the organization to ensure public health and safety but provides nospecificguidance onhowfar themandate must be pursued. NRC safety decisions have been criticized both by the nuclear industry and by environmental interests. The industry contends that NRC regulations have sometimes been unnecessary, counterproductive, and overly prescriptive in techniques for achieving safety. Environmental interests have asserted that the NRC has compromised safety to ensure the economic via- bility of nuclear projects. Safety concerns reached a peak when a reactor ac- cident occurred at the Three Mile Island nuclear plant in Pennsylvania in 1979. In response to investi- gations of the accident, the NRC reformed its licens- ing and regulatory processes. However, no new plants were begun, and anumberof nuclear projects then in progress were canceled. William C. Wood Web Site U.S. Nuclear Regulatory Commission http://www.nrc.gov/ See also: Atomic Energy Acts; Atomic Energy Com- mission; Energy economics; Nuclear energy; Nuclear waste and its disposal; Three Mile Island nuclear acci- dent. Nuclear waste and its disposal Category: Pollution and waste disposal The disposal of radioactive waste is a significant prob- lem for the nuclear power industry and society as a whole. Various methods of burying and destroying the material have been proposed. Background Unwanted radioactive materials are classified as low- level, transuranic, or high-levelwaste or spent nuclear fuel, depending on the concentration of radioactivity and the half-life of the radioactive material. In some cases radioactive tailingsfrom uranium minesare also of concern. Low-level waste (LLW), such as syringes contaminated by radioactive pharmaceuticals, is much less dangerous than the high-level waste (HLW) or spent fuel rods generated by nuclear reactors. Trans - uranic waste (TRU) is radioactive waste with a level of radioactivity greater than 100 nanocuries per gram, a half-life greater than twenty years, and a composition of elementswith atomicnumbers higher than92 (ura- nium). In the United States, the 1980 Low-Level Ra- dioactive Waste Policy Act makes individual states responsible for the development of low-level waste disposal sites in conformity with licensing rules estab- lished by the U.S. Nuclear Regulatory Commission. Disposal of transuranic waste, high-level waste, or spent fuel rods, however, must be accomplished on the national level, as specified by the Nuclear Waste Policy Act of 1982. Some countries, such as France and Japan,donot consider spentfuel rods aswastebe- cause they can be reprocessed into fuel (this process does generate TRU, however). Across the globe, trans- uranic waste and disposal of high-level nuclear waste and spent fuel rods continues to pose problems. Be- cause nuclear power is seen as a potential means for alleviating global warming, the disposal of nuclear waste has become of increasing concern. Spent Fuel Rods and High-Level Waste Most electricity continues to be produced through the burning of fossil fuels, predominantly coal. Some countries, such as France, rely heavily on nuclear en- ergy for their electric power; even the United States, a major consumer of fossil fuels, obtains 20 percent of its electric power from nuclear energy. Worldwide, 440 nuclear power reactors are in operation in thirty countries. Their operation is associated withtheaccu- mulation of large amounts of high-level waste and transuranic waste as well as with the production of highly radioactive spent fuel rods. The fuelfora typical reactor isarare isotopeofura- nium, uranium 235, which is mixed with common uranium (uranium 238); the enrichment ratio is 1 to 30. The fuel is consumed through a nuclear reaction, called fission,a processin which atomsof uraniumare broken into radioactive fragments. The energy re- leased in fission becomes heat, part of which is con- verted into electricity. Once or twice a year a reactor must be shut down for refueling. The removed waste is usually stored in a local isolated area for several years, because it is highly radioactive and its presence in the biosphere would be a great danger to all living organisms. However, this type of procedure is not sufficient: Tens of thousands of years are necessary to reduce the radioactivity in spent fuel to a nonthreat - ening level. 