Idea Transcript
Energy & Air Pollution Introduction Fossil Fuels: Oil & Gas Fossil Fuels: Coal Nuclear Energy Alternative Energy Resources Air Pollution Summary
At the heart of modern society lies an economy driven by energy use. Unfortunately, the same energy that brings us comfort, convenience, and prosperity also brings us pollution, impoverishment, and global warming. Our challenge is to maximize the benefits gained from energy consumption while minimizing the costs incurred. Douglas Foy A fuming smokestack is the perfect symbol of our national dilemma. On the one hand, it means the jobs and products we need. On the other, it means pollution. American Gas Association ad, 1991
Introduction • •
• •
Fossil fuels (oil, gas, coal) makeup most of the energy consumed in the U.S. Energy use increases with increasing population, land area, and industrial activity and energy use per capita is greatest in large, sparsely populated states. Fossil fuels are non-renewable resources with limited life span and their combustion contributes to global warming. Alternative energy sources such as solar and wind power are renewable and hold the promise of a sustainable energy future.
U.S. Energy Use Current U.S. energy use is weighted heavily toward fossil fuels (oil, natural gas, and coal) that account for approximately 90% of all energy used in the nation (Fig. 1). Environmental concerns over air pollution and the potential for global warming may encourage wider access to alternative energy sources such as nuclear power and wind or solar energy. Nuclear power accounts for about a fifth of U.S. electricity generation but only 5% of total energy consumption. Alternative energy sources (hydroelectric, wind, solar, geothermal) generate 5% of U.S. energy production but may expand that share in the decades ahead. Energy use within the U.S. varies with population size and character of energy demand (Fig. 2). States with large populations, large land area (greater distances to travel), and
Figure 1. U.S. energy consumption per energy type, 1949 to 1995. Graph courtesy of the Energy Information Administration.
2
energy-intensive industries (e.g., oil refining, chemicals), typically use the most energy. Large sparsely populated states such as Wyoming and Alaska rate highly in energy use per person because transportation consumes large volumes of fuel. Fossil fuels form from decayed organic material through a series of chemical reactions that occur gradually over millions of years under specific physical conditions in a select group of rocks. These conditions make it possible to predict where oil and gas may be found but also highlight the fact that fossil fuels are non-renewable resources that will not be replaced once used. Reserves of oil and natural gas will probably be stretched out for another century but we must face the inevitable conclusion that these finite resources will have to be replaced with an alternative form of energy in the next 50 years. The inevitable decrease in the availability of fossil fuels will be felt most acutely in transportation because there is no viable inexpensive replacement for the refined petroleum products that fuel automobiles and airplanes. Figure 2. Distribution of U.S. energy use. Energy use at home and industry is typically in the form of electricity generated by burning coal. Transportation is almost exclusively fueled by forms of gasoline refined from petroleum.
Coal represents an alternative fossil fuel with a potentially longer life span than either oil or gas but it has the unfortunate distinction of generating more pollution than the other fossil fuels. Furthermore, coal produces more carbon dioxide during combustion than either oil or gas, but all three have been fingered as the primary sources of the greenhouse gas that is the culprit for global warming. Advocates of a nuclear future have seized the potential threat of global warming and the nation's dependence on foreign oil to advance the nuclear cause. Fifty years ago, scientists working in the fledgling U.S. nuclear power industry (Fig. 3) predicted that electricity would be virtually free by the end of the century because of the electrical benevolence of nuclear energy. Today, only 17% of the world’s electricity is generated by nuclear power and that number is unlikely to grow because of concerns about the safety of nuclear reactors and anxiety over how to dispose of highly radioactive waste produced
3
during power generation. Rarely has a technology shown such early promise only to fall so rapidly from grace. Alternative energy resources (hydroelectric, wind, solar, biomass, geothermal) generate less than 10% of U.S. energy but have few of the drawbacks of fossil fuels or nuclear power and hold promise of a sustainable energy future. A veritable chorus of Pollyannas has sung the praises of alternative energy since the 1970s but their potential remains ambiguous because of uncertainties over the rate of technological development and operating costs. Some of these renewable energy sources have greater potential than others with solar energy and wind power holding the most hope for the future. The industrial air pollution that was once proudly viewed as a by-product of economic growth is now largely a thing of the past. No longer will thousands of people die during a weekend of lethal air pollution as they did in London in 1952. Air pollution is still widespread but its effects are muted, hidden among reports of greater incidence of asthma and other respiratory ailments and studies of acid rain downwind from industrial centers. The burning of fossil fuels represents a major source of air pollutants and cleaner air will therefore be an indirect by-product of any change in energy production in the years ahead.
