Lesson 3: Reading LessonNuclear Waste: What is It and Where is It?
Any power plant that produces electricity also produces some type of waste. This waste can range from the solid ash produced in coal-burning plants to exhaust gases that contribute to air pollution, greenhouse gas emission, and acid rain. Waste that results from using radioactive materials is nuclear waste. The U.S. government has programs under way to provide for the safe permanent disposal of all types of nuclear waste produced under its jurisdiction and has also been mandated by Congress to dispose of civilian high-level waste (mostly spent nuclear fuel). Sources and Types of Nuclear WasteNuclear waste comes from five major sources:
There are four basic types of nuclear waste: highlevel waste, low-level waste, transuranic waste, and mill tailings. How the waste is classified depends on its source, its level of radioactivity, and its potential hazard (or how likely it is to cause harm). High-Level WasteHigh-level nuclear waste is the most radioactive category of nuclear waste. It includes spent nuclear fuel (used fuel) from nuclear power plants and some wastes from our nation’s defense activities. Both spent fuel and high-level defense waste are now in storage awaiting disposal. Some radioactive elements in high-level waste lose radioactivity (decay) rather quickly. Others remain radioactive for thousands of years. High-level waste is handled using remote control equipment. Operators of the equipment work behind heavy protective shielding. This type of waste is transported in heavily shielded containers. Low-Level WasteLow-level nuclear waste consists of items that are contaminated by small amounts of radioactive material. This type of waste usually contains only a small amount of radioactivity within a relatively large amount of material. It is far less hazardous or dangerous than high-level waste. Most low-level waste becomes harmless to the environment in a relatively short time frame. Therefore, it does not require extensive protection. However, for certain low-level waste, some shielding may be necessary.
Hospitals, nuclear power plants, research labs, universities, and many industries produce low-level waste. Also, some of the waste produced by defense activities is low-level waste. Low-level waste from energy production and medical activities may include commonly used equipment such as empty containers, rags, papers, brooms, filters, tools, and used protective clothing. Low-level wastes are placed in containers and then buried at special landfills licensed by the federal government. The United States Department of Energy operates disposal facilities for its own low-level waste at several government facilities, including the following:
Industry low-level wastes are disposed of in commercial disposal facilities licensed by the U.S. Nuclear Regulatory Commission. Transuranic WasteLike low-level waste, transuranic waste is mostly discarded clothing, rags, equipment, containers, tools, etc.
Also, like low-level waste, it emits less penetrating radiation than high-level waste. In fact, some transuranic waste has no more radioactivity than certain low-level waste. However, unlike low-level waste, transuranic waste contains elements with very long half-lives. Therefore, these elements decay (lose radioactivity) slowly and remain radioactive for thousands of years. Most transuranic waste results from reprocessing nuclear fuel and making plutonium weapons as part of the nation’s defense activities. In March 1999, the federal government began disposing of this type of waste at the Waste Isolation Pilot Plant in Carlsbad, New Mexico. It is a geologic repository licensed to safely and permanently dispose of the transuranic wastes from about 20 locations nationwide. This waste will be shipped to WIPP over the next 35 years. WIPP is expected to receive about 170,000 cubic meters of waste in 37,000 shipments. It is regulated by the U.S. Environmental Protection Agency and the state of New Mexico. Mill Tailings
The fuel used at a nuclear power plant comes from uranium ore, which is found in the ground. Ore containing uranium is mined and then milled (crushed and treated to separate and remove the uranium). The leftover rocks and soil are mill tailings. The tailings contain a small amount of radium that decays to radon — a radioactive gas. Radon can be harmful to our health if we are exposed to it in concentrated amounts. For this reason, the mill tailings are covered over with enough soil to protect the public and the environment. Mining companies under supervision of the U.S. Environmental Protection Agency provide for the safety of mill tailings. However, where such companies no longer exist, or for sites in operation before 1970, the Department of Energy assures the stability and safety of mill tailing sites as directed by Congress. Spent Nuclear FuelThe fuel for nuclear power plants is uranium oxide formed into ceramic pellets.
Each pellet is about 10-centimeters around and 10-centimeters long – about the size of the tip of your little finger. The pellets are stacked and sealed in fuel rods – hollow metal tubes about twice the thickness of a pencil and about 4 meters long. Groups of fuel rods are spaced and bolted together to form a fuel assembly. One fuel assembly contains about 200 fuel rods. Finally, fuel assemblies are loaded into the nuclear reactor of a power plant. Over time, as the reactor operates, the fuel inside the assemblies becomes less efficient. After about three years in the reactor, the fuel assemblies are no longer doing their job. Coal, oil, and natural gas burn. Nuclear fuel does not burn. So how does nuclear fuel get used up? There are two types of uranium atoms in nuclear fuel: uranium-235 and uranium-238. When a uranium-235 atom absorbs a neutron, it fissions. The smaller atoms produced by the fissioning uranium atoms are called fission products. Uranium-238 does not fission readily. When a uranium-238 atom absorbs a neutron, it converts into a radioactive element called a transuranic element. (Transuranic elements have a higher atomic number than uranium on the periodic table of elements.) As uranium-238 atoms absorb neutrons, more and more of these transuranic elements form, including several plutonium isotopes (239 and 241) that fission as soon as they are formed. Toward the end of its useful life, about a third of the power produced comes from the fissioning of this plutonium produced inside the fuel. Where does all of this activity take place? It takes place inside the fuel rods. Fission products and transuranic elements accumulate within the fuel rods. Gradually, the uranium-235 in the nuclear fuel is almost used up and the nuclear chain reaction becomes less efficient.