838 • Nuclear waste and its disposal Global Resources High-level nuclear waste includes liquids derived from fuel reprocessing, solidified liquids and spent nuclear fuel that have not been reprocessed, and by- products from the nuclear weapons industry, includ- ing residues from the reprocessing of out-of-date nu- clear weapons.Like spentfuelrods, HLWposessevere environmental problems if not disposed of properly. Nuclear waste generated by the nuclear weapons industry poses a special set of problems. Some of this waste is either the same sort of high-level or low-level waste as that generated by other nuclear industries or a substantial amount of transuranic material. Nuclear warheads contain highly radioactive material that poses potential danger if not treated properly. Some weapons are reprocessed into fuel, although this pro- cess is not without hazard and produces substantial HLW and TRU wastes. Some people advocate repro- cessing the weapons into new weapons, and others call for destruction. Some “spent” uranium is repro- cessed into artillery shells. In addition, the U.S. De- partment of Energy’s Nuclear Weapons Complex has maintained sixteen major weapons sites throughout the country, generating a variety of nuclear wastes. Two of the oldest—at Oak Ridge, Tennessee, and Hanford, Washington—began operation during World War II, generating large quantities of nuclear waste that has been stored on-site. Some of the liquid storage tanks at Hanford are corroding; in others, chemical reactions are under way; and in some cases, knowledge of what is in the storage tanks no longer exists. The Russian military site at Mayak—which has been the source of several accidents and deaths in- volving substantial radioactive contamination of the land, water, and atmosphere—poses even more of a problem than U.S. weapons sites because of the poor safety and waste-disposal practices followed in the past. Some of the same problems as those experi- enced in the United States are also present in nuclear weapons sites in countries such as the United King- dom and France. Although there have been few accidents involving nuclear reactors, the 1986 accident at Chernobyl in the Ukraine (then part of the Soviet Union) pro- duced a large amount of nuclear waste. The reactor site and several hundred square kilometers of sur- rounding countryside became contaminated. In this case the contamination became so extensive that the Russian and Ukrainian governments sealed off the area, in essence leaving the region contaminated be - cause no other solution existed. The reactors were entombed in concrete, although some doubt exists as to how successful this process has been. Burying Nuclear Waste The nuclear industry faces the critical challenge of isolating radioactivewaste fromthehuman habitat.In the United States, for example, there are 131 sites in thirty-nine states storing HLW. One possible method of disposal is to bury spent fuel and HLW deep in geo- logically stableformations,a solutionthat theU.S.Na- tional Research Council advocates. A site has been se- lected for that purpose: Yucca Mountain, which is 145 kilometers northwest of Las Vegas, Nevada. Construc- tion is under way, and the U.S. government has spent millions of dollars developing the site, but it is not likely to be ready until 2015 or later. The Yucca Moun- tain site is geologically stable, with low precipitation. The water table lies, on average, 300 meters below the repository tunnels. According to the plan, high-level waste would be stored in corrosion-resisting titanium Global Resources Nuclear waste and its disposal • 839 The Hanford nuclear reservation in Richland, Washington, pre - pares for the acquisition of 2,000 metric tons of radioactive waste to be inserted into the holes in the floor pictured above. (AP/Wide World Photos) alloy containers with a drip shield, to prevent ground - water from damaging the containers, and monitored for fifty years. The depository would be sealed to pre- vent humaninterference. However,the project iscon- troversial. Some scientists think that an unacceptable number of radioactive atoms would leak into the bio- sphere with slowly percolating water or steam gener- ated by radioactive heat. Others fear the possibility of future volcanic activities and earthquakes. Several other countries are also engaged in the de- velopment of underground storage sites for nuclear waste, and this has become the likely method for dis- posal worldwide. Most European countries have not selected sites for the disposal of HLW. The United Kingdom continues to store nuclear waste on-site and has selected no permanent repository site, nor has France. Germany is considering a sites at an aban- doned iron-ore mine and a salt dome at Gorleben. Finland, on the other hand, is developing an under- ground site at Olkiluoto for HLW. Belgium is consid- ering a site for deep deposition in a clay formation. In the first decade of the twenty-first century, no country had a site for HLW in operation, and many, such as Canada, did not appear to have any possible sites. Some countries with small volumes of nuclear waste or no suitable geological formations ship their nu- clear waste to countries such as the United States. Although environmental questions have been raised concerning Yucca Mountain, the site is in- tended to provide a stable and safe repository for nu- clear waste. In the past, the former Soviet Union used a process of underground injection at three sites for a good deal of its LLW such as strontium 90 and cesium 137. Radioactive atomshave migrated from thesesites into nearby water supplies and soil, creating environ- mental problems. The Yucca Mountain tunnels are large enough to hold the radioactive waste already accumulated in the United States. New