Think about it . . . 1. Predict which of the following states consumes the most energy. a) California b) Illinois c) New York d) Texas 2. Examine the partially completed graph found at the end of the chapter that plots gross domestic product (GDP) per capita vs. energy consumption per capita. Label the points that represent where you think the eight named nations would plot on the graph. 3. Draw a time line for energy use before you read any further in this chapter. Label the time line to indicate how energy consumption has changed/will change from 1850 to 2050. Differentiate between domestic and industrial energy sources and transportation energy sources.
4
Figure 3. Perry nuclear reactor, 35 miles northwest of Cleveland, Ohio. Lake Erie is on the left of the image. Image courtesy of the Nuclear Regulatory Commission (NRC).
Fossil Fuels: Oil & Gas • • • • • •
Time and a specific temperature range are necessary for the generation of oil and gas. As hydrocarbons become mature they progress from heavy oils to light oils to natural gas. Hydrocarbons become concentrated in sedimentary rocks. The volume of the world’s oil reserves is approximately 1,070 billion barrels. The U.S. uses 25% of the world’s oil. Two-thirds of the world’s oil reserves are located in the Middle East.
Fossil fuels form from decayed organic material. Oil, coal, and natural gas are the most common products of this process. Oil and gas form from organic material in microscopic marine organisms, whereas coal forms from the decayed remains of land plants. Tar (oil) sands and oil shale are less common forms of fossil fuels and are less widely used because extraction of oil from these deposits is more expensive than producing other forms of fossil fuels.
Generation and Production of Oil and Gas The two principal requirements in the generation of oil and gas (also known as hydrocarbons - chemical compounds of carbon and hydrogen) are time and a specific range of temperature. The steps in the process are: 1. Organic-rich sediments are deposited and gradually buried to greater depths and converted to sedimentary rock (e.g., shale). 2. Chemical reactions occur during burial under conditions of increasing temperature and pressure. The reactions occur at temperatures of 50 to 100oC, higher temperatures "boil off" the hydrocarbons; lower temperatures are not sufficient to drive the chemical reactions. 3. The reactions change the organic molecules to hydrocarbon molecules. With increasing time (millions of years) the hydrocarbons become more mature changing from heavy oils to lighter oils to natural gas. Fossil fuels are considered
5
non-renewable resources because they are consumed much faster than they can be replaced. Oil and gas migrate upward through fractures and pore spaces in permeable rocks and/or sediments. Some hydrocarbons escape at Earth’s surface through features such as oil seeps. Others collect below the surface in sedimentary rocks when their path is blocked by low-permeability rocks (Fig. 4). Rock structures such as faults and folds may serve to juxtapose permeable and impermeable units. Oil and gas are trapped in the permeable rocks and will migrate upward to lie at the highest elevation in the rock unit.
When an oil field is first drilled the oil is driven into the well by pressures within the rocks. This primary recovery will extract about 25% of the oil. Additional oil can be extracted using enhanced recovery techniques that make it easier for the oil to enter the well. Such techniques may include artificially fracturing the rock to create passages for oil migration or pumping wastewaters from drilling operations into nearby wells to drive the oil toward the producing well.
