THE FUTURE OF NUCLEAR ENERGY



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Eighth Annual Symposium on Frontiers of Engineering THE FUTURE OF NUCLEAR ENERGY

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Eighth Annual Symposium on Frontiers of Engineering Advanced Nuclear Reactor Technologies JOHN F. KOTEK Argonne National Laboratory-West Idaho Falls, Idaho For more than a decade, when energy experts considered which technologies would be used to meet future U.S. energy needs, nuclear power was largely ignored. This has changed in the last five years, as improved performance at existing plants has shown that well run nuclear plants can be a very low-cost source of baseload electrical generation. Increasing concerns about the effects of human activity on climate, coupled with a growing desire to ensure the security of U.S. energy supplies, have led to a renewed interest in nuclear power. Other countries, including Japan, China, South Korea, Russia, and Finland are pressing ahead with plans to add new nuclear generating capacity. However, before new nuclear plants can gain a foothold in the U.S. market, the economics of constructing new plants must be improved. In addition, the standing of nuclear energy in public consciousness would be elevated if new plants offered improved nuclear-waste management strategies and were demonstrably safer than existing plants. Finally, the expansion of nuclear energy to developing countries would be greatly facilitated if stronger intrinsic barriers to proliferation were built into new nuclear energy systems. HOW NUCLEAR REACTORS WORK A nuclear reactor produces electricity by harnessing the energy released during the splitting, or fission, of a heavy isotope, such as uranium-235 or plutonium-239. Fission can be induced when the nucleus of one of these isotopes absorbs a free neutron. When the isotope fissions, it generally splits into two smaller isotopes (referred to as fission products) and releases two or three neutrons and about 200 MeV of energy, about 20 million times the energy released

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Eighth Annual Symposium on Frontiers of Engineering in a typical chemical reaction. The released neutrons can go on to fission another uranium or plutonium atom or can be captured in or escape from the reactor core. Power reactors are designed so the fraction of captured or escaped neutrons increases as the core temperature increases; the rate of fission reactions adjusts to maintain a nearly constant temperature, leading to a stable, self-sustaining chain reaction. The core of a typical water-cooled reactor contains the fuel, usually uranium oxide pellets sealed in zirconium alloy cladding tubes. About 290 of these tubes are contained in a fuel assembly, and a typical core of a light-water reactor contains about 200 of these 3.5 m-long assemblies. The uranium or plutonium in the fuel is allowed to fission, and the energy released during fission is used to heat water. The heated coolant is then used, either directly or after one or more heat-transfer steps, to generate steam to drive a turbine, which generates electricity. A typical fuel assembly contains about 500 kg of uranium; if electricity sells for around 3 cents per kilowatt-hour, about $6 million of electricity would be generated over the life of the fuel assembly. A SECOND LOOK AT NUCLEAR POWER Several attractive features of nuclear power have aroused renewed interest in the United States. First, of course, nuclear fuel is very inexpensive compared to coal or natural gas. The nuclear fuel needed to power a 1,000 MWe plant costs about $40 million per year; the fuel for a similar-sized coal plant costs about $110 million and for a natural gas plant about $220 million (Reliant Energy, 2001). The low fuel cost more than offsets the higher operations and maintenance costs of nuclear plants. The average operations, maintenance, and fuel cost in 1999 for the 103 U.S. nuclear power plants was 1.83 cents per kilowatt-hour, lower than for coal plants (2.07 cents) and natural gas-fired plants (3.52 cents) (Utility Data Institute, 2001). Another attractive feature is that nuclear plants do not release air pollutants or carbon dioxide. Today nuclear plants provide 20 percent of U.S. electrical generation without burning fossil fuels or causing air pollution or an increase in greenhouse gases. These and other benefits, such as a small footprint per unit energy and a secure fuel supply, have put nuclear power back on the drawing board. To get beyond the drawing board, however, new nuclear power plants must overcome several hurdles. The most challenging hurdle is the high capital cost of nuclear power plant construction. According to a 2001 report by the U.S. Department of Energy, overnight capital costs for a project must be contained at $1,500 per kWe or less (DOE, 2001). Capital costs for nuclear power plants completed in the 1980s and 1990s were in many cases several times higher, although a large fraction of the costs was interest that accumulated during construction delays. By comparison, capital costs for combined-cycle natural gas plants are currently about $500 per kWe.