sites, however, will be needed to store theHLWthat will beproduced inthetwenty-first century and beyond. Proliferation of such sites is not desirable. TRU waste in the United States is handled at the Waste Isolation Pilot Project (WIPP) site near Carls- bad, New Mexico, which began operation in 1999. WIPP and the Yucca Mountain site are under control of the U.S. Department of Energy. The WIPP disposes of TRU wastein salt-dome formations located 655me - ters below the surface. Questions remain about WIPP, most notablypertainingto the possibilityof migration of radioactive material to the surface through cracks in the salt. Mostnationsseem to be following a pathof underground disposition for TRU. Low-level nuclear waste presents its own set of problems, themostformidableof which is thevolume of material. Although not highly radioactive, LLWs— such as those generated by hospitals or contaminated clothing or building materials—may be solids, liq- uids, or even gases. In many cases, efforts are made to turn these materials into solids so that they can be dis- posed of more readily. They still require special han- dling because of their radioactivity and the possibility of contamination with other hazardous substances. LLWs are treated inavariety of ways. In manycases, countries are engaging in burial of LLWs, often in shallow trenches. Because LLWs have a high volume, their disposal requires a large site. LLWs have in- creased in volume as, for example, nuclear medicine facilities have expanded. In the United States, the Low Level Waste Policy Act provided that states would enter into compacts and develop LLW sites. Because the disposal of nuclear waste is a controversial politi- cal issue, state governments have done little to de- velop such disposal sites. By the early twenty-first cen- tury, the only two sites in operation were at Barnwell, South Carolina, and Richland, Washington, with Barnwell handling much of the LLW. One means of disposal that has been considered and rejected is dumping radioactive material in deep trenches in the ocean or even in lakes or rivers. The Soviet Union has engaged in such practices in the past, leading to substantial contamination. Two other means of disposal of HLW and TRU wastes have also been considered and rejected. One is to store this waste beneath polar ice caps. Another is to load the waste on rockets and send it into space. Both alternative pose enormous technological prob- lems. More important, they also pose substantial risks to human health and safety. Transforming Radioactive Waste Not everyone agrees that spent fuel should be buried without preliminary processing. In France, for exam- ple, where 80 percent of electricity is nuclear, the pol- icy is to process spent fuel so that some can be reused before the remainder is buried. Chemical processing is already usedto extract valuable by-products, such as plutonium, from spent fuel. One isotope of pluto - nium that is as fissionable as uranium 235 has already been used to manufacture new fuel. One by-product 840 • Nuclear waste and its disposal Global Resources of this process, however, is weapons-grade plutonium. France and the United Kingdom are developing a process to recycle spent control rods without produc- ing weapons-grade plutonium. The Russian Federa- tion stores most high-level nuclear waste on-site but has a small reprocessing facility at Chelyabinsk-65 and is developing a larger reprocessing facility at Krasno- yarsk, scheduled to be operational in 2015. Proposals have been developed to “incinerate” spent fuel. Nuclear incineration refers not to chemi- cal burning but to nuclear reactions by which long- lived radioactive atoms are transmuted into short- lived or nonradioactive atoms. Pyrometallurgical processes are intended to produce a mix of TRU ele- ments instead of plutonium, which is produced in conventional reprocessing processes. Because pluto- nium can be used to make nuclear weapons, the United States banned this sort of reprocessing in 1977, although it is done in some countries, most no- tably France. Pyrometallurgical recycling produces fission products and transuranics that are unsuitable for weapons and also current reactors. However, they are suitable for what are called fast neutron reactors. Because fast neutron reactors neither use nor gener- ate pure plutonium at any stage, they pose less of a threat for weapons production. China and India have considered fast reactors, but none are currently un- der consideration in the United States. Other means of destroying high-level wastes have been considered. Oneinvolvesthe use of a nuclear in- cinerator that would subject the waste material to a flux ofneutrons ofan intensity thatisseveral orders of magnitude higher than occurs in an ordinary reactor. Research groups in the United States, Switzerland, and Japan have engaged in research involving incin- eration, butthere are nocommercial applicationsyet. The prevailing means for the disposition of nu- clear waste remains underground disposition in sta- ble geologic formations. Such