Since nothing is burned in the generation of nuclear energy, no harmful emissions are vented into the atmosphere. This is why nuclear power plants do not have the smokestacks common to fossil fuel generation facilities. Some nuclear power plants use large cooling towers to remove excess heat from cooling water before it is returned to the waterways. The discharge from these towers is water vapor, not smoke or radioactive matter.
So how is nuclear power produced, especially without creating air pollution?
A nuclear power plant produces electricity in much the same way as other electric power plants: Water is heated to produce steam, and the steam turns turbines that turn a generator shaft to make magnetism, which is what electricity is. The difference in a nuclear power plant is how the water is heated. Most power plants today use coal or natural gas to heat the water to become steam. Nuclear power comes from a reactor where atoms are split to release their energy, which produces great amounts of heat.
The design of a nuclear reactor is complex, but basically, each reactor has six main elements: (1) Fuel, (2) Control rods, (3) Coolant, (4) Moderator (5) Shield, (6) Reflector, and (7) Nuclear Vessel. Let's look at each part.
Fuel Fuel is needed in all energy-producing processes, and fuel is at the heart of the reactor. For fuel to produce energy, it must be altered, which happens inside the reactor core. In most U.S. reactors, the fuel consists of pellets of enriched uranium dioxide. The pellets are held inside 12-foot-long metal tubes called “fuel rods.” These fuel rods are put together, or “bundled,” to form the fuel assembly. The process of preparing the fuel, burning the fuel, and disposing of the fuel is called the “fuel cycle.” A closed fuel cycle recycles the fuel from the last stage for reuse in the reactor. An open fuel cycle does not recycle the fuel.
Control rods The control rods are lowered or raised next to the fuel assembly to speed up or slow down the rate of the chain reaction by absorbing some of the neutrons released in fission. To speed up the chain reaction, the rods are pulled up away from the fuel assembly. To slow the reaction down, the rods are lowered next to the fuel assembly. Most control rods use the element boron to absorb neutrons.
Coolant The cooling process in a nuclear reactor is similar to the way a car radiator works to cool the engine. Because the fuel assembly gets very hot during a chain reaction, a coolant, usually water, is pumped through the reactor to carry the heat away. As with most power plants, two-thirds of the energy produced by a nuclear power plant goes into waste heat, and that heat is carried away from the plant in the coolant water (which remains uncontaminated by radioactivity).
In large reactors, as much as 330,000 gallons of water coolant flow through the reactor core every minute. The water that leaves the reactor is sent to either cooling towers or discharged into large bodies of water such as cooling ponds, lakes, rivers, or an ocean.
Coolants have very specific requirements: non-absorbant for neutrons, excellent resistance to high temperatures and high levels of radiation, non-corrosive, high boiling point for liquids to prevent evaporation from the high heat inside the reactor, low melting point for solids, and be easily circulated by a pump.
Moderator When a neutron causes fission, fast neutrons are released. These fast neutrons need to be slowed down to lower energy levels. Neutrons have a better chance of causing an atom to fission if they move considerably slower than their initial speed after being emitted from a fissioning nucleus. The material used to slow down the fast neutrons is called the “moderator.” Fast moving neutrons strike the moderator material, which is not efficient at absorbing them, and slows them down.
Moderators are made of various materials. Water is an excellent moderator because the water can also serve as a coolant. Normal water, known as “light water,” is used in most reactors simply because it is cheap and abundant. The only downside to using light water is that reactor fuel must be enriched when water is the moderator. Another commonly used moderator material is “heavy water”, which is chemically equivalent to normal water, but contains Hydrogen-2 – also known as Deuterium. Deuterium is an isotope of Hydrogen who’s nucleus contains both a neutron and a proton, and is thus twice as massive as Hydrogen-1. The neutrons that cause fission in reactors are not easily absorbed in heavy water, so more of them become available for fission in the fuel. Because of this, “Heavy Water Reactors” can operate with non-enriched (natural) Uranium fuel.
Shield Nuclear fission results in the release of neutrons and several other by-products such as alpha rays, beta rays, gamma rays, and fast moving neutrons. Radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting people and materials outside the reactor.
Typical reactor cores require an inner lining of steel that is almost half a meter thick. Even at that thickness, the steel is not enough protection, so it is reinforced with a few meters of concrete to make it safer. Concrete and steel are very good at absorbing radiation, and they are strong.
Reflector Fast moving neutrons are controlled with a moderator and reflectors to keep them inside the reactor core so that a sustained and controlled chain reaction takes place.
In the fission process, a bullet neutron is absorbed by the target nucleus, which causes the nucleus to divide into two nuclei and emit heat and two or three more neutrons. If all the neutrons keep hitting other nucleui, a chain reaction results. But some neutrons miss other nuclei and bump into the reactor core, which serves no useful purpose. To reduce neutron loss, the inner surface of the reactor core is surrounded by a material to reflect these escaping neutrons back to the reactor core. This lining is known as “reflecting materials.”
Various materials are used as reflecting material. Some, such as light water, heavy water, and carbon, can even serve the dual purpose of reflector and moderator. All reflecting materials must have low absorbtion of neutrons, be stable to withstand high levels of radiation, and resist oxidation.
The reflector helps make the reactor core more efficient because it reduces the consumption of the fissile material and therefore, the reactor core can be reduced to attain the amount of energy needed.