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Eighth Annual Symposium on Frontiers of Engineering Another hurdle, probably less important to the future deployment of new plants in the United States, but still significant, is the very small, but non-zero potential, for serious accidents. A third hurdle is the need for a repository to store used nuclear fuel. Finally, before nuclear power can be deployed on a wide scale, it may be necessary to reduce the potential for proliferation from civilian nuclear fuel cycles and to find better ways of managing used nuclear fuel. NEAR-TERM PROSPECTS No new nuclear power plants have been ordered in the United States since the 1970s, and no new plants have come on line since 1996. Despite this hiatus, work has continued on the development of more economical and safer reactor designs. Recent interest in new plants has been focused on two classes of new designs—water-cooled reactors and gas-cooled modular reactors. Because of extensive experience in the construction and operation of water-cooled reactors and the relative maturity of the technology, advanced water reactors are, in my view, the most likely to be constructed in the United States in the near term. One promising design is the Westinghouse AP1000, an evolution of the AP600 pressurized light-water reactor plant that received U.S. Nuclear Regulatory Commission (NRC) design certification in 1999. The AP600 is a 600 MWe plant that incorporates several passive safety features (i.e., safety systems that work without requiring operator action), including passive safety injection, passive residual heat removal, and passive containment cooling. These passive systems are designed to improve safety and reduce the need for operator response in the event of an accident. On April 2, 2002, Westinghouse submitted an application to the NRC for design certification of the AP1000 (Westinghouse hopes design certification can be achieved by the end of 2004). Westinghouse reports that more than 90 percent of the design for the plant has already been completed and that more than 80 percent of the AP600 Safety Analysis Report will remain unchanged for the AP1000. The thermal efficiency of the plant is about 32 percent, similar to existing pressurized-water reactors (Matzie, 1999). The second class of reactors under consideration for near-term deployment is gas-cooled modular reactors. The two designs of this type that have elicited the most industry interest are the gas-turbine modular helium reactor (GT-MHR) under development by General Atomics and the pebble-bed modular reactor (PBMR), which is being designed by a team that includes British Nuclear Fuels Ltd., the parent company of Westinghouse. Instead of using the steam cycle to generate electricity, these reactors couple the reactor directly to a gas turbine. By using a direct Brayton cycle, efficiencies approaching 50 percent can be achieved. The units are small enough (300-600 MWt) to be mass produced in standardized units, which reduces capital costs while retaining safety character

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Eighth Annual Symposium on Frontiers of Engineering istics. The small reactor and power-conversion units can be housed below ground, reducing the risk of man-made and natural hazards. The fuel for a gas reactor uses tiny particles of uranium or plutonium oxide coated with carbon and silicon carbide. The particles create a barrier to the release of fission products and can withstand maximum attainable accident temperatures. The GT-MHR has a three-year operating fuel cycle—half of the fuel in the reactor core is replaced every 18 months while the reactor is shut down. By contrast, the PBMR has continuous refueling with the reactor in operation. Both designs use inert helium gas as a coolant. LONG-TERM PROSPECTS The U.S. Department of Energy is leading a 10-country effort to develop the next generation of nuclear energy systems. This program, known as Generation IV, will be guided by a technology roadmap being prepared by representatives of the 10 countries. The roadmap technical teams have evaluated many innovative concepts, including integral pressure-vessel water reactors and liquid metal-cooled fast-spectrum reactors.1 In integral pressure-vessel water reactors, the integral vessel houses reactor core and support structures, the core barrel, control-rod guides and drivelines, steam generators, a pressurizer, and reactor coolant pumps. This arrangement eliminates the need for separate steam generators, as well as a separate pressurizer, connecting pipes, and supports. Although the vessel is large (~18 m height and 4.4 m outside diameter), it is well within state-of-the-art fabrication capabilities. A 300-MWt reactor has 21 fuel assemblies inside a 2.6 m core barrel. Each assembly has about 440 pins in a square lattice. Integral-vessel reactors have lower power densities than light-water reactors, which allows for higher safety margins and longer core life, although they also have larger vessels and other structures. Projected system efficiencies are on the order of 36 percent. The second class of innovative concepts is liquid metal-cooled fast-spectrum reactors (“fast-spectrum” refers to the energy of the neutrons in the reactor core). In a typical reactor, a moderator (usually water, which pulls double-duty as both neutron moderator and reactor coolant) is used to slow down neutrons because slower neutrons are more efficient at causing fission in U-235. In a fast-spectrum reactor, there is no moderator. Instead, it relies on higher energy neutrons, which are less effective at causing uranium to fission but are more effective at causing fission in plutonium and other heavy elements. For this reason, these reactors are not ideal for a uranium-based fuel cycle; but they are quite suitable for use with a fuel cycle based on plutonium and the other heavy 1   An overview of the roadmap was released in September 2002, and can be found at http://nuclear.gov.