a solution has some long-term problems, such as site maintenance and intergenerational equity questions. Challenges for the Future Many ofthetechnological issues facingthe disposal of nuclear waste are being resolved. Underground dis- posal stillhassome potential problems, butit is clearly superior toleaving spentcontrol rods,HLW, andTRU waste in temporary storage facilities on-site at power reactors or government research facilities. Long-term on-site disposition raises numerous questions, such as safety from terrorists, environmental contamination, and cost. Some remaining concerns regarding under- ground disposition include the potential for migra- tion of radioactive material through the surrounding material to the surface and some difficulties concern- ing transportation of radioactive material to disposal sites. The major obstacle to underground disposal re- mains societal. Many people are not confident in the technology for disposal or are concerned about long- term consequences of underground burial, such as migration of radioactive material into water tables or human access. Other people distrust governmen- tal bodies, especially when they have not been trans- parent in decision making in the past. However, above- ground storage on site is not a feasible long-term solution for HLW, LLW, spent nuclear fuel, and TRU waste. Failure to resolve questions of waste disposal are increasing potential costs and have hampered the development of the nuclear power industry, which has the potential for helping to resolve energy needs and provide an alternative fuel not linked to global warming. Ludwik Kowalski, updated by John M. Theilmann Further Reading Gerrard, Michael B. Whose Backyard, Whose Risk: Fear and Fairness in Toxic and Nuclear Waste Siting. Cam- bridge, Mass.: MIT Press, 1994. Hambin, Jacob Darwin. Poison in the Well: Radioactive Waste in Oceans at the Dawn of the Nuclear Age. New Brunswick, N.J.: Rutgers University Press, 2008. Hannum, William H., GeraldE. Marsh, and George S. Stanford. “Smarter Use of Nuclear Waste.” In Oil and the Future of Energy. Guilford, Conn.: Lyons Press, 2007. Johnson, Genevieve Fuji. Deliberative Democracy for the Future: The Case of Nuclear Waste Management in Can- ada. Toronto: University of Toronto Press, 2008. Lochbaum, David A. NuclearWaste Disposal Crisis.Tulsa, Okla.: PennWell Books, 1996. Long, Michael E. “America’s Nuclear Waste: The Search for Permanent Solutions Heats up as Tons of HighlyRadioactive Sludge, SpentFuel, andCon- taminated Soil Pile up Around the Nation.” Na- tional Geographic 202, no. 1 (2002): 2. Macfarlane, Allison M., and Rodney C. Ewing, eds. UncertaintyUnderground: Yucca Mountain andthe Na- tion’s High-Level Nuclear Waste. Cambridge, Mass.: MIT Press, 2006. Murray, Raymond L. Understanding Radioactive Waste. Global Resources Nuclear waste and its disposal • 841 5th ed. Edited by Kristin L. Manke. Columbus, Ohio: Battelle Press, 2003. National Research Council. Disposition of High-Level Waste and Spent Nuclear Fuel: The Continuing Societal and Technical Challenges. Washington, D.C.: Na- tional Academy Press, 2001. Risoluti, Piero. Nuclear Waste: A Technologicaland Politi- cal Challenge. Berlin: Springer Verlag, 2004. Rogers, Kenneth A., and Marvin G. Kingsley. Calcu- lated Risks: Highly RadioactiveWaste and HomelandSe- curity. Burlington, Vt.: Ashgate, 2007. Saling, James H., Audeen W. Fentiman, and Y. S. Tang, eds. Radioactive Waste Management.2ded. Philadelphia: Taylor & Francis, 2002. Savage, David, ed. The Scientific and Regulatory Basis for the Geological Disposal of Radioactive Waste. New York: John Wiley, 1995. Vandenbosch, Robert, and Susanne E. Vandenbosch. Nuclear Waste Stalemate: Political and Scientific Contro- versies. Salt Lake City: University of Utah Press, 2007. Web Sites Nuclear Energy Institute Nuclear Waste Disposal http://www.nei.org/keyissues/nuclearwastedisposal U.S. Nuclear Regulatory Commission Radioactive Waste http://www.nrc.gov/waste.html See also: Air pollution and air pollution control; Bio- sphere; Department of Energy, U.S.;Electricalpower; Energy economics; Food chain; International Atomic Energy Agency; Nuclearenergy; Plutonium; Resources as a source of international conflict; Three Mile Is- land nuclear accident; Uranium; Water pollution and water pollution control. 