Pressure Vessel The housing that contains all the components in the core is called the “vessel.” The vessel holds the coolant, provides a space for the rods, and acts as a buffer between the core and the environment outside the vessel. The material used to construct the vessel must be very strong and resilient so that it can withstanding great pressures; steel is commonly used for vessel construction. The structure around the pressure vessel is called the “containment.” It protects the reactor from outside intrusion and the people working inside the building from the effects of radiation in case of malfunction. The containment typically is a meter-thick concrete and steel structure.
Types Of Nuclear Reactors
Most nuclear electricity in the United States is generated in two types of reactors, pressurized water reactors (PWR) and boiling water reactors (BWR). Both types are “first generation reactors,” developed in the 1950s, that have maintained the design but with improvements. New reactor designs are coming forward, and some are in operation. The newer designs will be used to replace the first generation reactors as they come to the end of their operating lives.
What is the difference in PWR and BWR reactors?
Pressurized water reactor The most common of the reactor types, PWR reactors, originated as a submarine power plant. In PWRs, the water is kept under pressure so that it heats but does not boil. Coolant water from the reactor flows in a cooling circuit through the core of the reactor under very high pressure. A secondary circuit produces steam to drive the turbine. The water that is turned into steam travels in separate pipes and never mixes with the coolant water. PWRs use ordinary water as both coolant and moderator.
A PWR has fuel assemblies of 200 to 300 rods each that are arranged vertically in the core. Large reactors have about 150 to 250 fuel assemblies.
Boiling water reactor BWR reactors have many similarities to PWRs, except the water flows in only a single circuit under a lower pressure. The water that is heated by fission actually boils, and 12 to 15 percent of the water in the top part of the core is steam; therefore, the water has a lower moderating effect. A secondary control system involves restricting water flow through the core so that steam in the top part reduces its moderation effect.
The steam passes through drier plates (steam separators) above the core and then directly through a closed-loop circuit to the turbines. Because the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this protection balances with the savings gained from the simpler BWR design. Most of the radioactivity in the water is very short-lived, so the turbine hall can be entered soon after the reactor is shut down.
A BWR has fuel assemblies of 90 to 100 fuel rods each, and a reactor core has up to 750 assemblies.
Building a Nuclear Power Plant
Statistics show that U.S. commercial power plants have operated safely for more than a half-century. The success of nuclear power plants comes from continuing to apply technology advances to improve plant safety and health. It also results from the strict licensing, building standards, and regulations governing plant operations.
A nuclear power plant can only be built in the United States once the Nuclear Regulatory Commission (NRC) reviews both construction and operating plans. Before issuing a building permit and operating license for a nuclear plant, the NRC carefully reviews technical aspects of the proposed plan to verify that:
Constructing and operating the plant will not present undue risk to public health and safety;
Licensing the plant will not be harmful to naitonal defense and security;
The utility is technically qualified to design, construct, and opearte the proposed facility; and
The project complies with the National Environmental Policy Act.
The complete licensing and construction of a nuclear power plant requires a lengthy series of licenses and permits from Federal, State, and local government agencies. These permits and licenses determine where the plant can be located, whether the power is actually needed, and how excavation and construction will be carried out. They also ensure the protection of land, air, water, and local plant and animal life from pollution.
Nuclear power plants are designed and built to operate for at least 40 years; permits may be renewed for 20 years, following stringent reviews. After the plant begins operating, more than 200 workers handle its everyday operation and maintenance. These workers include operators and supervisors, mechanical mainteance crews, instrument technicians, electricians, laborers, experts in radiation protection called “health physicists,” and a security guard force.
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Handling Fuel
All operations involving radioactive materials-including nuclear power plants, hospitals, research centers, and industrial processes-create radioactive wasts that must be handled and disposed of safely. Because these wastes vary from slightly to intensely radioactive, they are handled in different ways depending on their level of radioactivity, their form, and other factors.
Industrial users that manufacture radiopharmaceuticals, smoke alrms, emergency exit signs, luminous watch dials, and other consumer goods produce low-level waste, consisting of machinery parts, plastics, and organic solvents. Most of this waste requires little or no shielding and no cooling, and may be handled by direct contact. About half of the total low-level waste generated today comes from nuclear power plants. This includes used resins from chemical ion-exchange processes, filters and filter sludges, lubricating oils and greases, and detergent wastes from laundry operations and from decontaminating personnel and equipment. Most of this waste is processed and packaged for disposal at a specially designed waste facility.
A 1-million kilowatt nuclear power plant typically contains about 100 tons of uranium fuel. Each year, about one-third of the fuel, or roughly 66 of its fuel bundles, are removed and replaced.
As the used fuel rods leave the plant, they are physically similar to the new fuel rods that were originally installed. The main difference is that the uranium fuel that released its energy in the reactor created radioactive fission products and other long-lived radioisotopes. Although they represent only a very small percentage, they continue to generate head and release radiation long after the fuel is removed from the reactor.
Most used fuel from nuclear power plants is stored in forty feet deep pools of water at the reactor site. The water cools the fuel rods to keep them from overheating, and it serves as an effect shield to protect workers from the radiation. The level of radiation begins to decline immediately, and within 10 years it has decayed by some 90 percent. Nevertheless, some fission products remain radioactive for many years. Recognizing that a permanent waste repository was necessary, Congress passed the Nuclear Waste Policy Act of 1982 to establish a national policy for the safe storage and disposal of high-level radioactive waste. The waste is shipped by truck or railroads to places specially designed to safely store the material. The NRC is responsible for licensing and regulating all commercial users and handlers of radioactive materials, including waste shippers and carriers. Please see the section, “What is Nuclear Waste?” to learn more.