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Eighth Annual Symposium on Frontiers of Engineering elements that accumulate in spent fuel. Therefore, fast reactors could play a vital role in a long-term nuclear energy system that recycles used fuel to minimize long-lived waste. Most fast-spectrum reactors operated around the world use liquid sodium metal as a coolant. Future fast-spectrum reactors may use lead or a lead-bismuth alloy, or even helium, as a coolant. One of the attractive properties of metals as coolants is that they offer exceptional heat-transfer properties; in addition, some (but not all) metal coolants are much less corrosive than water. However, because sodium is reactive with air and water, fast-spectrum reactors built to date have a secondary sodium system to isolate the sodium coolant in the reactor from the water in the electricity-producing steam system. The need for a secondary system has raised capital costs for fast reactors and has limited thermal efficiencies to the range of 32 to 38 percent. Novel steam-generator designs, direct gas cycles, and different coolants are options that may eliminate the need for this secondary sodium loop and improve the economics of fast reactors (Lake et al., 2002). The Generation IV group is looking at complete nuclear energy systems, not just reactors, which are only one part of the nuclear fuel cycle (the path of uranium from the mine through fuel fabrication and use and disposal or recycling). Because of concerns about proliferation, recycling of spent nuclear fuel was banned in the United States in April 1977; spent fuel is recycled in France, Japan, the United Kingdom, Russia, and elsewhere. The U.S. strategy for managing used nuclear fuel is to isolate it from the environment in canisters placed in a deep geologic repository; Yucca Mountain in Nevada has just been approved for this purpose. In an attempt to keep options for the long term open, Generation IV is revisiting the issue of recycling used fuel. Nuclear fission produces a lot of heat, which can be used for more than making electricity. Several nuclear power plants around the world also provide heating for homes or for desalinating water. In the future, nuclear power plants may make a larger contribution toward meeting nonelectrical energy needs by supplying heat for industrial processes and by producing hydrogen. The advantages of hydrogen as an energy carrier have been widely publicized, but we must find ways of producing enough hydrogen to meet our needs. Currently, more than 95 percent of the hydrogen produced for refineries and chemical plants comes from the cracking of natural gas, a process that releases carbon dioxide. In the near term, nuclear-generated electricity could be used to drive electrolyzers that split hydrogen from oxygen, which would release almost no carbon dioxide. In the long term, higher temperature reactors could be used to produce heat to drive thermochemical water-cracking cycles, a process that could be nearly twice as efficient as electrolysis.

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Eighth Annual Symposium on Frontiers of Engineering CONCLUSION Existing nuclear power plants are a cost-effective and nonemitting contributor to the world energy system. Advanced nuclear energy systems can make a large contribution to meeting future energy needs, but the economics of these systems must be improved without compromising safety. In the long term, next-generation systems could contribute not only to our electricity needs, but also to our need for clean fuels for transportation and industry. The successful deployment of new nuclear energy systems could be a key part of a sustainable energy system. REFERENCES DOE (U.S. Department of Energy). 2001. A Roadmap to Deploy New Nuclear Power Plants in the United States by 2010. Washington, D.C.: U.S. Department of Energy. Lake, James A., R.G. Bennett, and J.F. Kotek. 2002. Next generation nuclear power. Scientific American 286(1): 72-79. Matzie, Regis. 1999. Westinghouse’s advanced boiling water reactor program. Nuclear Plant Journal Editorial Archive. Available online at: <http://npj.goinfo.com/NPJMain.nsf> (October 30, 2001). Reliant Energy. 2001. Reliant Energy HL&P’s Nuclear Plant Has Lowest Fuel Costs of All Power Plants in the U.S. Available online at: <http//www.reliantenergy.com/news/pressreleases/press_release_225.asp> (July 25, 2001). Utility Data Institute. 2001. Nuclear Energy Surpasses Coal-Fired Plants as Leader in Low-Cost Electricity Production. Available online at: <http://www.nei.org/doc.asp?catnum=4&cat-id=304> (January 9, 2001).

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Eighth Annual Symposium on Frontiers of Engineering Licensing and Building New Nuclear Infrastructure PETER S. HASTINGS Duke Energy Charlotte, North Carolina Electricity demand is outpacing supply growth, and experts have calculated that the United States will need new baseload power generation (including nuclear power generation) by 2010 (NEI, 2002a). As part of the Nuclear Power 2010 Initiative, the U.S. Department of Energy (DOE) established the Near-Term Deployment Group (NTDG) to examine prospects for new nuclear plants in the United States in the next decade. According to a recent study by NTDG, a resurgence of the nuclear industry will be influenced by many factors, including economic competitiveness; deregulation of the energy industry; regulatory efficiency; existing infrastructure; the national energy strategy; safety; management of spent fuel; public acceptance; and nonproliferation (DOE, 2001). NTDG also noted that the nuclear industry is experiencing current shortfalls in several important areas (DOE, 2001): Qualified and experienced personnel in nuclear energy operations, engineering, radiation protection, and other professional disciplines. Qualified suppliers of nuclear equipment and components [including] fabrication capability and capacity for forging large components such as reactor vessels. Contractor and architect/engineer organizations with personnel, skills, and experience in nuclear design, engineering, and construction. Nuclear industry infrastructure can be defined in terms of technologies, facilities, suppliers, and regulatory elements; design and operational engineering and licensing tools; and (perhaps most important) human capital to sustain the