842 • Nuclear waste and its disposal Global Resources O Ocean current energy Category: Energy resources The use of ocean currents as an energy source carries great potential, but development has proceeded slowly because the cost is not competitive with that of other en- ergy sources. Background Just as winds flow through the Earth’s atmosphere, currents flow throughout the world’s oceans. These currents are a potential power source as great as wind, although winds harnessed for power have greater speed than the currents. The energy available in a fluid flow varies bothwithvelocity (by the square) and with density: Kinetic Energy = (Density) × (Velocity) 2 Because wateris nearlyeighthundredtimesdenser than air(1,000 and1.27 kilogramspercubic meter,re- spectively), a current of1.6kilometers per hour hasas much energy as a wind of 45 kilometers per hour, which is considered an excellent average speed for wind energy. Furthermore, currents are more de- pendable thanwinds andflow inaconstant direction. Ocean Temperature and Salinity Ocean currents are caused by differences in tempera- ture and salinity. For example, as water near the poles is cooled, its density increases, and much of this cooler water sinks toward the ocean floor. From there it flows toward the equator, displacing warmer water as it goes. Meanwhile, water near the equator is warmed, becoming less dense. It tends to flow along the surface toward the higher latitudes to replace the sinking denser water. The Gulf Stream is such a current. It starts from an area of warm water in the equatorial Atlantic and in the Gulf of Mexico. This warm water flows generally northward parallel to the coast of North America and bends gradually to the right due to the rotation of the Earth. This tendency to curve (right in the Northern Hemisphere, left in the Southern Hemisphere) is called the Coriolis effect, and it bends the flow north - east as the West Wind Drift, bringing warm, moist air to Western Europe. It continues south as the Canaries Current (carrying cooler water) past western North Africa. Finally, the bending turns back west toward North America as the North Equatorial Current. Similar circular patterns, or gyres, occur in all the world’s oceans, with many locations having great po- tential for electrical power generation. For instance, the Gulf Stream has more energy than all the world’s rivers combined. The area off Florida might yield 10,000 megawatts (10 billion watts) without observ- able change in the heat flow to Europe. Salinity differences also cause major flows. The most easily tapped salinity currents are those between a sea with high evaporation and the open ocean. High-salinity water flows along the bottom from the Mediterranean Sea, for instance, while less saline At- lantic water flows in to replace it. (German subma- rines used these currents during World War II for drifting silently past the major British base at Gibral- tar.) Two lesser potential sources of current power are tidal currents and the currents at the mouths of rivers. Methods for Harnessing Ocean Currents Electrical power generation from currents requires three things: mooring power stations to the ocean floor, generating power, and transmitting power to customers on shore. Mooring and transmitting power are related eco- nomic constraints on ocean current power. Although an underwater cable from a mid-Atlantic power sta- tion could technically supply power, deeper mooring lines and longer cables eventually cost more than the power delivered. Thus, ocean current stations, if built, will tend to be near shore on the continental shelf and slope before investors attempt to moor a plant to the depths of the ocean floor. Using currents in deeper and more distant waters will require some means of energy storage. This issue has been considered in design studies for ocean ther- mal energy conversion(OTEC)power stations, which would harness the temperature difference between warm tropicalwatersand thecolder deepwaters.Elec - tricity could be used for some energy-intensive pro - cess (such as refining aluminum) or for electrolyzing hydrogen from water. Hydrogen could be used to syn- thesize chemical products, such as ammonia or meth- anol. Once the potential of current power is proven, investors may consider the second set of risks inher- ent in such mid-ocean ventures. Among various proposals, two methods have been studied in detail: turbines and sets of parachutes on cables. Turbines were first proposed by William Mouton, who was part of a study team led by Peter Lissaman of Aerovironment, Inc. Their design is called Coriolis. In the study design, one 83-megawatt Coriolis station has two huge counter-rotating fan blades (so it does not pull to one side), roughly 100 meters in diameter. The blades move slowly enough for fish to swim through them. With blades so large, neither rigid blades nor the central hub couldbe made strong enoughwithoutbe- ing too heavy and expensive. However, a catenary (free-hanging, like the cables of the Golden Gate Bridge), flexible blade can be held in the proper shape by the current while the generators are in a rim around the blades. The rim also acts as a funnel to in- crease current speed past the blades and as an air res- ervoir for raising the station when necessary. Another concept is parachutes on cables, which was proposed by Gary Steelman. His water low-velocity energy converter (WLVEC) design is an endless loop cable between two pulleys, much like a ski-lift cable. Parachutes along the cable are opened by the current when going downstream and closed when coming back upstream. The WLVEC is cheaper than Coriolis, but there is a question of how well any fabric could withstand sustained underwater use. Ocean currents are sufficiently powerful and pre- dictable to supply electricity effectively. However, costs of competing fossil fuels must rise significantly before investors willovercometheirtimidity about construct- ing offshore power plants. However, test projects in the United States, China, Japan, and the European Union, in particular Britain, Ireland, and Portugal, continue with high expectations. Roger V. Carlson Further Reading Charlier, Roger Henri, and John R. Justus. “Ocean Current Energy Conversion.” In Ocean Energies: En - vironmental, Economic, and Technological Aspects of Al - ternative Power Sources. New York: Elsevier, 1993. Congressional Research Service. Energy fromthe Ocean. Honolulu: University Press of the Pacific, 2002. Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980. Lissaman, P.B.S. “The Coriolis Program.” Oceanus 22, no. 4 (Winter, 1979/1980): 23. _______. “Tapping the Oceans’ Vast Energy with Un- dersea Turbines.” Popular Science 221, no. 3 (Sep- tember, 1980). Noyes, Robert, ed. Offshore and Underground Power Plants. Park Ridge, N.J.: Noyes Data, 1977. Web Site Minerals Management Service, U.S. Department of the Interior Ocean Current Energy http://ocsenergy.anl.gov/guide/current/index.cfm See also: Energy storage; Ocean thermal energy con- version; Ocean wave energy; Oceans; Tidal energy. Ocean thermal energy conversion Category: Energy resources In some tropical regions of the Earth, there is virtually limitless energy in the ocean for possible conversion to electric power. The efficiency of the conversion is very low, however, and the engineering problems are chal- lenging. Development of ocean thermal energy conver- sion (OTEC) has been slow. Background In tropicaloceans, thetemperaturesof warm andcold layers of water may differ significantly even though the layers areless than 1,000 metersapart.This phenome- non results from globalcirculationcurrentscausedby the Sun. Solar energy warms water near the surface, and colder, more dense water moves to lower depths. At the same time, the rotation of the Earth causes the cold water to flow from the poles toward the tropics. As it is warmed, this cool water then rises toward the surface as its density decreases, causing the warm surface water to flow toward the polar regions, where it is cooled. Differences of 20° to 25° Celsius over a distance of 500 to 1,000 meters are found in the Caribbean Sea 844 • Ocean thermal energy conversion Global Resources and the Pacific Ocean near the Hawai - ian Islands. In accordance with the second law of thermodynamics, ther- mal energyfrom thewarmlayer canbe used as a “fuel” for a heat engine that exhausts energy to the coollayer.Typi- cally, the warm layer has a tempera- ture between 27° and 29° Celsius, and the cool layer is between 4° and 7°. The second law of thermodynamics indicates that themaximum efficiency of the conversion from thermal en- ergy to mechanical energy will be very low. For example, if the warm layer is at 25° Celsius and the cold layer is at 5°, the maximumefficiency willbeless than 7 percent; even this figure is be- tween two and three times the actual efficiency that can be achieved in an energy conversion plant. History The concept of OTEC was first sug- gested in 1882 by the French physicist Jacques Arsène d’Arsonval, but it was not until1926 thatthe Frenchscientist Georges Claude made an attempt to implement the idea at Matanzas Bay, Cuba. The facility in Cubawasa small, land-based plant which was so inefficient that it required more power to operate than it produced. It ran for only a few weeks. Beginning in the 1960’s, improvements in design and materials led to considerable research. Feasibility as a practical method of power generation was first demonstrated in the 1980’s. Advances in OTEC have depended ongovernmen- tal support. In the mid-1970’s, only the U.S. and Japa- nese governments were supporting research and de- velopment. The French government later became interested, and sponsorship followed in the Nether- lands, the United Kingdom, and Sweden. Basic Designs Broadly speaking, designs are either open cycle (OC) or closed cycle (CC). In the OC method, the incom- ing warm seawater is continuously sent into an evapo- rator operating at low pressure, where a small portion of the water “flashes” into steam. The steam in turn passes through a turbine connected to an electric power generator. The low-pressure steam leaving the turbine is then cooled and condensed in a heat ex - changer by the cold seawater stream. The condensed water is fresh water, the salt of the ocean having been left behindin theevaporator.Hence, thiswater can be used for drinking and other household uses. In the CC process, heat from the warm stream is transferred in a heat exchanger to a “working fluid” such as propane or ammonia. This fluid is vaporized and passed through a turbine generator in the same fashion as in the OC process. The vapor leaving the turbine is then condensed in a second heat exchanger. The condensate is recycled to the first exchanger, where it is again vaporized. Thus, the