That’s when the fuel assemblies are called spent, or used, fuel. The assemblies must be removed and replaced with fresh fuel. What Happens to Spent Fuel?Spent fuel must be removed and disposed of in a safe way. Approximately 20 percent of the nation’s electricity is produced by more than 100 nuclear power plants located around the country. As of 2004, nearly 49,000 metric tons of spent fuel were in storage at these power plants. Spent fuel is stored in specially treated water in a deep, steel-lined, concrete pool inside a building at the power plant. Here it begins to cool and becomes less radioactive as the fuel’s radioactive elements decay. Since 1986, more than a dozen U.S. nuclear power plants have supplemented their pool storage capacity by building above ground, dry storage facilities at their plant site. These facilities put the spent nuclear fuel in heavy containers made of steel, concrete, and lead, which together effectively shield radiation. They place the containers either upright on thick concrete pads or store them horizontally in concrete bunkers. During the first three months of storage, spent fuel loses about 50 percent of its radioactivity. In one year, it loses about 80 percent. In 10 years, radioactivity is reduced by 90 percent. But the remaining 10 percent is still intensely radioactive and could be a danger to health and the environment for thousands of years if improperly stored. In some counties, spent fuel is reprocessed to allow fissionable uranium and plutonium to be reused in fuel. In the United States, the concern over the potential misuse of this material, which can be used to make bombs if concentrated enough, has caused reproccessing to be halted as a matter of national policy, a policy now being reconsidered because of rising fuel prices and advancing nuclear technologies. Storing Defense High-Level Waste
As part of our country’s national defense program, nuclear materials are used as part of nuclear weapons. These materials were created in special “production” reactors owned and operated by the United States government, not in civilian reactors used to generate electricity. About 9,000 metric tons of defense high-level waste, including some unreprocessed production reactor fuel and all Navy fuels, are being stored temporarily at three U.S. Department of Energy sites:
Volume and RadioactivityThe volume (amount of space occupied) of waste does not tell you its level of radioactivity. Both its volume and the radioactivity it emits are important. A large volume of waste with little radioactivity presents less hazard than a smaller amount of waste with more radioactivity.
For example, spent nuclear fuel makes up less than 1 percent of the volume of radioactive waste in the United States, but it contains almost 95 percent of the total radioactivity. On the other hand, low-level waste makes up nearly 86 percent of all radioactive waste by volume, but contains less than one-tenth of 1 percent of the total radioactivity. Focus on High-Level WasteAs you now know, there are four different types of nuclear waste. Each type is stored and will be disposed of in a way that protects people and the environment. From this point on, our main focus will be on spent nuclear fuel and defense high-level waste. Why Spent Nuclear Fuel and High- Level Waste is DangerousWhen people hear the term “nuclear waste,” many think of it as a liquid that could leak into the environment or as an explosive that could blow up like a nuclear bomb. However, the wastes to be disposed of will not be in liquid form; they are, or will be made into, solid metals, ceramics, and glasses. Moreover, these wastes cannot cause an explosion.
Spent nuclear fuel and high-level waste are dangerous because they can emit extremely high levels of radiation for tens of thousands of years. If, over time, enough water contacted the solid waste, it could eventually cause it to corrode (much as rust affects a bicycle if you leave it out in wet weather for a long time). The corrosion products on nuclear waste would also be radioactive, and water could carry microscopic radioactive particles into the environment. Permanent Disposal OptionsFor the last five decades, scientists throughout the world have been working to find the safest way to permanently dispose of spent nuclear fuel and high-level waste. They have looked at many different options, such as shooting it into outer space, putting it into the ocean floor, or burying it in polar ice caps. Nearly 40 years ago, the international scientific community determined that the best option for isolating highly radioactive waste is to put it deep underground in facilities called geologic repositories. Scientists and researchers around the world agree that underground disposal is the best solution because it is technically feasible and because it
With this international consensus, the United States and many of the world’s other nuclear nations have decided upon deep underground repositories as the long-term solution for disposal of spent nuclear fuel and high-level radioactive waste.
In 1982 Congress passed a federal law called the Nuclear Waste Policy Act. In passing this law, Congress decided to permanently dispose of our high-level nuclear wastes in an underground repository, rather than to leave the problem for future generations. The Act directed the U.S. Department of Energy to study suitable sites for an underground repository. During the early eighties, the Department of Energy looked at many potential sites for a repository. Considering a wide range of criteria, the sites selected for further study were narrowed down to nine, then from nine to five, from five to three, and eventually, through a 1987 Congressional amendment to the Act, from three to one – Yucca Mountain in Nye County, Nevada. A Proposed Permanent Storage FacilityFor over two decades, the U.S. Department of Energy has conducted an extensive scientific effort to determine whether Yucca Mountain, Nevada, is a suitable site for a deep underground repository. On July 9, 2002, the U.S. Senate cast the final legislative vote approving the development of a repository at Yucca Mountain. The Yucca Mountain Project is currently focused on preparing an application to obtain a license from the U.S. Nuclear Regulatory Commission to construct a repository. If it receives this license, the department plans to begin operating the repository.
|
 |
 |
 |
 |
 |
 |
 |
 |
 |