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Eighth Annual Symposium on Frontiers of Engineering industry in the near and long terms. NTDG concluded that, although the industry has adequate industrial and human infrastructure today to build and operate a few new nuclear plants, we cannot be sure that this infrastructure can be expanded quickly enough to achieve the goal of 50 GWe in new nuclear plant installed capacity as laid out in the industry’s strategy for the future, Vision 2020 (NEI, 2002a). The nuclear industry faces infrastructure challenges, untested implementation of new Nuclear Regulatory Commission (NRC) regulations, and uncertainties about the economic competitiveness of new nuclear plants. The near-term deployment of new nuclear-power generation will be a good litmus test of the viability of a larger expansion in nuclear production. According to NTDG, even though the level of additional capacity in the industry goals in Vision 2020 is meager in terms of overall U.S. energy needs, achieving the goal presents major challenges (DOE, 2001). INDUSTRY AND GOVERNMENT INITIATIVES Various initiatives have been undertaken by the U.S. nuclear industry and DOE to encourage the domestic development of additional nuclear facilities. In 1998, DOE chartered the Nuclear Energy Research Advisory Committee (NER-AC) to advise the agency on nuclear research and development (R&D) issues. NERAC released the Long-Term Nuclear Technology R&D Plan in June 2000 (NERAC, 2000); NERAC is also responsible for overseeing the development of plans for both NTDG and Generation IV, a project to pursue the development and demonstration of one or more “next-generation” nuclear energy systems that offer advantages in economics, safety, reliability, and sustainability and that could be deployed commercially by 2030 (DOE, 2002). In 1999, DOE initiated the Nuclear Energy Research Initiative (NERI), an R&D program to address long-term issues related to nuclear energy; in 2000, the Nuclear Energy Plant Optimization Program was initiated to focus on the performance of currently operating nuclear plants. In 2000, NEI formed the industry-wide New Nuclear Power Plant Task Force to identify the market conditions and business structures necessary for the construction of new nuclear power plants in the United States. In April 2001, the task force published the Integrated Plan for New Nuclear Plants, which includes a discussion of nuclear infrastructure (NEI, 2001). More recently, NEI announced Vision 2020, an initiative with a goal of adding 50,000 MW of new nuclear generating capacity by 2020, along with increases in efficiency power uprates at existing plants equal to an additional 10,000 MW of generating capacity (NEI, 2002b). DOE funding has recently been allocated to advanced reactor development, specifically for the exploration of government/industry cost sharing for the demonstration of early site permitting as part of new NRC licensing processes (which

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Eighth Annual Symposium on Frontiers of Engineering also include provisions for combined licenses and design certifications) and for national laboratory activities associated with fuel testing, code verification and validation, and materials testing associated with new reactor designs (DOE, 2001). TECHNOLOGICAL INFRASTRUCTURE A number of elements are required to support a new or existing nuclear plant. Suppliers and fabricators of nuclear fuel and safety-related components are clearly essential, as are suppliers of balance-of-plant equipment, construction materials, electronics and instrumentation, and countless other components. The U.S. fuel-cycle industry has undergone significant changes in the past few years. Future fluctuations in uranium prices, the deployment of new enrichment technologies, significant consolidation of fuel-cycle supply companies, and the possible recycling of spent fuel could all affect the supply chain for nuclear fuel (i.e., mining/milling, conversion and enrichment, and fabrication into ceramic fuel pellets). The NTDG study recognized the sensitivity of new reactor deployment to these factors. NTDG solicited designs for nuclear plants that could be deployed by 2010 and attempted to identify generic issues that could impede their deployment. Proposals were received from reactor suppliers identifying eight candidate reactor designs. NTDG evaluated these designs to determine the prospects for deployment of a new nuclear plant in the United States by 2010. Candidate reactor technologies were required to demonstrate how they would operate “within credible fuel-cycle industrial structures” assuming a once-through fuel cycle using low-enriched uranium fuel and to “demonstrate the existence of, or a credible plan for, an industrial infrastructure to supply the fuel being proposed.” NTDG’s design-specific evaluations concluded that the candidates that would use existing fuel-cycle infrastructure could be built by 2010. However, NTDG also concluded that infrastructure expansion to achieve the industry’s goal of 50 GWe of new installed capacity by 2020 was a “generic gap” that warrants government and industry action (DOE, 2001). Siting for new reactors will be greatly influenced by how efficiently 10 CFR Part 52 (the NRC regulation for early site permits, standard design certifications, and combined nuclear plant licenses) can be implemented. NTDG concluded that the federal commitment to cost sharing via government/industry partnerships should include a demonstration of the NRC’s early site permit process for a range of likely scenarios. Recently a partnership to evaluate sites for new nuclear plants was announced, and DOE selected three utilities to participate in joint government/industry projects to pursue NRC approval for sites for new nuclear power plants. These projects, the first major elements of DOE’s Nuclear Power 2010 Initiative, are intended to “remove one more barrier to seeing the nuclear option fully revived” in the United States. All three companies intend to seek early site permit approvals that would enable them to locate new, advanced-

