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Review of recent advances of radioisotope power systems

Review of recent advances of radioisotope power systems Radioisotope power systems have demonstrated numerous advantages over other types of power supplies for long-lived, unattended applications in space and in remote terrestrial locations. Many especially challenging power applications can be satisfied by proper selec- tion, design, and integration of the radioisotope heat source and the power conversion technologies that are now available or that can be developed. This paper provides a brief review of the factors influencing selection of radioisotopes and design of power systems, and dis- cusses the current state of practice and future programmatic and technical challenges to continued use of radioisotope power systems in space. 2007 Published by Elsevier Ltd. Keywords: Radioisotope power systems; Radioisotope thermoelectric generator (RTG); Plutonium-238; Space power systems; General purpose heat source (GPHS) 1. Introduction has launched 45 radioisotope thermoelectric generators (RTGs) on 26 spacecraft for various NASA and Depart- Radioisotope power systems are nuclear power systems ment of Defense missions in high and low Earth orbit, on that derive their energy from the spontaneous decay of the surfaces of the moon and Mars, and fly-bys to and radionuclides, as distinguished from nuclear fission energy beyond the outer planets (see Table 1). created in reactor power systems. The two major compo- nents of any radioisotope power system, or generator, are 2. Radioisotope fuels a radioisotope heat source and an energy conversion sys- tem. Heat is produced during the decay process within Selection of a suitable radioisotope, commonly referred the heat source. This heat is partially transformed into elec- to as fuel, for use in space radioisotope power systems is tricity and the waste heat is transferred to space or the envi- the key to their acceptance and use. The characteristics of ronment surrounding the generator. Such power systems an acceptable fuel include a long half-life, low radiation are rugged, compact and highly reliable, and can be safely emissions, high power density and specific power and a sta- produced and used with minimal risk to operating person- ble fuel form with a high melting point. The fuel must be nel, the general public, and the Earth’s environment. safely producible in useful quantities and at a reasonable Since before the first Sputnik was launched into space, cost and must be capable of being used safely in all normal the Department of Energy (DOE) and its predecessor agen- and potential accident environments. cies have been developing the technology to fabricate and The size and weight of a heat source are directly related deliver radioisotope power systems for use on US military to the half-life of the fuel. If the half-life is too long, the and civilian space missions. Since 1961, the United States radioactive decay rate is slower and associated heat pro- duction rate is low. This results in a fuel loading that is too large and too heavy for space missions. If the half-life is too short, a great deal of heat may be produced initially, Corresponding author. Tel.: +1 301 903 3456; fax: +1 301 903 1510. E-mail address: Robert.Lange@nuclear.energy.gov (R.G. Lange). but the heat production rate will decay quickly. Because of 0196-8904/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.enconman.2007.10.028 394 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 Table 1 US spacecraft using nuclear power systems Power source # RTGs Spacecraft Mission type Launch Status SNAP-3 1 Transit 4A Navigational 1961 Currently in orbit SNAP-3 1 Transit 4B Navigational 1961 Currently in orbit SNAP-9A 1 Transit 5BN-1 Navigational 1963 Currently in orbit SNAP-9A 1 Transit 5BN-2 Navigational 1963 Currently in orbit SNAP-9A 1 Transit 5BN-3 Navigational 1964 Mission aborted; heat source burned up as designed SNAP-10A Reactor Snapshot Experimental 1965 Achieved orbit; was shut down after 43 days SNAP-19 2 Nimbus B-1 Meteorological 1968 Mission aborted; heat sources retrieved SNAP-19 2 Nimbus III Meteorological 1969 Currently in orbit ALRHU Heater Apollo 11 Lunar 1969 On lunar surface SNAP-27 1 Apollo 12 Lunar/ALSEP 1969 On lunar surface; station shut down SNAP-27 1 Apollo 13 Lunar/ALSEP 1970 Mission aborted; heat source jettisoned into ocean SNAP-27 1 Apollo 14 Lunar/ALSEP 1971 On lunar surface; station shut down SNAP-27 1 Apollo 15 Lunar/ALSEP 1971 On lunar surface; station shut down SNAP-19 4 Pioneer 10 Planetary 3/2/72 Operated to Jupiter and beyond solar system