Basic Nuclear Operation

The principles of any steam generated power system is basically the same.

Heat water to make steam
Inject steam into turbine
Steam pressure rotates turbine
Turbine rotates generator
Generator makes electricity

Nuclear power plants operate under this basic process but with a very different steam generator (vessel used to boil water) system. Water is heated through the use of a reactor. There are essentially two different types of reactors BWR (boiling water reactor) and PWR (pressurized water reactor).

Boiling Water Reactor (BWR)

BWR is the abbreviation for the Boiling Water Reactor. These reactors were originally designed by Allis-Chalmers and General Electric (GE). The General Electric design has survived, whereas all Allis-Chalmers units are now shutdown. The first GE US commercial plant was at Humboldt Bay (near Eureka) in California. Other suppliers of the BWR design world-wide have included; ASEA-Atom, Kraftwerk Union, Hitachi. Commercial BWR reactors may be found in Finland, Germany, India, Japan, Mexico, Netherlands, Spain, Sweden, Switzerland, and Taiwan.

The BWR reactor typically allows bulk boiling of the water in the reactor. The operating temperature of the reactor is approximately 570F producing steam at a pressure of about 1000 psi. Current BWR reactors have electrical outputs of 570 to 1300 MWe.

In the figure above, water is circulated through the Reactor Core picking up heat as the water moves past the fuel assemblies. The water eventually is heated enough to convert to steam. Steam separators in the upper part of the reactor remove water from the steam.

The steam then passes through the Main Steam Lines to the Turbine-Generators. The steam typically goes first to a smaller High Pressure (HP) Turbine, then passes to Moisture Separators (not shown), then to the 2 or 3 larger Low Pressure (LP) Turbines. In the drawing above there are 3 low pressure turbines, as is common for 1000 MWe plant. The turbines are connected to each other and to the Generator.

The Generator produces the electricity, typically at about 20,000 volts AC. This electrical power is then distributed to a Generator Transformer, which steps up the voltage to either 230,000 or 345,000 volts. Then the power is distributed to a switchyard or substation where the power is then sent offsite.

The steam, after passing through the turbines, then condenses in the Condenser, which is at a vacuum and is cooled by ocean, sea, lake, or river water. The condensed steam then is pumped to Low Pressure Feed water Heaters (shown but not identified). The water then passes to the Feed water Pumps which in turn, pump the water to the reactor and start the cycle all over again.

The BWR is unique in that the Control Rods, used to shutdown the reactor and maintain an uniform power distribution across the reactor, are inserted from the bottom by a high pressure hydraulically operated system. The BWR also has a Torus (shown above) or a Suppression Pool. The Torus or suppression pool is used to remove heat released if an event occurs in which large quantities of steam are released from the reactor or the Reactor Recirculation System, used to circulate water through the reactor.

The General Electric BWR designs are designated BWR-1 through BWR-6. Typical examples of each of these classes of BWR are:

BWR-1 Big Rock Point and Dresden-1 (both now permanent shutdown and/or decommissioned)
BWR-2 Oyster Creek
BWR-3 Monticello
BWR-4 Vermont Yankee
BWR-5 Columbia
BWR-6 Perry

Pressurized Water Reactor (PWR)

PWR is the abbreviation for the Pressurized Water Reactor. These reactors were originally designed by Westinghouse Bettis Atomic Power Laboratory for military ship applications, then by the Westinghouse Nuclear Power Division for commercial applications. The first commercial PWR plant in the United States was Shippingport, which operated for Duquesne Light until 1982.

In addition to Westinghouse, Asea Brown Boveri-Combustion Engineering (ABB-CE), Framatome, Kraftwerk Union, Siemens, and Mitsubishi have typically built this type of reactor throughout the world. Babcock & Wilcox (B&W) built a PWR design power plant but used vertical once-through steam generators, rather than the U-tube design used by the rest of the suppliers. Industry consolidation has occurred so that Framatome-ANP and Westinghouse are two key remaining manufacturers. Refuelings are done with the plant shutdown.

The Pressurized Water Reactor (PWR) has 3 separate cooling systems. Only 1 is expected to have radioactivity - the Reactor Coolant System.

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 530F to a temperature of about 590F. Boiling, other than minor bubbles called nucleate boiling, is not allowed to occur. Pressure is maintained by a Pressurizer (not shown) connected to the Reactor Coolant System. Pressure is maintained at approximately 2250 psi 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. Typically PWR's have 2, 3, or 4 reactor cooling system loops inside the containment.

In a Secondary Cooling System (which include the Main Steam System and the Condensate-Feed water Systems), cooler water is pumped from the Feed water System and passes on the outside of those steam generator tubes, is heated and converted to steam. The steam then passes through the a 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 Feed water Heaters, then to the Feed water Pumps, then to High Pressure Feed water Heaters, 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 (shown).

PWR's of varying sizes have been built since the late 60's.