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Eighth Annual Symposium on Frontiers of Engineering DOE (U.S. Department of Energy). 2002. Yucca Mountain Site Suitability Evaluation. DOE/RW-0549. Washington, D.C.: U.S. Department of Energy. Also available online at: <http://www.ymp.gov/documents/sse_a/index.htm>. Kotek. J. 2003. Advanced Nuclear Reactor Technologies. Frontiers of Engineering: Reports on leading-Edge Engineering from the 2002 NAE Symposium on Frontiers of Engineering. Washington, D.C.: The National Academies Press. NRC (National Research Council). 1995. Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options. Washington, D.C.: National Academy Press. Also available online at: <http://www.nap.edu/catalog/4754.html>. NRC. 1996. Nuclear Wastes: Technologies for Separations and Transmutation. Washington, D.C.: National Academy Press. Also available online at: <http://www.nap.edu/catalog/4912.html>. OECD (Organization for Economic Cooperation and Development). 1999. Status and Assessment Report on Actinide and Fission Product Partitioning and Transmutation. Nuclear Development Report No. 1507. Paris, France: Nuclear Energy Agency of the Organization for Economic Cooperation and Development. Also available online at: <http://www.nea.fr/html/trw/docs/neastatus99/>. OECD. 2002. Accelerator-Driven Systems (ADS) and Fast Reactors (FR) in Advanced Nuclear Fuel Cycles. Nuclear Development Report No. 3109 (2002). Paris, France: Nuclear Energy Agency of the Organization for Economic Cooperation and Development. Also available online at: <http://www.nea.fr/html/ndd/reports/2002/nea3109.html>. Rubbia, C., J.A. Rubio, S. Buono, F. Carminati, N. Fieter, J. Galvez, C. Geles, Y. Kadi, R. Klapisch, P. Mandrillon, J.P. Revol, and C. Roche. 1995. Conceptual Design of a Fast Neutron Operated High Power Energy Amplifier. CERN-AT-95-44ET. Geneva, Switzerland: European Organization for Nuclear Research. Till, C.E., Y.I. Chang, and W.H. Hannum. 1997. The integral fast reactor: an overview. Progress in Nuclear Energy 31: 3−11. UNDP (United Nations Development Programme). 2000. World Energy Assessment: Energy and the Challenge of Sustainability. New York: United Nations Development Programme. Also available online at: <http://www.undp.org/seed/eap/activities/wea/>. WEC (World Energy Council). 2001. Survey of Energy Resources. Part I: Uranium. Available online at: <http://www.worldenergy.org/wec-geis/publications/reports/ser/uranium/uranium.asp>. WEC. 2002. Global Energy Scenarios to 2050 and Beyond. Available online at: <http://www.worldenergy.org/wec-geis/edc/scenario.asp>.

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Eighth Annual Symposium on Frontiers of Engineering Stretching the Boundaries of Nuclear Technology JAMES P. BLANCHARD Department of Engineering Physics University of Wisconsin-Madison Most people are well aware that nuclear power can be used to produce electricity, but few are aware that it can be used to provide power in many other situations. Radioisotopes have been used for decades in commercial applications, such as pacemakers and smoke detectors, and recent trends indicate that other applications are on the horizon. Two technologies being actively investigated are space nuclear power and nuclear energy for microelectromechanical systems (MEMS). SPACE NUCLEAR POWER In 1989, a national space policy was approved that included the goal of putting a man on Mars by 2019. By most accounts, meeting this goal will require nuclear propulsion in order to shorten the mission time, thereby reducing exposure to zero gravity conditions and cosmic rays. Hence, nuclear propulsion will play a major role in space travel beyond the moon. This year, NASA announced a five-year, $1-billion program to develop nuclear reactors to power the next generation of spacecraft. History Early work on nuclear propulsion was primarily focused on nuclear-thermal technologies, in which a fission reactor is used to heat a gas and accelerate it through a nozzle. Research activity in this area began in 1944 and peaked in the 1960s. A typical design uses hydrogen as a propellant and graphite-moderated carbide fuel in the reactor core. One design, called Phoebus, achieved 5,000

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Eighth Annual Symposium on Frontiers of Engineering MW of thermal power and 1 MN of thrust, which is about half the thrust of a space-shuttle engine (Bower et al., 2002). Another early concept for nuclear propulsion was Project Orion, which relied on a series of nuclear blasts behind the payload to create shock waves that accelerated the device (Schmidt et al., 2002). Although this technology looked promising, it was abandoned in the 1960s because of a ban on nuclear testing. Fuel Efficiency and Mission Length The efficiency of the fuel used for propulsion is measured by a parameter called the specific impulse, which is defined as the ratio of the thrust produced to the rate at which fuel is consumed. The units of this parameter, typically seconds, are determined by dividing force by weight of fuel consumed per unit time. Hence, a specific impulse of N seconds can be interpreted as a capability for providing a unit thrust with a unit weight of fuel for N seconds. By comparing the specific impulses for different propulsion technologies, one can assess their advantages and disadvantages. Increased fuel efficiency is manifested in several ways—shorter trips, larger payloads for a fixed total launch weight, and flexibility for scientific activities at the destination. The specific impulses for several propulsion options are shown in Table 1. The specific impulse for nuclear fuels can be many times that of chemical fuels, while the thrust is correspondingly lower and the run time longer. Electrostatic thrusters have relatively low thrust but can run virtually continuously and, therefore, can provide short trip times and low launch weights for a given payload. In TABLE 1 Propulsion Parameters for Several Propulsion Technologies Technology Specific Impulse (sec) Thrust per Engine (N) Run Time (duration) Chemical 150–450 0.5–5 million A few seconds to hundreds of minutes Nuclear Thermal 825–925 5,000–50,000 A few minutes to several hours Electromagnetic 2,000–5,000 10–200 A few seconds to several hundred hours Electrostatic 3,500–10,000 1–10 A few minutes to several days or months   Source: Niehoff and Hoffman, 1996. Reprinted with permission of the American Astronautical Society.