Oil Reserves Oil and gas are not distributed uniformly within Earth's crust (Fig. 5). Hydrocarbons are initially formed as organic-rich sediments and the oil and gas subsequently migrate upward, into younger rocks that are also of sedimentary origin. Consequently, oil and gas reserves are generally absent in areas underlain by igneous or metamorphic rocks such as volcanic island chains like Japan or Hawaii. Even in areas where sedimentary rocks are present, they must fall within a specific age range to ensure that the rocks are mature enough to contain hydrocarbons but not so old that oil and gas would have long ago escaped. Oil reserves steadily increased since the first commercial oil well was drilled in Titusville, Pennsylvania, in 1859 but estimates of global reserves have remained relatively uniform 6
Figure 4. Oil and gas will migrate through permeable rocks to the highest available elevation. Examples of traps include folds (left), and faults (right).
Figure 5. Locations of principal North American oil fields (left) and other hydrocarbon resources (right). Most oil shales and oil sands are not economically viable now but may play a more significant role in energy production as supplies decrease.
at around a billion barrels over the last decade. Oil reserves remained stable despite the fact that global population has doubled in the last thirty years. Reserves haven't declined because of: •
Exploration of geologic formations in increasingly remote areas of the world, including the seafloor, using an array of new methods that utilize satellites and geophysical instruments to unravel the geology in regions where few rocks are visible.
•
Improved technology used by oil companies to extract greater volumes of oil through enhanced recovery techniques.
•
Greater efficiency in energy use as a result of higher fuel prices and stricter pollution standards that caused manufacturers to build more energy-efficient appliances and engines.
Further improvements in energy efficiency will continue to delay the inevitable decline in oil reserves. For example, recently introduced combination gas-electric cars can be driven 112 km (70 miles) on a gallon of gas. However, even with the best management and environmental stewardship we must anticipate that a world that continues to rely on oil will see this finite resource decline toward the second half of this century. Known world oil reserves are approximately 1,030 billion barrels (one barrel is equivalent to 42 gallons). These reserves would last for nearly 40 years at current global consumption 7
rates. The U.S. Geological Survey recently issued a more optimistic estimate that there actually may be double those reserves left to be discovered with a potential life span until the end of this century. The U.S. uses 25% of the world's oil, much more than any other nation, and imports over half of the oil it consumes. Consequently we are vulnerable to disruptions in oil supplies. Current fluctuations in gasoline prices that result from relatively modest changes in supply and demand will become much more exaggerated as the available reserves of oil decline. The future success of the U.S. economy may rely on the state of our political relationships with the relatively few nations that have abundant oil reserves. Figure 6. Distribution of global oil and gas reserves expressed as a percentage of global reserves. Twothirds of the world’s oil and one-third of all natural gas reserves are located in the Middle East. Russia has 33% of the world's natural gas and Saudi Arabia has 25% of the world's oil.
The majority of the oil and other petroleum products currently imported into the U.S. come from just four nations, Venezuela, Mexico, Canada, and Saudi Arabia. However, as two-thirds of all the world's oil reserves are located in the Middle East (Fig. 6), countries such as Saudi Arabia, Kuwait, Iran, and Iraq may play an increasingly important role in U.S. oil supply in the decades ahead.
Think about it . . . 1. Use the Venn diagram found at the end of the chapter to compare and contrast the similarities and differences between the characteristics of oil and coal resources. . . . continued on next page
8
2. Similar organic-rich source rocks are present in two locations. Oil deposits formed in the overlying rocks at the first location but did not form at the second location. Which of the following is the best explanation for this difference? a) The first location was more deeply buried than the second. b) The first location was subjected to lower temperatures than the second. c) The first location contains younger rocks than the second. d) Rocks at the first location had lower permeability than rocks at the second site.
Fossil Fuels: Coal • • • •
Figure 7. Progression of coal rank (maturity) from carbon-poor peat to carbon-rich anthracite. The relative proportion of U.S. coal production by rank is anthracite 2%, bituminous 53%, sub-bituminous 36%, and lignite 9%.
