What is Nuclear Energy?
Nuclear energy is energy that comes form the nucleus (core) of an atom. Atoms are the particles that make up all objects in the universe. Atoms consist of neutrons, protons, and electrons. Nuclear energy is released from an atom through one of two processes: nuclear fission. In nuclear fusion, energy is released when the nuclei of atoms are combined or fused together. This is how the sun produces energy. In nuclear fission, energy is released when the nuclei of atoms are split apart. Nuclear fission is the only method currently used by nuclear plants to generate electricity.
What is Uranium?
The fuel by nuclear power plants for fissioning is uranium. Uranium is the heaviest of the 92 naturally occurring elements and is classified as a metal. It is also one of the few elements that are easily fissioned. Uranium was formed when the earth was created and is found in rocks all over the world. Rocks that contain a lot of uranium are called uranium ore, or pitchblende. Uranium, although abundant, is a nonrenewable energy source.
Two forms (isotopes) or uranium are found in nature, uranium-235 and uranium-238. These numbers refer to the number of neutrons and protons in each atom. Uranium-235 is the form commonly used for energy production because, unlike uranium-238, its nucleus splits easily when bombarded by a neutron, causing its nucleus to split apart into two atoms of lighter weight.
At the same time, the fission reaction releases energy as heat and radiation, as well as releasing more neutrons. The newly released neutrons go on to bombard other uranium atoms, and the process repeats itself over and over. This is called a chain reaction.
The Nuclear Fuel Cycle
The steps-from mining the uranium ore, through its use in a nuclear reactor, to its disposal-is called the nuclear fuel cycle.
- Mining – The first step in the cycle is mining the uranium ore. Workers mine uranium ore much like coal miner mine coal-in deep underground mines or in open-pit surface mines. A tone of uranium ore in the United States typically contains three to ten pounds of uranium.
- Milling – After it has been mined, uranium or is crushed. The crushed ore is usually mixed with an acid, which dissolves the uranium, but not the rest of the crushed rock. The acid solution is drained off and dried, leaving a yellow powder called yellowcake, consisting mostly of uranium. This process of removing uranium form the ore is called uranium milling.
- Conversion – The next step in the cycle is the conversion of the yellowcake into a gas called uranium hexafluoride, or UF6. The uranium hexafluoride is then shipped to a gaseous diffusion plant for enrichment.
- Enrichment – Because less than one percent of uranium ore contains uranium-235, the form used for energy production, uranium must be processed to increase the concentration of uranium-235. This process-called enrichment-increases the percentage of uranium-235 from one to five percent. It typically takes place at a gaseous diffusion plant where the uranium hexafluoride is pumped through filters that contain very tiny holes. Because uranium-235 has three fewer neutrons and it one percent lighter than uranium-238, it moves through the holes more easily then uranium-238. This method increases the percentage of uranium-235 as the gas passes through thousands of filters.
- Fuel Fabrication – The enriched uranium is taken to a fuel fabrication plant where it is prepared from the nuclear reactor. Here, the uranium is made into a solid ceramic material and formed into small barrel-shaped pellets. These ceramic fuel pellets can withstand very high temperatures, just like the ceramic tiles on the space shuttle. Fuel pellets are about the size of your fingertip, yet each one can produce as much energy as 120 gallons of oil. The pellets are sealed in 12-foot metal tubes called fuel rods. Finally, the fuel rods are bundled into groups called fuel assemblies.
- Nuclear Reactor – The uranium fuel is now ready for use in a nuclear reactor. Fissioning takes place in the reactor core. Surrounding the core of the reactor is a shell called the reactor pressure vessel. To prevent heat or radiation leaks, the reactor core and the vessel are housed in an airtight containment building made of steel and concrete several feet thick. The reactor core houses about 200 fuel assemblies. Spaced between the fuel assemblies are movable control rods. Control rods absorb neutrons and slow down the nuclear reaction. Water also flows through the fuel assemblies and control rods to remove some of the heat from the chain reaction. The nuclear reaction generates heat energy just as burning coal or oil generates heat energy. Likewise, the heat is used to boil water into steam, which turns a turbine generator to produce electricity. Afterward, the steam is condensed back into water and cooled in a separate structure called a cooling tower. This way, the water can be used again and again.
- Spent Fuel Storage – Like most industries, nuclear power plants produce waste. One of the main concerns about nuclear power plants is not the amount of waste created, which is quite small compared to other industries, but the radioactivity of some of that waste. The fission process creates radioactive waste products. After about three cycles, these waste products build up in the fuel rods, making the chain reaction more difficult. Utility companies generally replace one-third of the fuel rods every 12 to 18 months to keep power plants in continuous operation. The spent fuel is usually stored near the reactor I a deep pool of water called the spent fuel pool. During storage, the spent fuel cools down and begins to lose most of its radioactivity through radioactive decay. In three months, the spent fuel will lose 50 percent of its radiation; in one year, 80 percent; in 10 years, 90 percent. The spent fuel pool is intended as a temporary method for storing nuclear waste. Eventually, the spent fuel will be reprocessed and/or transported to a permanent federal disposal site.