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Eighth Annual Symposium on Frontiers of Engineering contrast, chemical rockets tend to run at high thrust for short times, accelerating rapidly as the rocket fires and then coasting between necessary adjustments in trajectory. Nuclear-Electric Propulsion There are three basic types of electric propulsion systems: electrothermal, electrostatic, and electromagnetic. In electrothermal propulsion, the propellant is heated either by an electric arc or a resistance heater. The hot propellant is then exhausted through a conventional rocket nozzle to produce thrust. Electrostatic propulsion uses electric fields to accelerate charged particles through a nozzle. In electromagnetic propulsion, an ionized plasma is accelerated by magnetic fields. In all three types, electricity from a nuclear source, such as a fission reactor, is used to power the propulsion device (Allen et al., 2000; Bennett et al., 1994). The power flow for a typical nuclear-electric propulsion scheme is shown in Figure 1. The most mature of the electric propulsion concepts is electrostatic propulsion. NASA’s Deep Space 1 device (Figure 2), launched in 1998, relies on an ionized xenon gas jet for propulsion (Brophy, 2002). The xenon fuel fills a chamber ringed with magnets, which control the flow; electrons emitted from a FIGURE 1 Schematic drawing of power flow for a typical nuclear-electric propulsion device. Heat is produced in the nuclear source (typically a fission reactor core) and converted to electricity in the power converter. The electricity is then used to power the thruster. The radiator dissipates the waste heat. Source: NASA, 2001.

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Eighth Annual Symposium on Frontiers of Engineering FIGURE 2 Schematic drawing of the Deep Space 1 ion thruster. Source: NASA, 2002. cathode ionize the gas. The ions pass through a pair of metal grids at a potential of 1,280 volts and are thus accelerated out the back. A second electrode emits electrons to neutralize the charge on the device. The engine is capable of producing 90 mN of thrust while consuming 2,300 W of electrical power. This device is solar powered, but future designs anticipate using a fission reactor to produce the electricity. All electric propulsion systems require supplies of electricity, and fission reactors, which have high power density, are an excellent choice for meeting this need. Numerous projects are under way to develop fission reactors with low weight, high reliability, long life without refueling, and safety during launch. A wide variety of heat-transport and energy-conversion technologies are being investigated. One example of a fission reactor is the safe, affordable fission engine (SAFE-400), a 400-kW (thermal) reactor that is expected to produce 100 kW of electric power using heat pipes for energy transport and a Brayton cycle for energy conversion (Poston et al., 2002). The core consists of 381 uranium-nitride fuel pins clad with rhenium. The uranium-nitride fuel was chosen because of its high uranium density and high thermal conductivity. Molybdenum/ sodium heat pipes are used for heat transport to provide passive safety features in case of an accident.

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Eighth Annual Symposium on Frontiers of Engineering Plasma Propulsion An approach related to electric propulsion is the plasma rocket, exemplified by the variable specific impulse magnetoplasma rocket (VASIMR) (Diaz, 2000). Like an ion thruster, a VASIMR injects a propellant (usually hydrogen) into a cell and ionizes it. The resulting plasma is heated using radio-frequency injection and a magnetic nozzle that accelerates the gas to provide the propulsion. A second example is the gas dynamic mirror, a long, slender device in a magnetic mirror configuration. This device is powered by fusion reactions in the plasma; the thrust is produced by plasma ions exiting the end of the device. Accelerated by the mirror’s magnetic-field gradients, the ions provide efficient propulsion. One concept features a 50-m long, 7-cm radius plasma and produces 50,000 N of thrust at a specific impulse of more than 100,000 seconds (Kammash et al., 1995). SMALL-SCALE RADIOISOTOPE POWER MEMS have the potential to revolutionize many technologies, and the number of commercial applications is increasing rapidly. Many applications, such as pumps, motors, and actuators, can be improved with onboard power supplies, and various technologies are being explored to provide such power. Obvious choices, such as chemical batteries, fuel cells, and fossil fuels, show some promise, but none of them can match radioisotope power for long, unattended operation (Blanchard et al., 2001). This is because of the larger energy density available with nuclear sources. Radioisotopes can be used to produce power in a variety of ways. Thermo-electric and thermionic technologies convert the heat generated by the decay to electricity; other approaches make more direct use of the released energy. Thermoelectric conversion uses a thermal gradient between two different materials to create a current via the Seebeck effect. Thermionic conversion creates a current by boiling electrons off a cathode (at high temperature) and catching them at an anode. Techniques for more direct methods include simple collection of the emitted charged particles, ionization near a P-N or P-I-N junction in a semiconductor, and conversion of the decay energy to light and subsequent conversion to electricity in a photovoltaic. History Radioisotopes have been used as power sources for decades. Early pace-makers were powered by approximately 0.2 grams (3 Ci) of 238Pu, producing about 0.2 mW and delivering about 0.05 mW to the heart muscle (Parsonnet, 1972). Whereas pacemakers powered by chemical batteries have lives of less than 10 years and thus require replacement in most patients, the half-life of 238Pu