SNAP-27 1 Apollo 16 Lunar/ALSEP 1972 On lunar surface; station shut down Transit-RTG 1 Triad-01-1X Navigational 1972 Currently in orbit SNAP-27 1 Apollo 17 Lunar/ALSEP 1972 On lunar surface; station shut down SNAP-19 4 Pioneer 11 Planetary 1973 Operated to Jupiter, Saturn, beyond solar system SNAP-19 2 ea. Viking 1,2 Mars landers 1975 On Martian surface; landers shut down MHW–RTG 2 ea. LES 8, LES 9 Communication 1976 Currently in earth orbit MHW–RTG 3 ea. Voyager 1,2 Planetary 1977 Still operating; extended interstellar mission GPHS–RTG 2 Galileo Planetary 1987 Deliberately crashed into Jupiter at end-of-mission GPHS–RTG 1 Ulysses Planetary 1990 Successful ongoing mission to the Sun’s polar regions LWRHU Heaters Mars pathfinder Mars rover 1996 Mission successfully completed GPHS–RTG 3 Cassini Planetary 1997 Successful ongoing mission to Saturn and its moons LWRHU Heaters MER Mars rovers (2) 2003 Extended mission ongoing GPHS–RTG 1 New horizons Planetary 2006 Successfully operating en route to Pluto this, excess fuel must be added to maintain the amount of reactor or a very high-powered accelerator facility. Both of heat required at the end-of-mission (EOM). The half-life of these approaches require major investments in nuclear the radioisotope fuel should be at least as long as or longer facilities capable of processing highly radioactive spent than the mission lifetime to reduce the heat variation over reactor fuel or irradiated targets. Chemical processing tech- the mission. nology to produce the proper fuel compound with the nec- The levels of penetrating radiation (gamma, X-ray, and essary purity must also be available, along with fabrication neutron) emissions must be inherently low for any radio- processes and facilities to produce the final fuel form. These radioisotope fuel facilities must be operated under isotope fuel used in space applications. This will reduce the burden required to protect workers and the spacecraft the strictest safety and environmental standards and take from the potential damaging effects of radiation. This is into account the ultimate disposal of any radioactive also important for protection of the public and the environ- wastes generated. ment in the event of a launch accident. The radioisotope When it is determined that a proposed fuel form can be fuel must also be useable in a form with a high melting produced in adequate quantities and with the desired point that remains stable during postulated launch acci- nuclear characteristics, the production quality fuel form, dents. The fuel form must be chemically compatible with along with its encapsulation and other heat source compo- its containment material over the operating life of the heat nents, must be extensively tested to qualify it for space use source. It is also highly desirable that the fuel form have a and to support the necessary launch safety reviews and low solubility rate in the human body and in the natural approvals. Development and qualification of a new heat environment. Radioactive decay products must not source for flight use requires a large effort in terms of costs adversely affect the integrity of the fuel form, and the decay and schedule. In addition, there are only a limited number process should not degrade its properties. of radioisotope fuels that meet the requirements of half- Any radioisotope fuel selected for space power applica- life, radiation, power density, fuel form, and availability tions must be producible in sufficient quantities and on a for use in space power system applications. schedule to meet mission requirements. There are only two methods for obtaining radioisotopes in the quantities 3. Historical fuels perspective needed for space power applications. The first involves pro- cessing spent fuel from a nuclear reactor to isolate by-prod- Development of radioisotope power systems began in ucts of interest. The other is the deliberate production of the early 1950s and since then a variety of radioisotopes radioisotopes by irradiation of target materials in a nuclear has been evaluated for space and terrestrial applications. R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 395 The isotope initially selected for development was to Pu-238. However, this source has failed to materialize. cerium-144 (Ce-144) because it was one of the most plenti- The short-lived isotope curium-242 (Cm-242) was also ful fission products available from reprocessing defense initially selected to fuel an isotope power system for the nuclear reactor fuel. Its short half-life (290 days) made 90-day Surveyor mission to the Moon, but the Surveyor Ce-144 compatible with the 6-month military