working fluid is never in direct contactwith the seawater. Somehybrid plants have been designed which are combinations of OC and CC technology. Though the first plant was a land-based unit, some plant designs involve plants located offshore, possibly floating orsubmerged.One ofthekey elementsinthe process is the water pipe which carries the cold water to theplant. This pipeis typically between1 and 2kilo- meters long. Originally, Claude used a corrugated steel pipe, 1.6 meters in diameter, which was fragile and not corrosion-resistant. Steel has been replaced by fiberglass-reinforced plastic or high-density poly - ethylene. Diameters larger than this have been con - Global Resources Ocean thermal energy conversion • 845 The ocean thermal energy conversion plant in Keahole Point, Hawaii, opened in 1974 and became one of the top facilities of its kind in the world. (United States Depart- ment of Energy) sidered in some studies but are not feasible owing to a lack of flexibility. Engineering Problems Designs for OTEC plants with power capacities on the order of 10 megawatts or more have been made, but actual plantshave been muchsmaller, withoutputs on the order of tens of kilowatts. In spite of these rela- tively small outputs, the equipment and the engineer- ing problems are challenging. Both cold and warm water flow rates are large because the efficiency of the conversion process issolow. Theseawatercarries con- siderable dissolved gases, notably nitrogen and oxy- gen, and these gases must be vented if flash evapora- tion is used. The presence of noncondensable gases poses difficult problems both in the evaporator which precedes the power turbine and in the condenser which follows it. These gases not only increase the sizes of the units but also, because they are below at- mospheric pressure, mustbepumped out tomaintain the vacuum levels in the process. The CC method canavoidsome of these problems. The operating pressures in the cycle using propane are relatively high, so a turbine of reasonable size can be used. Moreover, because the pressures are greater than atmospheric, vacuum and deaeration problems are eliminated. TheCCprocess introduces additional problems, however, owing to the heat-transfer steps between the working medium and the hot and cold water. Advantages In viewof theverylow efficiency ofOTEC, itmayseem hard to imagine how the process can be profitable. However, the “fuel” is free and virtually unlimited. In addition, the OC process can produce sizable quanti- ties of fresh water, which is often valuable in places where OTEC plants are located. Some OC plants may even be profitable on thebasisof their freshwater pro- duction alone. Nevertheless, OTEC, even in the best of circumstances, poses both engineering and eco- nomic challenges that will continue to hamper its de- velopment for many years. Thomas W. Weber Further Reading Avery, William H., and Chih Wu. Renewable Energy from the Ocean: A Guide to OTEC. New York: Oxford Uni - versity Press, 1994. Charlier, Roger Henri, and John R. Justus. “Current Assessment of Ocean Thermal Energy Potential.” In Ocean Energies: Environmental,Economic,and Tech- nological Aspects of Alternative Power Sources. New York: Elsevier, 1993. Congressional Research Service. Energy fromthe Ocean. Honolulu, Hawaii: University Press of the Pacific, 2002. Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980. Krock, Hans-Jurgen, ed. OceanEnergyRecovery: Proceed- ings of the First International Conference, ICOER ’89. New York: American Society of Civil Engineers, 1990. Sorensen, Harry A. Energy Conversion Systems. New York: J. Wiley, 1983. Takahashi, Patrick, and Andrew Trenka. Ocean Ther- mal EnergyConversion. NewYork: JohnWiley,1996. Tanner, Dylan. “Ocean Thermal Energy Conversion: Current Overview and Future Outlook.” Renewable Resources 6, no. 3 (1995): 367-373. Web Sites State of Hawaii, Department of Business, Economic Development, and Tourism Ocean Thermal Energy http://hawaii.gov/dbedt/info/energy/renewable/ otec U.S. Department of Energy Ocean Thermal Energy Conversion http://www.energysavers.gov/renewable_energy/ ocean/index.cfm/mytopic=50010 See also: Electrical power; Ocean current energy; Ocean wave energy; Oceans; Tidal energy. Ocean wave energy Category: Energy resources A number of designs for harnessing wave energy have been proposed, and some are in use on various scales, but the vast potential of this power source has not been tapped because of the uncertainties and expense in - volved. 