Westinghouse 1 Loop Zorita (Spain)
Westinghouse 2 Loop Ginna (US); Krsko (Slovenia)
Westinghouse 3 Loop Turkey Point (older); North Anna (newer)
Westinghouse 4 Loop San Onofre-1 (original and shutdown); Zion (older and shutdown); Callaway (newer)
Combustion Engineering 2 Loop Calvert Cliffs
Backcock & Wilcox Davis-Besse

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A B C D E F G H I K M N P R S T U X Z

ALARA - Acronym for As Low as Reasonably Achievable: one of the three basic criteria recommended by the International Commission on Radiological Protection to minimize radiation risks.

Alpha Radiation - A positively charged particle made up of two neutrons and two protons. It is the least penetrating of the three common forms of radiation, and can be stopped by a sheet of paper.

Atom - The basic building block of all matter. An atom has a nucleus made up of positively charged protons and neutral neutrons surrounded by orbiting electrons whose negative charge balances that of the protons in the nucleus.

Atomic Number - The number assigned to each element on the basis of the number of protons found in the element's nucleus.

Atomic Weight (Atomic Mass) - Approximately the sum of the number of protons and neutrons found in the nucleus of an atom.

Background Radiation - The naturally occurring nuclear radiation coming from outer space as cosmic radiation, or from naturally occurring radioactive elements such as uranium and radium in the materials of the earth.

Becquerel (Bq) - The unit of radioactive decay equal to 1 disintegration per second. 37 billion becquerels is equal to 1 curie (Ci). There are 30,000 disintegrations per second taking place inside a household smoke detector.

Beta Radiation - An electron (or a particle of equal weight to an electron but with a positive electrical charge) that is emitted from a radionuclide. It is less damaging than the same dose of alpha radiation but more penetrating. Beta radiation can be stopped by a thin sheet of metal or plastic.

Chain Reaction - A reaction that initiates its own repetition. In a fission reaction, free neutrons are produced which fly off and strike other nuclei, causing them to split and send off yet more free neutrons. The fission will continue as long as there are enough free neutrons carrying the right amount of energy.

Cladding - The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents the corrosion of the fuel by the coolant and the release of fission products in the coolants. Aluminum, stainless steel and zirconium alloys are common cladding materials.

Containment - Most reactors are enclosed in a thick, concrete, domed building, called the containment. In the event of a release of radioactive material into the reactor building, the containment traps the emissions and prevents their escape.

Contamination - Radioactive material deposited or dispersed into materials or places where it does not belong.

Control Rods - A rod containing neutron-absorbing materials, such as boron or cadmium. Control rods are moved in and out of the core of the reactor to control the rate of the nuclear reaction.

Coolant - The liquid or gas used to transfer the heat of nuclear fission to a heat exchanger in which steam is produced to drive the electrical generator. The cooled liquid or gas is then returned to the reactor.

Core - The central region of a nuclear reactor, containing the fuel assemblies, coolant and moderator, in which the fission chain reaction takes place.

Critical Mass - The minimum amount of fuel needed in the core of a nuclear reactor in order to start a self-sustaining chain reaction. When a reactor starts up it is said to "go critical".

Curie (Ci) - A unit used to measure the rate of radioactive decay. One curie equals 37 billion disintegrations per second, or approximately the radioactivity of one gram of radium.

Decay (Radioactive) - The change of one radioactive nuclide into a different nuclide by the spontaneous emission of alpha, beta, or gamma radiation, or by electron capture. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life.

Decay heat - The heat produced by the decay of radioactive fission products after the reactor has been shut down.

Decontamination - The reduction or removal of contaminating radioactive material from a structure, area, object, or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination; (2) letting the material stand so that the radioactivity is decreased by natural decay; and (3) covering the contamination to shield the radiation emitted.

Deuterium - A stable, naturally occurring hydrogen isotope. It is used as a moderator in the form of deuterium oxide or heavy water.

Dose - A general term denoting the quantity of radiation or energy absorbed in a specific mass.

Electromagnetic Radiation - Electric or magnetic waves that travel at the speed of light. Examples: light, heat, radio waves, microwaves, gamma radiation, x-rays.

Electron - An elementary particle carrying one unit of negative electrical charge. Electrons surround the atom's positively charged nucleus and determine the atom's chemical properties.

Emergency feedwater system - Backup feedwater supply used during nuclear plant startup and shutdown; also known as auxiliary feed water.

Enrichment - The physical process of increasing the concentration of the uranium-235 isotope relative to the predominant uranium-238 isotope in natural uranium.

Feedwater - Water supplied to the steam generator that removes heat from the fuel rods by boiling and becoming steam. The steam then becomes the driving force for the turbine generator.

Fission - The break-up of the nucleus of an atom into two major fragments, plus smaller fragments and free neutrons, when the nucleus is struck by a fast-moving free neutron.

Fuel rod - A long, slender tube that holds fuel (fissionable material) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.

Gamma Radiation - Highly penetrating, short wavelength radiation emitted from the nuclei of atoms. It is stopped by an adequate thickness of lead, concrete or other materials.

Geiger Counter - An instrument for detecting and measuring ionizing radiation. It contains a gas-filled tube which discharges electrically when ionizing radiation passes through it.