reconnais- program later decided not to use isotope systems. None of sance satellite mission envisioned as its primary application these alternative fuels has been used in space by the United at that time. The cerium oxide fuel form and its heavy fuel States. capsule met all safety tests for intact containment of the In the final analysis, Pu-238 is clearly superior to other fuel during potential launch abort fires, explosions, and ter- radioisotope fuels for use in long-duration space missions minal impacts. However, the high radiation field associated especially for deep space exploration. The technology for with the beta/gamma emission of Ce-144 caused handling producing and processing Pu-238 fuel forms has been and payload interaction problems as well as safety prob- clearly demonstrated over more than 40 years. Pu-238 lems upon random reentry from orbit. The Ce-144 fueled fueled heat sources have been through rigorous flight qual- SNAP-1 (SNAP stands for Systems for Nuclear Auxiliary ification testing and have performed safely and reliably in Power) power system was never used in space. all of the radioisotope power systems employed in the US By the late 1950s, large amounts of polonium-210 space program to date. The availability of Pu-238 fuel for (Po-210) became available, also as a byproduct of the future space missions is a continuing concern. For nearly nuclear weapons program. Po-210 is an alpha emitter with 30 years the production and processing of Pu-238 fuel a very high power density and low radiation emissions. Po- was accomplished as a byproduct of the production of 210 metal was used to fuel the small SNAP-3 technology materials for nuclear weapons. Changes in the Nation’s demonstration RTG that was first displayed at the White nuclear weapons program eliminated this traditional capa- House in January 1959. Several SNAP-3 RTGs were fueled bility to produce Pu-238 in 1986. Therefore, for Pu-238 fuel with Po-210 and used in various exhibits. However, the to continue to be available for use in the space program, a short half-life of Po-210 (138 days) makes it suitable for reliable source or sources must be established. Since 1996, only limited duration space power applications and no Pu-238 has been purchased from Russia for use in US SNAP-3 RTGs were deployed in space. space missions. Alternative domestic production capabili- In order to provide a longer-lived radioisotope fuel, ties for Pu-238 are currently being investigated by DOE. strontium-90 (Sr-90), an abundant fission product with a 28.6-year half-life, was recovered from defense wastes at 4. General purpose heat source (GPHS) Hanford. A very stable and insoluble fuel form, strontium titanate, was developed and widely used in terrestrial Once the appropriate isotope is selected for a space mis- power systems. Sr-90 and its daughter Yttrium-90 give off sion, it must be combined with other components to create significant radiation that requires heavy shielding, but a heat source. The heat source must efficiently and reliably shield weights are not as critical in most terrestrial power transfer the isotope heat to the conversion system while systems as for space power systems. withstanding routine mission environments and postulated By 1960, plutonium-238 (Pu-238) had been identified as accident scenarios. Numerous heat source designs have an attractive radioisotope fuel that could be made by irra- been used for terrestrial and space applications. However, diating neptunium-237 (Np-237) targets in the defense pro- since the launch of the Galileo spacecraft in 1989, all duction reactors. Its availability was extremely limited due NASA missions powered by radioisotopes have used some to shortage of Np-237 target material that must be recov- version of the general purpose heat source (GPHS). The ered from recycling high burnup, enriched uranium fuel. GPHS module has undergone an extensive safety analysis However, Pu-238 has all of the necessary nuclear character- and test program and was the only radioisotope heat istics for a space power system fuel: long half-life (87.74 source to be qualified for and used in a launch on the Space years), low radiation emissions, high power density, and Shuttle. All current-generation radioisotope power systems useful fuel forms (metal or oxide). After flight qualification designed for use on US space missions incorporate the of its heat source, a Pu-238 fueled SNAP-3 RTG was GPHS. launched on the Transit 4A Navy navigation satellite in The GPHS module is shown in Fig. 1. Each module is June 1961 – the first use of nuclear power in space. designed to deliver up to 250 thermal watts (W )atBOM Because of the limited availability of Pu-238, several and has a mass of 1.43 kg (3.16 lb). The module size and other radioisotopes have been thoroughly evaluated for shape were selected to survive orbital reentry through the space use. Sr-90 and Po-210 fuels were considered for use atmosphere and impact the Earth at a modest terminal in higher powered military satellite constellations for which velocity of 50 m/s (164 ft/s). Each module is a cuboid with there were insufficient quantities of Pu-238 available, but the overall dimensions of 9.72 cm  9.32 cm  5.31 cm these projects were all cancelled before launch. Curium- (3.83 in.  3.67 in.  2.09 in.). 