Basics

Energy from the sunThe sun has produced energy for billions of years and is the ultimate source for all of the energy sources and fuels that we use today. People have used the sun's rays (solar radiation) for thousands of years for warmth and to dry meat, fruit, and grains. Over time, people developed devices (technologies) to collect solar energy for heat and to convert it into electricity.

Radiant energy from the sun has powered life on earth for many millions of years.

Source: NASA

Collecting and using solar thermal (heat) energyAn example of an early solar energy collection device is the solar oven (a box for collecting and absorbing sunlight). In the 1830's, British astronomer John Herschel used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into useful heat energy for a variety of purposes.

We use solar thermal energy systems to

● heat water for use in homes, buildings, or swimming pools● heat the inside of homes, greenhouses, and other buildings● heat fluids to high temperatures in solar thermal power plants

Solar photovoltaic systems convert sunlight into electricitySolar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes.

Solar energy has benefits and some limitationsThe two main benefits of using solar energy are

● Solar energy systems do not produce air pollutants or carbon dioxide.

● Solar energy systems on buildings have minimal impact on the environment.

The main limitations of solar energy are● The amount of sunlight that arrives at the earth's surface is not

constant. The amount of sunlight varies depending on location, time of day, season of the year, and weather conditions.

● The amount of sunlight reaching a square foot of the earth's surface is relatively small, so a large surface area is necessary to absorb or collect a useful amount of energy.

Solar Energy and the Environment – BasicsAn array of solar photovoltaic panels supplies electricity for use at Marine Corps Air Ground Combat Center in Twentynine Palms, California

Source: U.S. Marine Corps photo by Pfc. Jeremiah Handeland/Released (public domain)

Solar energy does not produce air or water pollution or greenhouse gases. However, using solar energy may have some indirect negative impacts on the environment. For example, some toxic materials and chemicals are used to make the photovoltaic (PV) cells that convert sunlight into electricity. Some solar thermal systems use potentially hazardous fluids to transfer heat. U.S. environmental laws regulate the use and disposal of these types of materials.As with any type of power plant, large solar power plants can affect the environment near their locations. Clearing land for construction and the placement of the power plant may have long-term affects on habitat areas for native plants and animals. Some solar power plants may require water for cleaning solar collectors and concentrators or for cooling turbine generators. Using large volumes of ground water or surface water in some arid locations may affect the ecosystems that depend on these water resources. In addition, the beam of sunlight a solar power tower creates can kill birds and insects that fly into the beam.

Wind – Basics

Energy from moving airHow uneven heating of water and land causes wind

Source: Adapted from National Energy Education Development Project (public domain)

Wind is caused by uneven heating of the earth's surface by the sun. Because the earth's surface is made up of different types of land and water, it absorbs the sun's heat at different rates. One example of this uneven heating is the daily wind cycle.

The daily wind cycleDuring the day, air above the land heats up faster than air over water. Warm air over land expands and rises, and heavier, cooler air rushes in to take its place, creating wind. At night, the winds are reversed because air cools more rapidly over land than it does over water.In the same way, the atmospheric winds that circle the earth are created because the land near the earth's equator is hotter than the land near the North Pole and the South Pole.

Wind energy for electricity generationToday, wind energy is mainly used to generate electricity, although water pumping windmills were once used throughout the United States.

Electricity Generation from Wind – Basics

How wind turbines workDiagram of wind turbine components

Source: National Renewable Energy Laboratory, U.S. Department of Energy (public domain)

Wind turbines use blades to collect the wind’s kinetic energy. Wind flows over the blades creating lift (similar to the effect on airplane wings), which causes the blades to turn. The blades are connected to a drive shaft that turns an electric generator that produces the electricity.

Electricity generation with wind

In 2015, wind turbines in the United States were the source of nearly 5% of total U.S. electricity generation. Although wind's share of the country's total electricity production is small, it was equal to the electricity use of about 17.5 million U.S. households in 2014.The amount of electricity generated from wind has grown significantly over the past 15 years. Electricity generation from wind in the United States increased from about 6 billion kilowatthours (kWh) in 2000 to about 191 billion kWh in 2015.New technologies have decreased the cost of producing electricity from wind, and growth in wind power has been encouraged by government and industry incentives.

