Atoms and Nuclear Energy: The Underlying Science
Atoms to Energy: Producing Electricity with Nuclear Energy
Electricity is produced in power-generating plants and transmitted through high-voltage power lines.
             
All About Nuclear -
Making Electricity Some large plants use dams, rivers, or other water (hydro) sources to generate electricity; however, the majority of non-hydro power plants use coal, fuel oil, or natural gas to boil water to produce steam. The steam pressure turns large turbine blades, which then turn a generator. A generator is a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. After the steam passes through the turbines and begins to cool, it changes back to water. The water is cooled further in a separate structure, called a “cooling tower,” at the power plant. The water can be used again and again. Nuclear power plants do not use coal, fuel oil, or natural gas to boil water, but instead, they capture the heat released during fission in the nuclear reactor to heat water to make steam. Capturing Heat during FissionNuclear reactors contain and control fission chain reactions. The intense heat released during fission is captured and used to make steam. Most nuclear power plants use uranium for nuclear fission. Uranium is a common metal found in rocks all over the world. The best type of uranium for nuclear chain reactions is U-235 because its atoms are easily split during fission. Although uranium is a common metal-about 100 times more common than silver-U-235 is relatively rare. Most uranium in the United States is mined in the western states. After uranium is mined, the U-235 must be extracted and processed before it can be used as a fuel. A nuclear reactor uses pellets of uranium that are approximately the size of a dime and an inch in length, or about the size of your fingertip. These energy-rich pellets are stacked end-to-end in 12-foot metal fuel rods. The rods are collected into bundles called the “fuel assembly.” Each uranium pellet produces the same amount of energy as 150 gallons of oil. A pound of uranium is smaller than a baseball and is roughly equivalent to a million gallons of gasoline, which would fill a cube 50 feet per side (50 feet is as tall as a five-story building). Typically, the fuel assembly is submerged in water, which acts as a coolant, inside a pressure vessel. For the reactor to work, the submerged bundle must be slightly supercritical. Left uncontrolled, the uranium would overheat and melt. To prevent the bundle from overheating, operators insert control rods into the bundle. The control rods, which are made of a material that absorbs neutrons, are raised and lowered to control the rate of the nuclear reaction. When the rods are raised out of the uranium bundles, the uranium core produces more heat. When the rods are lowered into the bundles, the rods produce less heat. The rods can also be lowered completely into the uranium bundles to shut the reactor down for repairs or to change the fuel. A nuclear reactor's pressure vessel typically is surrounded in a concrete liner that acts as a radiation shield. That concrete liner is housed in a much larger steel containment vessel that holds the reactor core and the cranes and hardware needed to refuel and maintain the reactor. The steel containment vessel prevents radioactive gases or fluids from leaking from the plant. The steel containment vessel is protected by an outer concrete building that is strong enough to survive a crashing jet airliner or an earthquake. These secondary containment structures prevent the escape of radiation and radioactive steam. Types of Nuclear ReactorsEngineers have developed different types of nuclear power plants. Two types in use in the United States are boiling-water reactors (BWRs) and pressurized-water reactors (PWRs). In BWRs, the water heated by the reactor core turns directly to steam in the reactor vessel, and then it powers the turbine generator. In PWRs, the water that passes through the reactor core is under pressure, and it does not turn to steam; it remains liquid. A separate piece of equipment, a steam generator, produces steam to drive the turbine. A steam generator is a giant cylinder with thousands of tubes carrying the hot radioactive water. The tubes pass through clean (non-radioactive) water outside the steam generator, heating the clean water to boiling, which produces steam. The clean water may come from an ocean, a lake, or a river. The radioactive water, which has cooled as it passes through the tubes, flows back to the reactor core, where it is reheated before it flows back to the steam generator. About 70 percent of reactors in the United States are PWRs. Nuclear Energy and the EnvironmentCompared with electricity generated by burning fossil fuels, nuclear energy is extremely clean. Nuclear power plants produce no air pollution or carbon dioxide, but a small amount of emissions result from processing the uranium that is used in nuclear reactors. Like all industrial processes, nuclear power generation has by-product wastes-spent (used) fuels, other radioactive waste, and heat. Most nuclear waste is low-level radioactive waste that consists of ordinary tools, protective clothing, wiping cloths, and disposable items that have been contaminated with small amounts of radioactive dust or particles. These materials are subject to special regulations that govern their disposal so they will not come in contact with the outside environment. The spent fuel assemblies are highly radioactive and initially must be stored in specially designed pools (rather like large swimming pools) or in specially designed dry storage containers. An increasing number of reactor operators now store older and less spent fuel in dry storage facilities using special outdoor concrete or steel containers with air cooling. The United States Department of Energy's long-range plan is for this spent fuel to be stored deep in the earth in a geologic repository at Yucca Mountain, Nevada.
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