One of the types of nuclear reactors is the pressurized water reactor, abbreviated as PWR. The PWR were initially produced by Westinghouse Bettis power laboratory, and were intended for use in military ships (Stacey, 2007), and then for the commercial applications of the company’s nuclear power division. The plants were later produced by Asea Brown-Combustion Engineering (ABB-CE), Kraftwerk Union, Framatonne, Mitsubishi and Siemens. Due to industry consolidation, the two key manufacturing companies are the Westinghouse and the Framatonne-ANP.
On the pressurized water reactor, there are three separate cooling systems, and only one of these is expected to have radio-activity. This is the reactor’s coolant system, and it consists of toe, three, or four cooling loops that are connected to the reactor (Rajput, 2007). Each loop contains a coolant pump and a heat exchanger. Water is heated in the reactor, and moves up passing through the fuel assemblies. The range of temperature of water is between 530F and 590F. Except some minor bubbles usually called nucleate boiling, the actual boiling of water is not allowed to take place.
Pressure in the system is regulated using pressurizer which is included in the reactor’s cooling system. The pressurizer maintains the pressure to about 2250psi using a heater and spray system. Water is pumped from the reactor to the heat exchangers, passing through the tubes. The reactor’s cooling system is the only one containing radio-active materials. A typical PWR has two to four cooling loops within the containment.
In the secondary cooling system of the reactor, there is the main steam system and a condensate feed water system. In this secondary coolant system, water from the feeding system is pumped to pass outside the tubes of the heat exchanger, where it is heated to produce steam. This steam moves to the turbine through the main steam line. The turbine operates a generator. The steam then condenses in the condenser, where the condensate water is pumped using condensate pumps back the steam generator (Rajput, 2007). From the condenser to the steam generator, the condensate moves via s series of feed water heaters including low pressure, high pressure and finally to the steam generator.
The condensers are kept at vacuum pressure by use of either air ejectors or vacuum pumps. The steam is cooled by the cooling water from condenser, which is pumped by the circulating pumps through the condenser. The pump take suction from the water delivered from the selected source (lakes, rivers, ocean, sea or cooling towers).
The PWR have fuel assemblies of between 200 to 300 rods, which are vertically arranged inside the core. Large reactors mainly have up to 250 fuel assemblies and up to 100 tons of uranium. Inside the core of the reactor, water temperature reaches up to 325 degrees Celsius, and therefore it must be maintained below 150 times the atmospheric pressure so that it does not boil (Stacey, 2007). The pressure inside the reactor is controlled by steam in the pressurizer. Inside the primary cooling circuit, water also serves as the moderator and the presence of steam would lower the rate of fission reaction. This creates a negative feed-back effect, which is a safety feature of the reactor. Boron can also be added to the primary circuit to create a secondary shutdown.
In the secondary circuit, the pressure of water is relatively low and therefore it boils in the steam generators, or the heat exchangers. The steam rotates the turbine which is connected to a generator to produce electrical power. As described previously, the steam then condenses and returns to the steam generators.
- Rajput, R. K. Textbook Of Power System Engineering. New Delhi, Laxmi Publications (P) Ltd., 2007,.
- Stacey, Weston M. Nuclear Reactor Physics. Weinheim, Wiley-VCH, 2007,.