In This Issue
Expanding Frontiers of Engineering
December 1, 2002 Volume 32 Issue 4

Stretching the Boundaries of Nuclear Technology

Wednesday, December 3, 2008

Author: James P. Blanchard

Radioisotopes are being used in numerous commercial applications, and many more are on the horizon.

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 person 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 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 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 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.


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. Thermoelectric 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 pacemakers 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 (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 mW 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 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).


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 1W 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 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
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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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Diaz, F. 2000. The VASIMR rocket. Scientific American 283(5): 90-97.
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Li, H., A. Lal, J. Blanchard, and D. Henderson. 2002. Self-reciprocating radioisotope-powered cantilever. Journal of Applied Physics 92(2): 1122-1127.
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TABLES

TABLE 1 Propulsion Parameters for Several Propulsion Technologies

TechnologySpecific Impulse (sec)Thrust per Engine (N)Run Time (duration)
Chemical150-4500.5-5 millionA few seconds to hundreds of minutes.
Nuclear thermal825-9255,000-50,000A few minutes to several hours.
Electromagnetic2,000-5,00010-200A few seconds to several hundred hours
Electrostatic3,500-10,0001-10A few minutes to several days or months
About the Author:James P. Blanchard is a professor in the Department of Engineering Physics, University of Wisconsin-Madison.