•
The carbon content and heat content of coal increase with increasing maturity. The volume of ash residue after burning decreases with increasing coal maturity. The two principal regions of coal production in the U.S. are the Appalachian basin and the Great Plains. Sulfur content of coal is lower in the Great Plains and higher in the Appalachian basin. Air pollution, medical expenses, and landfill fees are external costs of coal use.
Coal, the carbon-rich residue of plants, can be classified by rank or carbon content. Coal matures by increasing rank with increasing burial pressure (Fig. 7).
9
Peat is the least-mature form of coal, containing a large volume of fibrous plant matter. With increasing compaction, water is driven out and carbon becomes increasingly concentrated. Both carbon content and the amount of heat released during burning increase with maturity. The carbon content ranges from around 30% in peat to 99% for anthracite. The higher the carbon content, the more heat that is released when the coal is burned. Small amounts of high-carbon coals produce the same heat as large volumes of low-carbon coal. The volume of ash that remains after burning decreases with increasing rank. The ash must be disposed off in a landfill thus increasing expense.
Figure 8. Coalbearing areas of the U.S. Image courtesy of Energy Information Administration.
10
There are three principal coal-producing regions in the U.S. (Fig. 8). The first two, Appalachian basin states (Ohio, eastern Kentucky, West Virginia, Pennsylvania) and interior states (Illinois, Indiana, western Kentucky) produce high-rank bituminous coals and anthracite. These coals are produced from both surface and underground mines. Unfortunately, some of the bituminous coals have a high sulfur content (Fig. 9) and therefore contribute to air pollution. Given the stringent regulations on pollutants, some companies prefer to use lowergrade sub-bituminous coals to avoid costs associated with installing pollution control devices. Figure 9. Comparison of sulfur content and heat content of coals from principal U.S. coal-producing regions. Western coals have less sulfur and lower heat content.
Figure 10. Thick seam of subbituminous coal in the Powder River basin, northeast Wyoming. This seam is 60 meter (200 foot) thick for much of its length and is less than 15 meters(50 feet) below the surface at this location.
Great Plains and Rocky Mountain states (Montana, Wyoming, North Dakota, South Dakota, Colorado) produce lignite and sub-bituminous coals from surface mines (Figs. 8, 10). These coals may occur in especially thick seams making the mining process much less expensive than for underground mines. Larger volumes of these lower-grade coals must be burned to generate the same heat as bituminous coal or anthracite. Companies pay more to haul the extra coal but save money on production and labor costs. Sub-bituminous coals were not heavily mined prior to 1970. Subsequent to that date surface mines have produced more coal than underground mines and the western coal production has steadily risen to a
11
point today where coal production is approximately equal east and west of the Mississippi River (Fig. 11). Figure 11. Principal coal reserves of the U.S. Lower map shows top-10 states for coal reserves that can be divided between lignite and sub-bituminous coals in the West, and mainly bituminous coals and anthracite east of the Mississippi River.
Air pollution represents one of the external costs associated with the combustion of fossil fuels. External costs are the price we pay indirectly - in taxes, health insurance, medical bills, landfill fees - because of the use of fossil fuels. The use of coal would become less economically attractive if these costs were applied to the original (internal) cost of coal. Electric utilities account for approximately 90% of all U.S. coal consumption and are the major source of nitrogen dioxide and sulfur dioxide, two key air pollutants. The most potentially significant external cost of using fossil fuels is the build up of carbon dioxide in Earth's atmosphere. Scientists predict that fossil fuel emissions will lead to a warmer "greenhouse" world, initiating a potential cascade of negative economic repercussions. Consequently, future energy policy may not be concerned with how much fuel is left, but may instead focus on how to use it without prompting changes in global climate.