846 • Ocean wave energy Global Resources Background Waves crashing against a beach are a vast, almost mys- tical, display of mechanical power. For centuries peo- ple have sought ways of tapping it. In 1799, a father and son named Girard applied to the French govern- ment for a patent on awave-powerdevice.They noted that waves easily lifted even mighty ships. Hence, a le- ver from a ship to shore could power all manner of mills. (There are records of Girard mills on rivers, but the wave machine was probably never built.) Hun- dreds of patents later, wave power is still largely a dream,althougha dream thatis approaching reality. The Nature of Waves Waves are the product of wind blowing on the ocean surface. The energy available comes from the wind speed and the distance (or “fetch”) that the wind blows: A breeze blowing on a small bay produces rip- ples, whereas a hurricane blowing across several hun- dred meters builds hill-sized waves. Waves hitting a beach canbethe resultofa storm on theopposite side of an ocean. From that standpoint, waves are a collect- ing and concentrating mechanism for wind power. However, there is some loss of wave energy over great distances, so the best places to take advantage of wave potential are along high-wind coasts of the temperate and subpolar latitudes. Specific regions of the world with strong wave actions include the western coasts of Scotland, northern Canada, southern Africa, Austra- lia, andthe northwestern coastsof theUnitedStates. Water waves mostly consist of a circular motion of the water molecules as the wave energy continues until it meets a barrier, such as a shoreline. Then the energy hurls water and pieces of the shore until grav- ity pulls them back. Ultimately, the energy is trans- formed into heat, hardly noticed in the water. Along the way, the energy is vast. The North Pacific is esti- mated to have a flux of 5 to 50 megawatts of mechani- cal energy per kilometer. One limitation of wave energy is that timing and power are variable (although not as much as with winds). The crests and troughs of one storm may be out of phase with another, in which case they largely cancel each other out. Winds may be low, or they may be directly against waves approaching the power plant. Any of these factors can limit power production at unpredictable times. Conversely, waves from two or more storms maybein phase andstack,creating mon - ster waves that have been observed as high as 34 me - ters in the open ocean. Extraordinary waves have been the destruction of countless ships and of more than one wave power station. They are probably the greatest obstacle to widespread use of wave energy. Methods for Harnessing Ocean Waves Electrical power generation from waves requires three things: mooring the power stations to the ocean floor or building along the coast, generating power, and transmitting the power to customers inland. As with wind energy, a useful fourth item would be storage to deal with low-wave days. Building and power transmission are straightfor- ward operations, because most wave-harvesting de- signs are on or nearshore. Even thoughtheseinstalla- tions must be reinforced against especially strong waves, they do not have the cost and complexity of deepwater structures. Proposed energy-harvesting techniques have great variation because many researchers have been at- tempting to harness wave potential. The researchers face three major problems. First, generators face the previously mentionedfluctuations in awesome power. Second, wave power is large but moves at a slow pace, and the machinery to obtain high speed (needed for an electric generator) is expensive. Third, complex hinges, pistons, and other moving parts need fre- quent replacement in the salty ocean environment. The simplest approach is a ramp and dam facility that traps water splashing above sea level. Draining water goes down a pipe (penstock) to turbines, just as in a hydroelectric dam. The “Russelrectifier” is sort of a dam with chambers and flaps so that both rising and falling waves cause water in a turbine to flow continu- ously in the same direction. The various dam schemes are familiar and can be built on land. The disadvan- tage is that power dams must be large and capable of surviving the surf; thus they are expensive. The “dam atoll” is an open-ocean variant of the ramp. A half-submerged dome in the ocean bends waves around it so that waves come in from all sides, just as with coral atolls. The water sloshes to a central drain at the top and drains back through a penstock. The central collection increases efficiency, and being a floating structure allows submerging below the waves during major storms. However, increased dis- tance fromshoreincreasespower transmissioncosts. Air pressure can translate slow wave motion into a fast spin.In manyschemes,waves riseand falleither in a series of open rooms at the bottom of a floating structure or in cylinders at the end of funnel-shaped Global Resources Ocean wave energy • 847 . type of procedure is not sufficient: Tens of thousands of years are necessary to reduce the radioactivity in spent fuel to a nonthreat - ening level. 838 • Nuclear waste and its disposal Global Resources High-level. manyschemes,waves riseand falleither in a series of open rooms at the bottom of a floating structure or in cylinders at the end of funnel-shaped Global Resources Ocean wave energy • 847 . poses a special set of problems. Some of this waste is either the same sort of high-level or low-level waste as that generated by other nuclear industries or a substantial amount of transuranic material.