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There are two types of nuclear reactors operating in the United States: the pressurized water reactor (PWR) and the boiling water reactor (BWR). Both types of reactors use fission to heat water and create steam.
How do Nuclear Power Plants Work?
Electricity is produced by using the heat from fission to create mechanical energy, which turns an electric generator. The nuclear energy released by fission is about 1,000,000 times greater than the chemical energy released by combustion, so a small amount of uranium fuel produces an enormous amount of heat, which is used to heat water and turn it into steam. The steam moves the blades of a turbine that then turns the generator.
Types of Reactors
Heat for the BWR is produced in the same way as for a PWR: water within the reactor core is heated by fission. In a BWR, the water is allowed to boil and the resulting steam proceeds directly to the turbine. In a PWR, the heated water is sent through a radiator-like exchanger that uses the heat to turn a separate supply of water into steam. The steam then proceeds to the turbine. In both types of reactors, once the steam has passed through the turbine, it is condensed back to water. This water is then pumped back to the reactor to be heated again, continuing the process.
In the United States, the Nuclear Regulatory Commission must certify the designs for proposed nuclear power plants before a construction and operation license can be issued. Construction is under way on two reactors in Georgia, two in South Carolina and one in Tennessee and another 67 new reactors are being built in 14 countries. Some of these countries, such as the United Arab Emirates, are building their first reactors. Others, such as China and India, already have made a significant commitment to nuclear energy. Generation IV (or Gen IV) designs are nuclear reactor designs still in the conceptual phase and represent the future of nuclear power.
Pressurized Water Reactors (PWRs)
When you pour hot cocoa into a mug, you may notice that the mug soon becomes warm, perhaps even too hot to hold. This is because heat will always flow from a hot material into a cooler one. This scientific law helps us understand how to move the heat energy from inside a reactor to a place where it can be changed into electrical energy. Because of the heat produced by the fission reaction, water that is circulated through the core becomes extremely hot. Generally, when water reaches 100 Celsius (212 Fahrenheit), it boils and turns into a gas called steam. Gases take up more space than liquids. But inside a reactor, there is only a limited amount of space and the water cannot turn into steam. As a result, it can be heated to 315 Celsius (600 Fahrenheit) while still remaining a liquid. We say that the water is under pressure. Because the water in the core is under enough pressure to remain a liquid, the reactor is called a pressurized water reactor or PWR for short.
Boiling Water Reactors (BWRs)
In a typical U.S. BWR water inside a thick steel reactor vessel passes over a reactor core constructed of long cylindrical fuel rods. Inside each fuel rod is a stack of ceramic fuel pellets that contain fissile elements.
The energy of the fission reaction inside these rods is transferred to the water as heat, evaporating the water and producing steam.
The amount of energy produced is controlled — and can be shut off entirely — by small changes in the position of control rods inside the core that absorb the neutrons which fission the fuel.
The high temperature, high-pressure steam rapidly expands between turbine blades, causing the turbine to turn and rotate an electric generator. This produces electricity.
The steam exits the turbine and then passes through a condenser, where it is cooled by cold water from a cooling tower or another source, such as a lake. When cooled, the steam condenses to liquid water and is returned to the reactor core to begin the process again.
DID YOU KNOW?
The first nuclear reactor on Earth was made by Mother Nature. In 1972, scientists found the remains of a natural nuclear reactor in a uranium mine in Oklo, Gabon, Africa. They determined that about 1 1/2 billion years ago a nuclear chain reaction occurred in the mine. It generated heat off and on for 500,000 years.
The real importance of this find was that the radioactive wastes, left over from the “reactor”, were found right where they were left more than a billion years ago.
The waste had not moved or contaminated the area outside the mine. This was without having any man-made containers or the knowledge we have today to safely store and dispose of solidified nuclear wastes.
Center for Nuclear Science and Technology Information of the American Nuclear Society
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