Wind Energy and the Environment – Basics

Wind is an emissions-free source of energyWind turbines at the Cerro Gordo Project, west of Mason City, Iowa

Source: National Renewable Energy Laboratory (public domain)

Wind is a renewable energy source. Overall, using wind to produce energy has fewer environmental impacts than many other energy sources. Wind turbines do not release emissions that can pollute the air or water (with rare exceptions), and they do not require water for cooling. Wind turbines may also reduce the amount of electricity generated from fossil fuels, which reduces total air pollution and carbon dioxide emissions.An individual wind turbine has a relatively small physical footprint. Groups of wind turbines, sometimes called wind farms, are located on open land, on mountain ranges, or offshore in lakes or the ocean.

Wind turbines have some negative impacts on the environmentModern wind turbines are large machines, and they have a visual impact on the landscape. A small number of wind turbines have also caught fire, and

some have leaked lubricating fluids, but these occurrences are rare. Some people do not like the sound that wind turbine blades make as they turn in the wind. Some types of wind turbines and wind projects cause bird and bat deaths. These deaths may contribute to declines in the population of species also affected by other human-related impacts. The wind energy industry and the U.S. government are researching ways to reduce the impact of wind turbines on birds and bats.Most wind power projects on land also require service roads that add to the physical impact on the environment. Wind turbines also require the use of rare earth minerals. These minerals are often located in countries with less stringent environmental standards than the United States, and mining these minerals can have negative impacts on the environment. Producing the metals and other materials used to make wind turbines and the concrete used for their foundations requires energy that may have been produced by fossil fuels.

Basics

Energy from moving water

Source: Adapted from National Energy Education Development Project (public domain)

Hydropower generates electricity

Hydropower is the largest renewable energy source for electricity generation in the United States. In 2015, hydropower accounted for about 6% of total U.S. electricity generation and 46% of electricity generation from all renewables.Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source.

Hydropower relies on the water cycleUnderstanding the water cycle is important to understanding hydropower. The water cycle has three steps:

● Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate.

● Water vapor condenses into clouds and falls as precipitation (rain, snow, etc.).

● Precipitation collects in streams and rivers, which empty into oceans and lakes, where it evaporates and begins the cycle again.

The amount of precipitation that drains into rivers and streams in a geographical area determines the amount of water available for producing hydropower. Seasonal variations in precipitation and long-term changes in precipitation patterns, such as droughts, have a big impact on hydropower production.

Hydroelectric power is produced from moving waterThe volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water. Swiftly flowing water in a big river, like the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, like Niagara Falls in New York, also has substantial energy in its flow.At both Niagara Falls and the Columbia River, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity. In a run-of-the-river system, the force of the current applies pressure on a turbine. In a storage system, water accumulates in reservoirs created by dams and is released as needed to generate electricity.

Hydropower and the Environment – Basics

Hydropower generators produce clean electricity, but hydropower does have environmental impactsMost dams in the United States were built mainly to control floods and to help supply water for cities and irrigation. Although many of these dams have hydroelectric generators, only a small number of dams were built specifically for hydropower generation. Although hydropower generators do not directly produce emissions of air pollutants, dams, reservoirs, and the operation of hydropower electric generators can affect the environment.

Fish ladder at the Bonneville Dam on the Columbia River separating Washington and Oregon

Source: Stock photography (copyrighted)

Did you know?

The Safe Harbor Dam on the Susquehanna River in Pennsylvania has elevators that lift migrating shad from the base of the dam to the top of the reservoir.

A dam that creates a reservoir (or a dam that diverts water to a run-of-river hydropower plant) may obstruct fish migration. A reservoir and dam can also change natural water temperatures, water chemistry, river flow characteristics, and silt loads. All of these changes can affect the ecology and the physical characteristics of the river. These changes may have negative impacts on native plants and on animals in and around the river. Reservoirs may cover important natural areas, agricultural land, or archeological sites. A reservoir and the operation of the dam may also result in the relocation of people. The physical impacts of a dam and reservoir, the operation of the dam, and the use of the water can change the environment over a much larger area than the area covered by a reservoir.

Although no new hydropower dams have been built recently in the United States, they are being built in other countries like China. Manufacturing the concrete and steel used to construct these dams requires equipment that may produce emissions. If fossil fuels are used as the energy source to make these materials, then the emissions from the equipment could be associated with the electricity that hydropower facilities generate. However, given the long operating lifetime of a hydropower plant (50 years to 100 years) these emissions are offset by the emissions-free electricity that is generated.Carbon dioxide and methane may also form in reservoirs and be emitted into the atmosphere. The exact amount of greenhouse gases produced in hydropower reservoirs is uncertain. The emissions from reservoirs in tropical and temperate regions, including the United States, may be equal to or greater than the greenhouse effect of the carbon dioxide emissions from an equivalent amount of electricity generated with fossil fuels. Scientists at Brazil's National Institute for Space Research designed a system to capture methane in a reservoir and burn it to produce electricity.

