation IV design. There is no set time table for the introduction of the generation IV design and no guarentee that this design will be safe or economical, but it is hoped to be the future of the industry. Until the generation IV design makes more significant progress, the nuclear industry will work at making strides to improve on prior designs and create a generation III+ model.
Nuclear power as a whole, and specifically PWRs, have a very bright future in both the United States and world wide. While the United States has yet to begin reprocessing nuclear fuel, other countries have acceptted this idea and use reprocessed fuel in PWRs. Recycling nuclear fuel is a positive. Less fuel needs to be mined when you reprocess the fuel that has already been mined. It also is a very good way to control nuclear waste. With less waste, less space is needed to store used, radioactive material. Nuclear power and PWRs have great sustainability as nuclear fuel will ultimtely become a renewable resource once reprocessing can be refined.