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Eighth Annual Symposium on Frontiers of Engineering (approximately 86 years) permits radioisotope-powered devices to last the life of the patient. Although a smoke detector is not strictly a power source, many smoke detectors contain radioisotopes (usually 1 to 5 microcuries of 241Am). The source ionizes air between a pair of parallel plates, and a chemical battery (or house current) is used to collect these charges and thus measure the degree of ionization in the gap. When smoke enters the gap, the increased ionization trips the sensor. Radioisotope thermoelectric generators (RTGs) are used in many applications, including underwater power and lighting in remote locations, such as the Arctic (Lange and Mastal, 1994). RTGs were also used to provide power for the Cassini and Voyager missions. Much like the pacemakers mentioned above, RTGs create power by thermoelectric conversion. Most RTGs are modular, with each module containing approximately 2.7 kg of Pu (133 kCi) and measuring approximately 42 cm in length and 114 cm in diameter. The modules produce 276 W of electric power at the beginning of life and, despite decay of the isotope, will produce approximately 216 W after 11 years of unattended operation. Current research is focused mostly on the miniaturization of RTGs for many applications, such as MEMS; in addition, efforts to improve the efficiency of existing RTGs are ongoing. Nuclear Microbatteries Thermal devices, such as RTGs, are difficult to reduce to the microscale because, as the size is decreased, the surface-to-volume ratio increases, thus increasing the relative heat losses and decreasing the efficiency of the device. Hence, microbattery designs have tended to focus on direct methods of energy conversion. For example, one can construct a diode from silicon using a layer of P-type silicon adjacent to a layer of N-type silicon and a radioactive source placed on the top of the device. As the source decays, the energetic particles penetrate the surface and create electron-hole pairs in the vicinity of the P-N junction. This creates a potential across the junction, thus forming a battery. Figure 3a is a schematic drawing of such a device, and Figure 3b is a photograph of one concept created at the University of Wisconsin. The device shown in Figure 3b is fairly large, measuring approximately 0.5 cm on each side, but one can easily imagine using a single pit from the device as a power source. This would provide a microbattery measuring approximately 400 microns by 400 microns by 50 microns; using a beta emitter (63Ni), it could produce approximately 0.2 µW of electrical power. An early prototype of the device pictured in Figure 3b, loaded with a weak source (64 microcuries of 63Ni), produced approximately 0.07 nW of power. Given that the thermal energy of 64 microcuries of 63Ni is 6.4 nW, this device is about 1 percent efficient. Placing a second diode on top of the source would nearly double the efficiency (because the decay

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Eighth Annual Symposium on Frontiers of Engineering FIGURE 3(a) A schematic drawing of a simple device using a P-N junction and radioactive source to produce the potential. (b) Photograph of a device using pits rather than trenches to hold the source. products are produced isotropically). Work is also under way to improve the efficiency by optimizing the design. When an alpha source is used in such a battery, the available energy for a fixed activity level is increased by several orders of magnitude. Unfortunately, the high-energy alpha particles damage the silicon lattice as they pass through and quickly degrade the power. Attempts are being made to overcome this limitation by using materials that are resistant to damage. Materials being considered include wide-band-gap semiconductors, such as gallium nitride, which might improve radiation stability and device efficiency (Bower et al., 2002).

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Eighth Annual Symposium on Frontiers of Engineering Thermoelectric devices are another approach to using alpha sources. These devices use the heat from the source to produce a temperature gradient across the thermoelectric device to produce power. Thus, there is no risk of radiation damage, and alpha particles could be used. Hi-Z Technology, Inc. (San Diego), developed a 40 mW device using a radioisotope heater unit (RHU) that was produced by NASA several years ago. The RHU uses about 2 grams of 238Pu to produce 1 W of thermal power. Using this as a heat source, Hi-Z produced a thermoelectric device that established a temperature difference of approximately 225°C throughout the device and provided 40 mW of power. The efficiency of the device was approximately 4 percent. Some improvement can be gained through improved insulation and thermoelectrics. A third approach to creating a micropower device uses radioisotopes to excite phosphors that emit photons, which can then be collected in a standard or modified solar cell. This protects the photovoltaic from damage but increases losses in the system. In addition, the phosphor may be damaged. Typical organic scintillators have energy-conversion efficiencies of 1 percent, whereas inorganic crystals can achieve efficiencies of up to 30 percent. TRACE Photonics, Inc. (Charleston, Illinois), has built a scintillation glass using sol-gel processes with high light-conversion efficiency under radiation exposure (Bower et al., 2002). Current overall efficiencies are approximately 1 percent, but device integration can probably be improved because of the low weight and direct conversion. Applications of Micropower Sources All current MEMS devices sold commercially are passive devices. Hence, there is no existing market for micropower sources. Nevertheless, one can envision many future applications of MEMS devices with onboard micropower sources, such as small drug dispensers placed directly into the bloodstream and laboratories-on-a-chip that can carry out real-time blood assays. Researchers at UCLA and UC Berkeley have been investigating so-called “smart-dust” concepts for using wireless communications to create large-scale sensor networks (Kahn et al., 1999). This approach involves distributed sensors that can communicate with each other through a network and thus “provide a new monitoring and control capability for transportation, manufacturing, health care, environmental monitoring, and safety and security” (Asada et al., 1998). These devices will require power for data collection and storage, as well as for the delivery of information between neighboring devices. A new application of nuclear power is the self-powered cantilever beam produced at the University of Wisconsin (Li et al., 2002). This device, shown in Figure 4, places a conducting cantilever beam in the vicinity of a radioisotope, in this case 63Ni. As the beam collects the electrons emitted from the source, it becomes negatively charged, and the source becomes positively charged. The