Fish ladders help salmon reach their spawning groundsHydropower turbines kill and injure some of the fish that pass through the turbine. The U.S. Department of Energy has sponsored the research and development of turbines that could reduce fish deaths to lower than 2%, in comparison to fish kills of 5% to 10% for the best existing turbines.There are many species of fish, such as salmon, that swim from the sea up rivers and streams to reproduce in their spawning grounds in the beds of rivers and streams. Dams block their way. Different approaches to fixing this problem have been used, including the construction of fish ladders and elevators that help fish move through or around dams to the spawning grounds upstream.

Basics

Biomass—renewable energy from plants and animals

Source: Adapted from The National Energy Education Project (public domain)

Source: Adapted from The National Energy Education Project (public domain)

Biomass is organic material that comes from plants and animals, and it is a renewable source of energy.Biomass contains stored energy from the sun. Plants absorb the sun's energy in a process called photosynthesis. When biomass is burned, the chemical energy in biomass is released as heat. Biomass can be burned directly or converted to liquid biofuels and biogas that are burned as fuels. Examples of biomass and their uses for energy:

● wood and wood processing wastes—burned to heat buildings, to produce process heat in industry, and to generate electricity

● agricultural crops and waste materials—burned as a fuel or converted to liquid biofuels

● food, yard, and wood waste in garbage—burned to generate electricity in power plants or converted to biogas in landfills

● animal manure and human sewage—converted to biogas, which can be burned as a fuel

Converting biomass to other forms of energyBurning biomass is only one way to release its energy. Biomass can be converted to other useable forms of energy like methane gas or transportation fuels like ethanol and biodiesel.Methane gas is a component of landfill gas or biogas that forms when garbage, agricultural waste, and human waste decompose in landfills or in special containers called digesters.Crops such as corn and sugar cane are fermented to produce fuel ethanol for use in vehicles. Biodiesel, another transportation fuel, is produced from vegetable oils and animal fats.

How much biomass is used for fuel?Biomass fuels provided about 5% of the energy used in the United States in 2015. Of that 5%, about 43% was from wood and wood-derived biomass, 46% was from biofuels (mainly ethanol), and about 11% was from municipal waste. Researchers are trying to develop ways to use more biomass for fuel.

Biomass and the Environment – Basics

Using biomass for energy has positive and negative effectsBiomass as an energy source is an alternative to fossil fuels (coal, petroleum, and natural gas). Burning either fossil fuels or biomass releases carbon dioxide (CO2), a greenhouse gas. However, the plants that are the source of biomass capture a nearly equivalent amount of CO2 through photosynthesis while they are growing, which can make biomass a carbon-neutral energy source.

Burning woodUsing wood, wood pellets, and charcoal for heating and cooking can replace fossil fuels and may result in lower CO2 emissions overall. Wood can be harvested from forests, woodlots that have to be thinned, or from urban trees that fall down or have to be cut down.Wood smoke contains harmful pollutants like carbon monoxide and particulate matter. Modern wood-burning stoves, pellet stoves, and fireplace inserts can reduce the amount of particulates from burning wood. Wood and charcoal are major cooking and heating fuels in poor countries, but if people harvest the wood faster than trees can grow, it causes deforestation. Planting fast-growing trees for fuel and using fuel-efficient cooking stoves can help slow deforestation and improve the environment.

Burning municipal solid waste (MSW) or wood wasteBurning municipal solid waste (MSW, or garbage) to produce energy in waste-to-energy plants means that less waste is buried in landfills. On the other hand, burning garbage produces air pollution and releases the chemicals and substances in the waste into the air. Some of these chemicals can be hazardous to people and the environment if they are not properly controlled.The U.S. Environmental Protection Agency (EPA) applies strict environmental rules to waste-to-energy plants, and requires that waste-to-energy plants use air pollution control devices such as scrubbers, fabric filters, and electrostatic precipitators to capture air pollutants.Scrubbers clean emissions from waste-to-energy facilities by spraying a liquid into the combustion gases to neutralize the acids present in the stream of emissions. Fabric filters and electrostatic precipitators also remove particles from the combustion gases. The particles—called fly ash—are then

mixed with the ash that is removed from the bottom of the waste-to-energy furnace.A waste-to-energy furnace burns at high temperatures (1,800°F to 2,000°F), which breaks down the chemicals in MSW into simpler, less harmful compounds.