Coal Reserves Over 80% of the world's recoverable coal is found in just seven nations (Fig. 12). The U.S. has the greatest reserves, accounting for 25% of the world's coal, enough to last for 270 years at current consumption rates. This suggests that we will have a plentiful supply of electricity into the distant future but it is of little help as a replacement fuel for refined oil products (gasoline) unless we can assume that automobiles of the future 12
Figure 12. The U.S. has a quarter of the world's available coal reserves and 83% of all reserves are divided among just seven nations.
will run, at least partially, on electricity. Even in this scenario, we are still left with the potential for additional air pollution and the threat of global warming.
Think about it . . . 1. Use the Venn diagram found at the close of the chapter to compare and contrast the characteristics of oil and coal resources. 2. Examine the map of U.S. coal resources found at the end of the chapter and predict where the five numbered points on the graph of sulfur content vs. BTU might plot on the map.
Nuclear Energy • • •
•
Nuclear reactors generate 17% of the world’s electricity and 5% of total energy. Nuclear power has fallen from favor because of accidents like Three Mile Island (1979) and Chernobyl (1986). There are over 100 operating nuclear reactors in the U.S., approximately a quarter of all nuclear power plants worldwide. The benefits of nuclear energy are: no air pollution, no greenhouse effect, and a reduction in dependence on foreign oil.
13
•
• •
The potential problems are: U.S. reactors are getting old and there is no currently available site for permanent nuclear waste disposal. A potential storage site for nuclear waste is being investigated at Yucca Mountain, Nevada. The Yucca Mountain site is isolated, has a dry climate, in rocks with low porosity and permeability, and is located far above the groundwater table. However, the area around Yucca Mountain has experienced earthquakes and volcanism.
Approximately 17% of the world’s electricity is generated by nuclear power but that represents only 5% of the world’s consumption of energy. Clearly there is room for improvement. Current concerns about global warming have caused some governments to give nuclear energy another look and has increased optimism within the nuclear power industry prompting a series of ads that tout nuclear energy as the environmentally friendly alternative to dirty fossil fuels. Most technologies evolve into increasingly sophisticated and cheaper forms following their introduction and will continue to grow in popularity until they are replaced by a better alternative. Not so nuclear power. After a meteoric rise, the nuclear power industry hit a wall in the latter part of the last century as a result of problems with their own product. Nuclear energy originated in the nuclear weapons programs of World War II. Following the war, control of nuclear research passed from military to civilian control with the creation of the Atomic Energy Commission. Early plans to use nuclear weapons for mega-engineering projects (e.g., excavating a harbor on the coast of Alaska) were dismissed amid concern over potential radioactive contamination. The first commercial nuclear power plants generated electricity in the late 1950s. Nuclear power generation increased steadily until the 1970s and appeared to be on the road to acceptance as fuel costs increased during the 1973 oil crisis. However, the honeymoon Figure 13. Three Mile Island Unit 1 reactor, Pennsylvania, with Susquehanna River in background. The Unit 2 reactor is nearby but is no longer in use. Image courtesy of the Nuclear Regulatory Commission (NRC).
14
ended amidst with construction costs and a widely reported accident at the Three Mile Island Unit 2 reactor (1979), near Harrisburg, Pennsylvania (Fig. 13). Furthermore, the demand for energy decreased as energy conservation and efficiency gained popularity. A dangerous nuclear accident at Chernobyl in the former Soviet Union (now the Ukraine) in 1986 lessened the chances for a rebound in nuclear fortunes. The accident resulted from an unauthorized experiment by operators who were testing the capabilities of the reactor. Two explosions blew the top of the power plant. The reactor did not have a containment vessel (unlike U.S. reactors) allowing the escape of radioactive debris into the atmosphere. The accident was revealed when Sweden detected an increase in wind-borne radiation. As a result of the accident, over 200,000 people had to be moved from the area surrounding the damaged reactor; 31 workers and emergency personnel died immediately after accident and an unknown number of people died later because of exposure to lesser levels of radioactivity. A concrete "sarcophagus" was built over the damaged reactor in an unsuccessful effort to contain any further leaks. The nuclear industry argues that improved reactor design and the absence of airborne pollutants associated with fossil fuels make nuclear power an ideal source for future energy.