244 (Cm-244) was expected to become available in signifi- Each GPHS module contains four pressed and sintered cant quantities from the US breeder reactor fuel cycle and Pu-238 oxide fuel pellets, each with nominal output of was investigated as a potential alternative long-lived fuel 62.5 W . The cylindrical fuel pellet is 2.75 cm (1.08 in.) in t 396 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 Fig. 1. General purpose heat source (GPHS). diameter and 2.75 cm (1.08 in.) in length. Each fuel pellet is aeroshell between the two GISs. This modification, known individually encapsulated in a welded iridium alloy con- as a Step 1 GPHS module, was used on the New Horizons tainment shell or cladding with a minimum wall thickness mission to Pluto. Systems currently being designed will of 0.05 cm (0.02 in.). The iridium alloy is capable of resist- incorporate another improvement to the aeroshell, which ing oxidation in the post-impact environment while it also makes the broad ablation faces thicker. This variation of provides chemical compatibility with the fuel and graphite the GPHS is known as Step 2. The Step 2 GPHS module is components during high-temperature operation and postu- 9.96 cm  9.32 cm  5.82 cm (3.92 in.  3.67 in.  2.29 in.) lated accident conditions. The iridium fuel cladding is and weighs 1.60 kg (3.53 lbs.). The more robust GPHS aero- equipped with a frit vent that allows release of helium pro- shell provides additional protection for the fueled clads dur- duced by the decay of the Pu-238 without releasing pluto- ing potential launch pad accidents and higher-velocity nia particles. The combination of fuel pellet and cladding is reentries into the atmosphere. called a fueled clad. Two fueled clads are encased in a Graphite Impact Shell 5. Energy conversion systems (GIS) made of Fine Weave Pierced Fabric (FWPF) car- bon–carbon composite material. The GIS is designed to The radioisotope heat source delivers its heat to some limit the damage to the iridium clads during free-fall type of energy conversion system that converts part of impacts. Two of these GISs are inserted into an aeroshell the heat into useful electrical power. There are two general that is also made of FWPF graphite. A thermal insulation classes of energy conversion systems, static and dynamic. layer of Carbon Bonded Carbon Fiber graphite surrounds Static systems include thermoelectric, thermionic, and ther- each GIS to limit the peak temperature of the iridium clad- mophotovoltaic conversion devices that can convert heat ding during atmospheric reentry heating and to maintain to electricity directly with no moving parts. Dynamic sys- its ductility during the subsequent impact. The aeroshell tems involve heat engines with working fluids that trans- is designed to contain the two GISs under severe reentry form heat to mechanical energy that in turn is used to conditions and to provide additional impact protection generate electricity. Both rotating and reciprocating against hard surfaces at its terminal velocity. It also pro- engines may be used. Potential dynamic systems include vides protection for the fueled clads against overpressures Rankine, Brayton, and Stirling engines that operate on var- and fragment impacts during postulated missile explosion ious types of working fluids. events. The aeroshell serves as the primary structural mem- Efficiency is an important consideration in selecting an ber to maintain the integrity and position of a stack of energy conversion system because of its effect on the radio- GPHS modules within a power system during normal oper- isotope inventory and its impact on cost, availability, size ations including testing, transportation, and launch. and weight. System reliability is also important. Since mis- The original GPHS module (shown in Fig. 1) was used on sion success depends on having sufficient electrical power the Galileo, Ulysses, and Cassini missions. It has been over the life of the mission, the selection of