Disposing ash from waste-to-energy plantsAsh can contain high concentrations of various metals that were present in the original waste. Textile dyes, printing inks, and ceramics, for example, may contain lead and cadmium.Separating waste before burning can solve part of the problem. Because batteries are the largest source of lead and cadmium in municipal waste, they should not be included in regular trash. Florescent light bulbs should also not be put in regular trash because they contain small amounts of mercury.The EPA tests ash from waste-to-energy plants to make sure that it is not hazardous. The test looks for chemicals and metals that could contaminate ground water. MSW landfills use ash that is considered safe as a cover layer for their landfills, or they add it to concrete to make cement blocks.

Collecting landfill gas or biogasBiogas forms as a result of biological processes in sewage treatment plants, waste landfills, and livestock manure management systems. Biogas is composed mainly of methane (a greenhouse gas) and CO2. Many facilities that produce biogas capture it and burn the methane for heat or to generate electricity. This electricity is considered renewable and, in many states, contributes to meeting state renewable portfolio standards (RPS). This electricity may replace electricity generation from fossil fuels and can result in a net reduction in CO2 emissions. Burning methane produces CO2, but because methane is a stronger greenhouse gas than CO2, the overall greenhouse effect is lower.

Liquid biofuels: ethanol and biodieselThe federal government promotes ethanol use as a transportation fuel to help reduce oil imports and CO2 emissions. In 2007, the government set a

target to use 36 billion gallons of biofuels by 2022. As a result, nearly all gasoline now sold in the United States contains some ethanol.Biofuels may be carbon-neutral because the plants that are used to make biofuels (such as corn and sugarcane for ethanol and soy beans and palm oil trees for biodiesel) absorb CO2 as they grow and may offset the CO2 emissions when biofuels are produced and burned.Growing plants for biofuels is controversial because the land, fertilizers, and energy for growing biofuel crops could be used to grow food crops instead. In some parts of the world, large areas of natural vegetation and forests have been cut down to grow sugar cane for ethanol and soybeans and palm oil trees for biodiesel. The U.S. government supports efforts to develop alternative sources of biomass that do not compete with food crops and that use less fertilizer and pesticides than corn and sugar cane. The U.S. government also supports methods to produce ethanol that require less energy than conventional fermentation. Ethanol can also be made from waste paper, and biodiesel can be made from waste grease and oils and even algae.Ethanol and gasoline-ethanol blends burn cleaner and have higher octane ratings than pure gasoline, but they have higher evaporative emissions from fuel tanks and dispensing equipment. These evaporative emissions contribute to the formation of harmful, ground-level ozone and smog. Gasoline requires extra processing to reduce evaporative emissions before it is blended with ethanol. Biodiesel combustion produces fewer sulfur oxides, less particulate matter, less carbon monoxide, and fewer unburned and other hydrocarbons, but it does produce more nitrogen oxide than petroleum diesel.

Basics

What is geothermal energy?The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is heat within the earth. People can use this heat as steam or as hot water to heat buildings or to generate electricity.Geothermal energy is a renewable energy source because the heat is continuously produced inside the earth.

Geothermal energy comes from deep inside the earth

Source: Adapted from a National Energy Education Development Project graphic (public domain)

The slow decay of radioactive particles in the earth's core, a process that happens in all rocks, produces geothermal energy. The earth's core is hotter than the sun's surface.The earth has a number of different layers:

● The inner core is solid iron and is surrounded by an outer core of hot molten rock called magma.

● The mantle surrounds the core and is about 1,800 miles thick. The mantle is made up of magma and rock.

● The crust is the outermost layer of the earth. The crust forms the continents and ocean floors. The crust can be 3 to 5 miles thick under the oceans and 15 to 35 miles thick on the continents.

The earth's crust is broken into pieces called plates. Magma comes close to the earth's surface near the edges of these plates, which is where many volcanoes occur. The lava that erupts from volcanoes is partly magma. Rocks and water absorb heat from magma deep underground. The rocks and water found deeper underground have the highest temperatures.People around the world use geothermal energy to heat their homes and to produce electricity by drilling deep wells and pumping the hot underground water or steam to the surface. People can also use the stable temperatures near the surface of the earth to heat and cool buildings.