The Nuclear Fuel Cycle The nuclear fuel cycle represents the series of steps that begin with the mining of uranium, continue through the generation of electricity, and end with the disposal of nuclear waste. Uranium Mining and Milling: Uranium is approximately 500 times more abundant in Earth’s crust than gold. The top-five sources of uranium are Canada (12,029 tonnes, 34% of world production), Australia, Niger, Namibia, and U.S. (Fig. 14). Over half of uranium is produced from open-pit mines. The original uranium ore contains 0.1 to 1% uranium. Uranium is removed from ore by milling to produce a refined ore that contains approximately 60% uranium. During the milling process the uranium is dissolved from the ore and reprecipitated in a concentrated form known as “yellowcake.” Uranium Enrichment: Additional processing is required before the uranium is in a form that can be used in a reactor. 15
Natural uranium consists of two isotopes of uranium. The bulk of natural uranium is U238. Only 0.7% of natural uranium is the isotope U235 that is capable of undergoing fission, the process by which energy is produced in a nuclear reactor. Enrichment increases the concentration of U235 to approximately 4% of the uranium mixture by removing much of the U238 isotope. The uranium is formed into pellets that are placed in metal tubes to form the fuel rods in a reactor fuel assembly. Nuclear Power Generation: Nuclear reactors generate electricity from heat much the same way coal- or oil-fired power plants do. The heat converts water to steam, steam spins a turbine, and the spinning turbine generates electricity. The big difference is in how the heat is generated. In power plants using fossil fuels the fuel of choice is simply burned. In a nuclear plant, nuclear fission, the splitting of the nucleus of an atom, is the heat source. Neutrons ejected from the split atom hit adjacent atoms, causing them to fission. Uranium undergoes nuclear fission in the fuel rods of a nuclear reactor. Neutronabsorbing control rods may be inserted in the reactor to slow down the rate of the reaction and produce less heat. Both fuel rods and control rods are stored in water that serves to cool the rods and moderate the nuclear fission reactions. The radioactive material in fuel rods is not sufficiently enriched to cause a nuclear explosion but a runaway reaction could result
16
Figure 14. U.S. uranium mining and production plants. Image courtesy of Energy Information Administration.
Figure 15. Map of the distribution of U.S. nuclear reactors. Image courtesy of the Nuclear Regulatory Commission (NRC).
in overheating of the surrounding water and cause a steam explosion. Nuclear Reactors: A typical nuclear power plant in the U.S. is granted a 40-year license for operation but many are taken out of service (decommissioned) before the end of that time interval. The oldest currently operating nuclear reactors in the U.S. started up in 1969. There are over 100 nuclear power plants operating in the U.S. (Fig. 15; 104 as of November, 1999) but no new plants have been ordered in the last 20 years. Consequently, as the current plants are decommissioned the total number of operating nuclear plants will inevitably decline. Figure 16. Graph of proportion of electricity from nuclear power for France (58 reactors), Belgium (7), Sweden (12), Japan (52), and U.S. (104). There are 428 nuclear power plants worldwide (1999).
Some nations rely heavily on nuclear power to supply the bulk of their electricity (Fig. 16). France generates over threequarters of its electricity from 58 nuclear power plants and Lithuania generates 77% of its electricity from just two plants. In contrast, the U.S. has 104 nuclear reactors that produce much more electricity than France (96,977 megawatts vs. 61,723 megawatts). However, this represents a smaller 17
proportion (19%) of national electricity production than several other nations. Europe is home to more nuclear reactors than any other continent (173), and Africa and South America have only 5 between them.