an energy con- improved since then by adding an internal web in the version system must be consistent with mission power R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 397 levels and lifetimes. Graceful power degradation over the the cover gas. Helium gas buildup within the converter life of a mission is acceptable as long as it is within predict- must be controlled by using a separate container around able limits. Other characteristics important in selecting an the heat source or permeable seals in the generator design. energy conversion unit for a radioisotope space power sys- Gas management considerations in the generator housing tem are weight, size, ruggedness to withstand shock and design and the use of bulk insulation materials increase vibration loads, survivability in hostile particle and radia- the size and weight of the generator. However, this type tion environments, scalability in power levels, flexibility of RTG is equally useful for space vacuum or for planetary in integration with various types of spacecraft and launch atmospheric applications. vehicles, and versatility to operate in the vacuum of deep SiGe materials can be operated at hot junction temper- space or on planetary surfaces with or without solar energy atures up to 1000 C. Their sublimation rates and oxida- inputs. tion effects, even at these higher temperatures, can be controlled by use of sublimation barriers around the ele- 6. Radioisotope thermoelectric generators ments and an inert cover gas within the generator during ground operation. The advantage of SiGe RTGs is clearest Of the various static conversion technologies, thermo- in space vacuum applications. In this case, a pressure electric energy conversion has received the most interest, release device opens on reaching altitude to vent the cover both in development and use, for radioisotope power sys- gas to space. This allows the use of lightweight multifoil tems. Thermoelectric converters are highly reliable over thermal insulation and operation of the unicouples without extended operating lifetimes, compact, rugged, radiation a cover gas, eliminating significant mass when compared to resistant, easily adapted to various applications, and pro- telluride RTGs. duce no noise, vibration, or torque during operation. Ther- The efficiency of current RTGs using either PbTe/TAGS moelectric converters require no start-up devices to or SiGe thermoelectric elements can be improved by using operate. They start producing electrical power as soon as more efficient thermoelectric materials capable of operating the heat source is installed. Power output is easily regulated up to 1273 K. Since the 1990s, investigation of materials at design level by maintaining a matched resistive load on with low thermal conductivity has been a key aspect in the converter. A limiting feature of thermoelectric conver- the research and development of advanced thermoelectric sion is its relatively low conversion efficiency, typically less materials at the NASA Jet Propulsion Laboratory (JPL). than 10%. A comprehensive review of recent development in thermo- Thermoelectric materials, when operating over a tem- electric materials has been provided in a recent volume of perature gradient, produce a voltage called the Seebeck the Materials Research Bulletin [1]. Exciting results have voltage. When connected in series with a load, the inter- been obtained on thin-films, superlattices and quantum nally generated voltage causes an electron current to flow dot materials. These materials may, however, be more through the load producing useful power. Power is pro- appropriate for small-scale electronics applications where duced in a thermoelectric element (thermocouple) placed spot cooling or low levels of power generation may be between a heat source and a heat sink. Typically, thermo- required. Half-Heuslers intermetallic alloys have also couples are low voltage devices so a number of them must received increasing attention as potential high-temperature be connected in series to produce normal load voltages. thermoelectric materials. Thermocouples can be connected in a series-parallel Efforts at NASA JPL have primarily focused on skutter- arrangement to enhance reliability by minimizing the effect udites, zintl, and zinc antimonide materials. Filled and on total power due to failure of a single thermocouple. unfilled skutterudites have been intensively investigated The most widely used thermoelectric materials, in order mostly because of the possibility of independently manipu- of increasing temperature capability, are bismuth telluride lating the thermal and electronic transport