Geothermal Power Plants – BasicsA geothermal power plant emitting steam

Source: Stock photography (copyrighted)

Geothermal power plants use hydrothermal resources that have both water (hydro) and heat (thermal). Geothermal power plants require high-temperature (300°F to 700°F) hydrothermal resources that come from either dry steam wells or from hot water wells. People use these resources by drilling wells into the earth and then piping steam or hot water to the surface. The hot water or steam powers a turbine that generates electricity. Some geothermal wells are up to two miles deep.

Types of geothermal power plantsThere are three basic types of geothermal power plants:

● Dry steam plants use steam directly from a geothermal reservoir to turn generator turbines. The first geothermal power plant was built in 1904 in Tuscany, Italy, where natural steam erupted from the earth.

● Flash steam plants take high-pressure hot water from deep inside the earth and convert it to steam to drive generator turbines. When the steam cools, it condenses to water and is injected back into the ground to be used again. Most geothermal power plants are flash steam plants.

● Binary cycle power plants transfer the heat from geothermal hot water to another liquid. The heat causes the second liquid to turn to steam, which is used to drive a generator turbine.

Geothermal Energy and the Environment – BasicsThe environmental effects of geothermal energy depend on how geothermal energy is used or on how it is converted to useful energy. Direct use

applications and geothermal heat pumps have almost no negative effects on the environment. In fact, they can have a positive effect by reducing the use of energy sources that have negative effects on the environment.

Grand Prismatic Spring, Yellowstone National Park, Wyoming

Source: Stock photography (copyrighted)

Geothermal power plants have low emission levelsGeothermal power plants do not burn fuel to generate electricity, so the levels of air pollutants they emit are low. Geothermal power plants emit 97% less acid rain-causing sulfur compounds and about 99% less carbon dioxide than fossil fuel power plants of similar size. Geothermal power plants use scrubbers to remove the hydrogen sulfide naturally found in geothermal reservoirs. Geothermal power plants inject the geothermal steam and water that they use back into the earth. This recycling helps to renew the geothermal resource.

Many geothermal features are national treasures

Geothermal features in national parks, like geysers and fumaroles in Yellowstone National Park, are protected by law.

BasicsDid you know?

All nuclear power in the United States is used to generate electricity.

Source: Adapted from National Energy Education Development Project (public domain)

Nuclear energy is energy in the core of an atomAtoms are the tiny particles in the molecules that make up gases, liquids, and solids. Atoms themselves are made up of three particles called protons, neutrons, and electrons. An atom has a nucleus (or core) containing protons and neutrons, which is surrounded by electrons. Protons carry a positive electrical charge and electrons carry a negative electrical charge. Neutrons do not have an electrical charge. There is enormous energy present in the bonds that hold the nucleus together. This nuclear energy can be released when those bonds are broken. The bonds can be broken through nuclear fission, and this energy can be used to produce electricity.In nuclear fission, atoms are split apart, which releases energy. All nuclear power plants use nuclear fission, and most nuclear power plants use uranium atoms. During nuclear fission, a neutron hits a uranium atom and splits it, releasing a large amount of energy in the form of heat and radiation. More neutrons are also released when a uranium atom splits. These neutrons go on to hit other uranium atoms, and the process repeats itself over and over again. This is called a nuclear chain reaction. This reaction is controlled in nuclear power plant reactors to produce a desired amount of heat.Nuclear energy can also be released in nuclear fusion, in which atoms are combined or fused together to form a larger atom. This is the source of energy in the sun and stars. Nuclear fusion is the subject of ongoing research as a source of energy for heat and electricity generation, but it is not yet clear whether or not it will be a commercially viable technology because of the difficulty of controlling a fusion reaction.

Nuclear fuel—uranium

Uranium is the fuel most widely used by nuclear plants for nuclear fission. Uranium is considered to be a nonrenewable energy source, even though it is a common metal found in rocks worldwide. Nuclear power plants use a certain kind of uranium, referred to as U-235, for fuel because its atoms are easily split apart. Although uranium is about 100 times more common than silver, U-235 is relatively rare.Most U.S. uranium ore is mined in the western United States. Once uranium is mined, the U-235 must be extracted and processed before it can be used as a fuel.

Nuclear Power Plants – BasicsNuclear reactors in the United States may have large concrete domes covering the reactors, which are required to contain accidental releases of radiation. Not all nuclear power plants have cooling towers. Some nuclear power plants use water from lakes, rivers, or the ocean for cooling.