Nuclear Energy and the Future There has recently been renewed interest in the use of nuclear power in some quarters (mainly from advocates in the nuclear industry). They cite three principal benefits of the use of nuclear energy: 1. Air pollution and global warming, associated with fossil fuels, are not produced by nuclear power plants. 2. Electricity from nuclear power would reduce the nation's dependence on foreign oil which is growing increasingly scarce. 3. New reactors have safer standardized reactor designs that markedly reduce the potential for an accident. However, for nuclear power to become a viable energy alternative in the immediate future it must first deal with the following issues: 1. Many existing nuclear power plants are entering old age and will have to be decommissioned, reducing the energyproduction capacity in the U.S. 2. More nuclear power plants mean more high-level nuclear waste. The nation still has no repository for this waste and will not have a disposal site until at least 2010.
Nuclear Waste Nuclear waste comes in a variety of forms, each with different storage requirements but it is the disposal of high-level nuclear waste that presents the greatest challenge for the future. Although high-level radioactive waste (e.g. used fuel rods) composes a relatively small volume of all nuclear waste it represents nearly all (95%) of the radioactivity nuclear wastes and may remain dangerous for over 10,000 years. Like several other nations that rely on nuclear energy, the U.S. is attempting to find a suitable site where it can store nuclear waste safely for thousands of years. The potential site is located below Yucca Mountain, Nevada (Fig. 17), about a one hour drive north of Las Vegas.
18
The Department of Energy (DOE) initially identified nine potential nuclear dump sites but later shortened the list to three (Fig. 17; Hanford, Washington; Deaf Smith County, Texas; Yucca Mountain, Nevada). The DOE hoped to investigate the geology of each site thoroughly to determine which would be the safest repository for the dangerous waste. However, in December 1987, Congress saw a chance to save some money and directed DOE to study just the Yucca Mountain site. Nevada, which has no nuclear power plants, has fought vainly against hosting the site. Figure 17. Three proposed sites for a possible high-level nuclear waste disposal facility were considered (map right) before Congress chose to locate the site below Yucca Mountain, Nevada (left). Image courtesy of the Yucca Mountain Project.
Not only must a high-level nuclear waste disposal facility be safe from accidental entry and sabotage, potentially for a few hundred thousand years, it must also be safe from geologic hazards that may release the radioactive materials. The ideal site would be geologically stable to ensure that groundwater could not infiltrate through the waste, and neither earthquakes nor volcanic eruptions would rupture the containment structure.
Geologic Setting of Yucca Mountain The waste would be stored in sealed containers in an underground vault approximately 300 m (1,000 feet) below the surface (Fig. 18). The site at Yucca Mountain is favorable for waste disposal because: •
It is located in the desert of southern Nevada far from population centers (Las Vegas is ~100 km south).
19
•
The vault would be hollowed out of a layer of volcanic tuff, a resistant igneous rock with very low porosity (spaces within the rock that may contain water) and low permeability (the ability of water to flow through the rock).
•
In addition, the site gets ~15 cm (6 inches) of precipitation a year, most of which evaporates in the desert heat. Project scientists believe that it is unlikely that water could inundate the disposal facility and transport radioactive materials into the surrounding environment.
•
Furthermore, the local groundwater source is 240 meters (~750 feet) below the site, making it difficult for any leaks to pass quickly (before detection) to the groundwater supply. Figure 18. Approximate position of the nuclear waste repository in impermeable volcanic tuff rocks below Yucca Mountain, Nevada.
However, some scientists point out that certain geologic features point toward potential problems in the future: •
Groundwater flow may be accelerated along fractures and faults that exist in the region, and that evidence points to an elevated water table (groundwater) in the relatively recent geologic past (~10,000 years ago).
•
Nevada is one of the most seismically active states after Alaska and California. Some have suggested that the threat of a damaging earthquake is too great to take the risk of building the disposal facility in Nevada. However, although there have been numerous small earthquakes near the site, few have been of sufficient magnitude to pose any threat and a structure could be engineered to withstand the moderate-size earthquakes that occasionally occur in southern Nevada.
•
Geologically recent (