Half-life - The time over which the atoms of a particular radioactive nuclide decay to half their original intensity of emitted radiation. The half-life is a characteristic property of each radioactive isotope.

Heavy Water - Heavy water or deuterium oxide (D₂0) is a natural form of water used to lower the energy of neutrons in a reactor. It is heavier than normal water by about 10 per cent, and occurs in minute quantities (about one part heavy water per 7,000 parts water).

IAEA - The International Atomic Energy Agency is an agency set up by the United Nations to monitor and promote the peaceful uses of nuclear energy.

Ion - An atomic particle, atom or molecule that is electrically charged.

Ionizing Radiation
Any type of radiation that can, directly or indirectly, change the electric charges of atoms or molecules. It is produced when radionuclides decay.

Isotope - Different forms of atoms of the same element. They have the same number of protons in their nuclei but a different number of neutrons (the same atomic number but different atomic weights). Uranium-238 and uranium-235 are isotopes of uranium. Isotopes may be stable (not spontaneously decaying) or unstable (spontaneously decaying, emitting ionizing radiation).

Kilowatt - Kilowatt (KW) is a metric measurement of power, and is equal to 1,000 watts.

Kilowatt-Hour - The basic unit of electric energy equal to one kilowatt of power supplied to or taken from an electric circuit steadily for one hour. A kilowatt-hour (KWH) is 1,000 watt-hours.

Megawatt - Megawatt (MW) is a unit of power, is equal to one million watts, and refers to the heat output of a reactor. MWe refers to electrical output.

Moderator - Moderators are used to lower ("moderate") the energies of a portion of the neutrons emitted by fissioning uranium atoms, to increase their probability of hitting another uranium atom to cause further fissioning. Graphite and light water are frequently used as moderators.

Nuclear Energy - Nuclear energy is the energy stored in the bonds of the sub-atomic particles in the nucleus of atoms.

Nuclear Reactor - A device in which nuclear fission may be sustained and controlled in a self-supporting nuclear reaction. There are several varieties, but all incorporate certain features, such as fissionable material or fuel, a moderating material (to control the reaction), a reflector to conserve escaping neutrons, provisions for removal of heat, measuring and controlling instruments, and protective devices

Nucleus - The core of the atom, where most of its mass and all of its positive charge is concentrated. Except for hydrogen, it consists of protons and neutrons.

Nuclide - Any species of atom that exists for a measurable length of time. A nuclide can be distinguished by its atomic weight, atomic number, and energy state.

Neutrino - An electrically neutral particle with negligible mass. It is produced in many nuclear reactions such as in beta decay.

Neutron - One of the basic particles which make up an atom. A neutron and a proton have about the same weight, but the neutron has no electrical charge.

Pressure Vessel - A strong-walled container housing the core of most types of power reactors.

Pressurizer - A tank or vessel that controls the pressure in a certain type of nuclear reactor.

Primary System - The cooling system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine.

Proton - One of the basic particles which makes up an atom. The proton is found in the nucleus and has a positive electrical charge equivalent to the negative charge of an electron and a mass similar to that of a neutron.

Radioactive Dating - A technique for estimating the age of an object by measuring the concentrations of various radioisotopes in it.

Radiation - Energy given off by atoms when they are moving or changing state. It can take the form of electromagnetic waves, such as heat, light, X-rays, or gamma rays, or streams of particles such as alpha particles, beta particles, neutrons or protons.

Radioactivity - The emission of alpha particles, beta particles, neutrons and gamma or x-radiation from the disintegration of an atomic nucleus.

Radioisotopes - Atoms of chemical elements may have many isotopes (different forms) with different atomic numbers and different atomic weights. If an isotope is radioactive, it is sometimes referred to as a radioisotope or a radionuclide.

Radionuclide - An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.

Scintillation Counter - An instrument that detects and measures gamma radiation by counting the light flashes (scintillations) induced by the radiation.

Secondary System - The steam generator tubes, steam turbine, condenser and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation.

Shielding - A protective barrier for reducing or eliminating the transfer of radiation from radioactive materials to the surroundings. Depending on the radiation level, shielding can range from paper, to water, to lead, to concrete.

Sievert (Sv) - A unit that is used for describing the absorption of radiation by the human body.

Source - A radioactive material that produces radiation for experimental or industrial use.

Spent Fuel - Fuel assemblies taken out of a nuclear reactor after a period of useful energy production. Also referred to as used fuel.

Turbine - A rotary engine made with a series of curved vanes on a rotating shaft. Usually turned by water or steam. Turbines are considered to be the most economical means to turn large electrical generators.

Tracer - A small amount of radioactive isotope introduced into a system in order to follow the behavior of some component of that system.

Uranium - The heaviest naturally occurring element, with an atomic number of 92.

X-ray - An X-ray is a form of electromagnetic radiation with a very short wave length. (see radiation)

Zirconium - Zirconium is a metallic element with atomic number of 40. An alloy of zirconium known as Zircaloy is extensively used for the cladding of nuclear fuel elements.

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