Nuclear power comes from nuclear fissionMany power plants, including nuclear power plants, heat water to produce electricity. These power plants use steam from heated water to spin large turbines that generate electricity. Nuclear power plants use heat produced during nuclear fission to heat water.In nuclear fission, atoms are split apart to form smaller atoms, releasing energy. Fission takes place inside the reactor of a nuclear power plant. At the center of the reactor is the core, which contains uranium fuel.The uranium fuel is formed into ceramic pellets. Each ceramic pellet produces roughly the same amount of energy as 150 gallons of oil. These energy-rich pellets are stacked end-to-end in 12-foot metal fuel rods. A bundle of fuel rods, sometimes hundreds, is called a fuel assembly. A reactor core contains many fuel assemblies.The heat produced during nuclear fission in the reactor core is used to boil water into steam, which turns the turbine blades. As the turbine blades turn, they drive generators that make electricity. Afterward, the steam is cooled back into water in a separate structure at the power plant called a cooling tower. The water can then be reused.

Nuclear power plants generate about 20% of U.S. electricityThe United States has 100 nuclear reactors at 60 operating nuclear power plants located in 30 states.1 Thirty-six of the plants have 2 or more reactors, and 46 plants are located east of the Mississippi River. Nuclear power has supplied about one-fifth of annual U.S. electricity since 1990.

The United States generates more nuclear power than any other country

Of the 31 countries in the world that have commercial nuclear power plants, the United States has the most nuclear capacity and generation. France has the second-highest nuclear electricity generation and obtains about 75% of its total electricity from nuclear energy. Fifteen other countries generate more than 20% of their electricity from nuclear power.

Nuclear Power and the Environment – BasicsDid you know?

Steam coming out of nuclear power plant cooling towers is just hot water.

Nuclear power reactors do not produce direct carbon dioxide emissionsUnlike fossil fuel-fired power plants, nuclear reactors do not produce air pollution or carbon dioxide while operating. However, the processes for mining and refining uranium ore and making reactor fuel all require large amounts of energy. Nuclear power plants also have large amounts of metal and concrete, which require large amounts of energy to manufacture. If fossil fuels are used for mining and refining uranium ore, or if fossil fuels are used when constructing the nuclear power plant, then the emissions from burning those fuels could be associated with the electricity that nuclear power plants generate.

Nuclear energy produces radioactive wasteThe primary environmental concern related to nuclear power is the creation of radioactive wastes such as uranium mill tailings, spent (used) reactor fuel, and other radioactive wastes. These materials can remain radioactive and dangerous to human health for thousands of years. Radioactive wastes are subject to special regulations that govern their handling, transportation, storage, and disposal to protect human health and the environment. The U.S. Nuclear Regulatory Commission (NRC) regulates the operation of nuclear power plants.

Radioactive wastes are classified as low-level waste or high-level waste. The radioactivity in these wastes can range from just above natural background levels, like in uranium mill tailings, to much higher levels, like in used (spent) reactor fuel or in the parts inside a nuclear reactor. The radioactivity of nuclear waste decreases over time through a process called radioactive decay. The amount of time it takes for the radioactivity of radioactive material to decrease to half its original level is called the radioactive half-life. Radioactive waste with a short half-life is often stored temporarily before disposal to reduce potential radiation doses to workers who handle and

transport the waste. This storage system also reduces the radiation levels at disposal sites.

By volume, most of the waste related to the nuclear power industry has a relatively low level of radioactivity. Uranium mill tailings contain the radioactive element radium, which decays to produce radon, a radioactive gas. Most uranium mill tailings are placed near the processing facility or mill where they come from. Uranium mill tailings are covered with a sealing barrier of material like clay to prevent radon from escaping into the atmosphere, and then the barrier is covered by a layer of soil, rocks, or other materials to prevent erosion of the sealing barrier.

The other types of low-level radioactive waste are the tools, protective clothing, wiping cloths, and other disposable items that become contaminated with small amounts of radioactive dust or particles at nuclear fuel processing facilities and nuclear power plants. These materials are subject to special regulations that govern their handling, storage, and disposal so they will not come in contact with the outside environment.

High-level radioactive waste consists of irradiated or spent nuclear reactor fuel (i.e., fuel that is no longer useful for producing electricity). The spent reactor fuel is in a solid form, consisting of small fuel pellets in long metal tubes called rods.