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Eighth Annual Symposium on Frontiers of Engineering FIGURE 4 Schematic drawing of a self-oscillating cantilever beam. Devices can be modeled as capacitors in parallel with leakage resistors, with most of the leakage resulting from ionization in the gap. beam is thus attracted to the source until contact is made and the device discharges. This causes the beam to be released and return to its original position. The process then repeats itself. Hence, the beam undergoes a repetitive bending and unbending; the period of the oscillation is determined by the strength of the source, the beam stiffness, and the initial separation between the beam and the source. Work is ongoing to produce wireless communication devices based on this design. CONCLUSIONS Nuclear power is the best, perhaps the only, realistic power source for both long-distance space travel and long-lived, unattended operation of MEMS devices. Much more research will have to be done to optimize the currently available technologies for future applications, but nuclear technologies will clearly provide viable, economic solutions, and they should be given continued attention and support as they approach commercialization REFERENCES Allen, D.T., J. Bass, N. Elsner, S. Ghamaty, and C. Morris. 2000. Milliwatt Thermoelectric Generator for Space Applications. Pp. 1476-1481 in Proceedings of Space Technology and Applications International Forum-2000. New York. American Institute of Physics Press. Asada, G., T. Dong, F. Lin, G. Pottie, W. Kaiser, and H. Marcy. 1998. Wireless Integrated Network Sensors: Low Power Systems on a Chip. Pp. 9-16 in Proceedings of the 1998 European Solid State Circuits Conference. Paris: Seguir Atlantica. Bennett, G., H. Finger, T. Miller, W. Robbins, and M. Klein. 1994. Prelude to the Future: A Brief History of Nuclear Thermal Propulsion in the United States. Pp. 221-267 in A Critical Review of Space Nuclear Power and Propulsion, 1984–1993, edited by M. El-Genk. New York: American Institute of Physics Press. Blanchard, J., R.M. Bilboa y Leon, D.L. Henderson, and A. Lai. 2001. Radioisotope Power Sources for MEMS Devices. Pp. 87-88 in Proceedings of 2001 ANS Annual Meeting. Washington, D.C.: American Nuclear Society.

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Eighth Annual Symposium on Frontiers of Engineering Bower, K., X. Barbanel, Y. Shreter, and G. Bohnert. 2002. Polymers, Phosphors, and Voltaics for Radioisotope Microbatteries. Boca Raton, Fla.: CRC Press. Brophy, J. 2002. NASA’s Deep Space 1 ion engine. Revue of Scientific Instruments 73(2): 1071-1078. Diaz, F. 2000. The VASIMR rocket. Scientific American 283(5): 90-97. Kahn, J.M., R.H. Katz, and K.S.J. Pister. 1999. Mobile Networking for Smart Dust. Pp. 271-278 in Proceedings of the ACM/IEEE International Conference on Mobile Computing and Networking. New York: ACM Press. Kammash, T., M. Lee, and D. Galbraith. 1995. High-Performance Fusion Rocket for Manned Space Missions. Pp. 47-74 in Fusion Energy in Space Propulsion, edited by T. Kammash. Progress in Astrophysics and Aeronautics 167. Lange, R., and E. Mastal. 1994. A Tutorial Review of Radioisotope Power Systems. Pp. 1-20 in A Critical Review of Space Nuclear Power and Propulsion 1984–1993, edited by M. El-Genk. New York: American Institute of Physics Press. Li, H., A. Lal, J. Blanchard, and D. Henderson. 2002. Self-reciprocating radioisotope-powered cantilever. Journal of Applied Physics 92(2): 1122-1127. NASA (National Aeronautics and Space Administration). 2001. The Safe Affordable Fission Engine (SAFE) Test Series. Available online at: <http://www.spacetransportation.com/ast/presentations/7b_vandy.pdf>. NASA. 2002. DS1: How the Ion Engine Works. Available online at: <http://www.grc.nasa.gov/WWW/PAO/html/ipsworks.htm>. Niehoff, J., and S. Hoffman. 1996. Pathways to Mars: An Overview of Flight Profiles and Staging Options for Mars Missions. Pp. 99-125 in Strategies for Mars: A Guide for Human Exploration, edited by C.R. Stoker and C. Emmart. Paper no. AAS 95-478. Science and Technology Series Vol. 86. San Diego, Calif.: Univelt. (Copyright © 1996 by American Astronautical Society Publications Office, P.O. Box 28130, San Diego, CA 92198; Website: <http://www.univelt.com>. All Rights Reserved. This material reprinted with permission of the AAS.) Parsonnet, V. 1972. Power sources for implantable cardiac pacemakers. Chest 61: 165-173. Poston, D., R. Kapernick, and R. Guffee. 2002. Design and Analysis of the SAFE-400 Space Fission Reactor. Pp. 578-588 in Space Technology and Applications International Forum. New York: American Institute of Physics Press. Schmidt, G., J. Bonometti, and C. Irvine. 2002. Project Orion and future prospects for nuclear propulsion. Journal of Propulsion Power 18(3): 497-504.