Pressurized Water Reactors
Primary Systems of A PWR
These water reactors also use steam to turn a turbine and hence generate steam. However, the mode of cooling the steam is different from that of boiling water reactors. Steam is directed to the condenser where cool water is used in the removal of excess heat. This system comprises of two systems: the primary and secondary systems. Primary system comprises of the steam generators, reactor vessel, coolant pumps, pressurizer and connecting piping. The primary system (reactor coolant system) functions to transfer heat to the steam generators from the fuel. It also serves to house any wastes from fission of the fuel. Reactor vessel houses the reactor core and other vital support devices. This mainly comprises of the core barrel, the reactor core, and some internal packages.
The Core
The reactor pressure level houses the core of the PWR. The outer housing is made of seven inches of forged steel. The plates of steel form a hollow cylinder for housing the core. The core contains approximately 100 tons of nuclear fuel. The fuel is usually uranium dioxide that is loaded in metal fuel rods placed in a square array. The lower and upper heads are domed as is structurally necessary. The upper head can be unbolted and removed to allow entrance to the core during refueling. The power levels of the core have to be controlled to avoid any accidents from high temperature and pressure (Nero, 1979).
These control rods are made up of elements, such as boron, that can receive electrons from the fission process. Long metal poles, used as control rod drive mechanisms, inject and withdraw the rods to and from the space in the core. To achieve shutdown, the control rods are inserted fully into the core for a short duration to interrupt the nuclear chain reaction. Power levels are also controlled by adjusting the concentration of boron dissolved at the bottom of the reactor power vessel (RPV). Water flows in loops, in the reactor power vessel, and is heated by energy emitted from the splitting of atoms. However, due to the high pressure the water does not boil.
The pressurizer is a part of the reactor coolant system that is used to control the pressure of the system. This utilizes relief valves, electrical heaters, and safety valves to maintain the required system pressure. The relief valve vents fluid from the pressurizer to avoid over-pressurization and hence bursting of the pressurizer. The operation makes use of an equilibrium amount of water and steam. The whole system is a control mechanism that aims at restoring a desired pressure in the system. Pressure deviations usually arise due to temperature variances in the RCS (Nero, 1979).
An increase in temperature of the reactor coolant system causes the density of the reactor coolant to increase. This means that more water takes space as compared to the steam. This water expands into the pressurizer via a surge line. Steam in the pressurizer will be compressed and hence increase the pressure. If pressure decreases, the water takes up less space and becomes denser. This decrease in the level of water in the pressurizer causes a pressure decrease.
Secondary Systems of A PWR
Secondary systems of a PWR consist of the feed-water system and the main steam system. The secondary system is isolated from the primary system and hence contains little radioactive material. The steam system is connected to the outlet of the steam generator. After the steam has turned the high-pressure turbine, it is directed to the main separators and reheators. At this stage, the steam is dried of most of moisture and then reheated. The steam is then directed to the low-pressure turbines and thereafter to the main condenser. The water circulating in the pipes condenses the steam (Lahey and Moody, 1977).
The main condenser removes most energy from the steam since it is operated at a vacuum. This is the starting point of the feed-water system. The condensed steam is collected in a hot well and pumped to a cleanup system. Failure to remove any impurities may result in damage of the steam generator or poor heat transfer capability of the steam. The condensate is then heated and directed to the suction of the feed-water pumps. The water passes through several high-pressure heaters that are heated by steam from the high-pressure turbine. This heated water is then directed to the steam generators.
The steam generators produce steam that is then transported to the turbine. The core’s power level determines the amount of steam that is generated. An increase in the core’s power level causes an increase in the amount of steam generated. The steam then turns the turbine at a speed of 1800 or 3600 revolutions per minute (rpm). Relief valves that can automatically open to the atmosphere are used to protect the main steam piping from over-pressurization. The valves close upon a decrease in the pressure.
The turbine is coupled to an electrical generator. This is the central purpose for having the nuclear power plant. The main generator generates electricity from the turning of the turbine. This electricity is then routed to step-up transformers to boost the voltage to make it synchronized to the transmission grid. Once the team has turned the turbines, it is cooled and condensed in the condenser using water from a nearby lake or river. The condensed water is then used to generate additional steam.
Chemical Manipulation
Under normal operation, water in the primary system flows continuously through, the demineralizers and the volume control tank. This system is used to control the chemical composition of water in the primary system. The primary and secondary systems do not mix. This precautionary measure aims at ensuring that no radioactive materials are emitted by the plant.
Boiling Water Reactor
Boiling water reactor consists of production of a steam –water mixture through heating of the latter in a core. The resultant mixture is then directed to a steam line where the water is removed from the steam before the steam is directed to the turbines. The turbine rotates mechanically and so does the generator coupled on it. Any steam that is unused in the rotary movement is condensed to water for use in production of more steam. This is facilitated by the use of recirculation pumps.
Reactor Vessel Assembly
The BWR consists of several components that aid in removal of water vapor from the steam. It also provides support to the control and fuel rods. The water used in the reactor must be of high quality especially as concerns purity levels. Soluble and insoluble impurities, fission products and other impurities are removed from the water through the reactor water clean-up system. The water is subjected to filtration and demineralization after the recirculation. Emergency core cooling systems also exist to provide for a cooling mechanism of the core in case of failure of the normal coolants. This immensely impedes damage to the fuel cladding. These emergency systems consist of both high and low pressure systems.
Structure of BWRs
Reactor Core and Internals
The reactor core is normally housed in a reactor vessel that has fuel assemblies and corresponding control rods. There are equipments used to generate steam such as a steam dryer and steam-water separator. There are equipments used to control the reactor power such as control rod drive housing and guide tubes. The BWR fuel assembly may consist of 62 fuel rods, a spacer holding water rod, and a water rod. The fuel rods are made of uranium dioxide pellets plus accompanying accessories such as the plenum spring that creates an accommodation for the discharged fission gas. Control rods may be made of different materials: boron carbide, hafnium, or several combinations of elementary materials (Kramer, 1958).
Several control drive mechanisms are used to vary power: hydraulic pressure drive and motor drive. In case of an anomaly in the plant, all the rods are inserted into the reactor core to shut down the operation of the nuclear plant. Furthermore, a boron acid solution (neutron absorber) is available at the bottom of the reactor core in case the control rods cannot be inserted, or their mechanism fails. Refueling is done once in about 12 to 24 months. The refueling takes about 20 days and about 20% of the fuel assemblies are replaced. The decay heat of the fuel that has been spent is removed using heat exchangers of the reactor. The fuel pool cooling and cleanup are achieved through several components such as filter demineralizers, pumps, and heat exchangers.
References
Kramer, A. W. (1958). Boiling Water Reactors. Reading, Mass.: Addison-Wesley Pub. Co.
Lahey, R. T., & Moody, F. J. (1977). The Thermal Hydraulics of Boiling Water Nuclear
Reactor. Hinsdale, Ill.: The Society.
Nero, A. V. (1979). A Guidebook to Nuclear Reactors. Berkeley: University of California Press.