- Reprocessing – Spent fuel contains both radioactive waste products and unused nuclear fuel. In fact, about one-third on the nuclear fuel remains unused when the fuel rod must be replaced. Reprocessing separates the unused nuclear fuel from the waste products so that it can be used in a reactor again. Currently, American nuclear power plants store the spent fuel in spent fuel pools-without reprocessing. Why? Mainly because reprocessing is more expensive than making new fuel from uranium ore. If uranium prices rise significantly or storage becomes a bigger problem, reprocessing may gain favor in the industry.
Most scientists believe the safest way to store nuclear waste is in rock formations deep underground called geological repositories. In 1982, the U.S. Congress agreed and passed the Nuclear Waste Policy Act. This law directed the U.S. Department of Energy to site, design, construct, and operate America’s first repository by 1998. The repository stores radioactive waste from nuclear power plants and from defense weapons plants. The same law also established the Nuclear Waste Fund to pay for the repository.
People who use electricity from nuclear power plants pay 1/10 of a cent for each kilowatt-hour of nuclear-generated electricity they use. An average household, which uses about 7,500 kilowatt-hour a year, would contribute $7.50 a year to the fund if it got all its electricity from nuclear power.
The nation has collected $14 billion into the fund since 1982. In 1987, Congress passed the Nuclear Waste Policy Amendments Act. Among others things, this act proposed Yucca Mountain, Nevada as the nation’s first repository site. If the current plan is approved, nuclear waste will be sealed in steel canisters and stored in vaults located 1,000 feet below the surface. Current projections are that it may open about 2010. Yucca Mountain is being studied as a repository site because it is dry and geologically stable (the chance of erupting volcanoes or earth-quakes is very slim). Yucca Mountain site is also isolated. Few people live in the area. Although utility companies currently sore their nuclear waste in pools of water at the power plant, some companies will run out of storage space in the next year or two.
Utility companies are asking the Department of Energy to accept responsibility for the waste. The Department of Energy would need to store the waste in a temporary facility prior to its final burial at the repository.
Nuclear Energy Use
Nuclear energy is an important source of electricity-second only to coal-providing almost 18 percent of the electricity in the U.S. At the end of 1997m there were 107 nuclear power plants operating in the U.S. No new plants are planned for the future. Worldwide, however, nuclear energy is a growing source of electrical power. Nuclear energy now provides about 17 percent of the world’s electricity. The U.S., France, Japan, and Germany are the world leaders. France gets 75 percent of its electricity from nuclear power.
Nuclear Energy and the Environment
Nuclear power plants have very little impact on the environment. Generating electricity from nuclear power produces no air pollution because no fuel is burned. Most of the water used in the cooling processes is recycles. In the future, using nuclear energy may become an important way to reduce the amount of carbon dioxide produced by burning fossil fuels and biomass. Carbon dioxide is considered the major greenhouse gas. People are using more and more electricity. Some experts predict that we will have to use nuclear energy to produce the amount of electricity people need at a cost they can afford. Whether or not we should use nuclear energy to produce electricity has become a controversial and sometimes highly emotional issue.
The greatest potential risk from nuclear power plants is the release of high-level radiation. In the United States, plants are carefully designed to contain radiation, and emergency plans are in place to alert and advise nearby residents if there is an accident. Two serious accidents have occurred since the industry began over 30 years ago-Three Mile Island in the Unites States (1979) and Chernobyl in the Soviet Union (1986).
At Three Mile Island, about half the uranium fuel melted when water to the reactor core was inadvertently cut off. A small amount of radioactive material escaped into the surrounding area before the mistake was discovered. But due to the safety design features of the plant-multiple barriers contained most of the radiation-no one was injured or died as a result of this accident.
However, the accident at Chernobyl was far more serious. It happened when two explosions blew the top off the reactor building. A lack of containment structures and other design flaws caused the release of a large amount of radioactive material into the surrounding area. More than 100,000 people were evacuated from their homes and about 200 workers were treated for radiation sickness and burns; 31 or them died.
Could a Chernobyl-type accident occur at an American nuclear plant? Many experts say no. Old soviet nuclear plants do not have the safety systems and containment chambers that are standard on all American plants. American operators are also better trained than their Eastern European counterparts to respond to any problems that may occur.