properties. (BiTe); lead telluride (PbTe); tellurides of antimony, ger- While much development work remains before any of these manium, and silver (TAGS); lead tin telluride (PbSnTe); materials could be successfully integrated into a power gen- and silicon germanium (SiGe). All of these except BiTe erator either for space or terrestrial applications, recent have been used in RTGs that have been flown on space results clearly indicate that the figure of merit (ZT)  1 limit missions. Many more materials have been, and are still observed for most of the thermoelectric materials used to being, investigated in hopes of producing higher-efficiency, date in practical thermoelectric generators can be broken, lower weight power systems with stable performance over offering future prospects for higher efficiency generators. long operating lifetimes. The telluride materials are limited to a maximum hot 7. Dynamic conversion systems junction temperature of 550 C. Due to the deleterious effects of oxygen on these materials and their high vapor While RTGs have a long history of providing reliable pressures, the tellurides must be operated in a sealed gener- power for long-duration space missions, the higher conver- ator with an inert cover gas to retard sublimation and sion efficiencies of dynamic systems would allow for better vapor phase transport within the converter. Bulk-type, use of limited radioisotope fuel and would offer higher sys- fibrous thermal insulation is used due to the presence of tem power per unit mass. System efficiencies of 25% or 398 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 more are achievable, reducing the radioisotope inventory Stirling cycle systems differ from Rankine and Brayton required to less than one-third of what would be required systems in that they do not include turbines, pumps or for a comparable RTG. However, a great deal of testing compressors. The Stirling cycle uses a working gas that will be required in order to demonstrate a reliability level expands by absorption of heat on the hot-side and con- comparable to that of an RTG. Dynamic conversion radio- tracts by rejection of heat on the cold side causing rapidly isotope power systems have not yet been used in space. changing pressure cycles across a piston, forcing it to move The difference between static and dynamic systems is the in a reciprocating fashion. The most promising type of Stir- thermal-to-electric power conversion mechanism. For ling engine for radioisotope generator applications is the dynamic systems, the thermal energy in the working fluid free-piston Stirling engine, which requires no lubricating is partially transformed into mechanical work to drive an fluids and produces electricity by means of a linear alterna- alternator to produce electricity. The three different tor within a hermetically sealed engine housing. The piston dynamic conversion concepts that have received the most moves back and forth at a resonant frequency on a cushion attention for use in radioisotope power systems employ of working gas between it and the surrounding cylinder Rankine, Brayton or Stirling cycles. The Rankine cycle is wall. A permanent magnet is attached to the power piston based on a two-phase working fluid that requires special to produce electrical currents in surrounding alternator vapor–liquid boiler and condenser designs for use in the coils as it oscillates back and forth. Since the reciprocating microgravity of space. Development programs have con- motion of the piston would cause unbalanced vibration sidered Rankine systems using liquid metal and organic loads, Stirling engines for space applications are generally working fluids. Brayton and Stirling cycles use a single designed in pairs with dynamically opposed pistons to min- gas working fluid such as a helium–xenon mixture for imize the net load transmitted to the spacecraft. Brayton and pure helium for Stirling. The Stirling cycle provides higher conversion efficiencies In a Brayton or Rankine system, the gaseous working than the Rankine and Brayton cycles at the same cycle tem- fluid from the heat source heat exchanger turns a turbine peratures. System efficiencies of 30% or more are possible rotor that is mounted on a common shaft with the alterna- at operating temperatures achievable with isotope heat tor rotor and the compressor or liquid pump rotor. The sources and oxidation-resistant superalloy structural mate- remainder of the power conversion system includes the tur- rials. The Stirling cycle also retains its high performance bine housing, turbine nozzles, bearings, alternator coils, characteristics at lower power levels than the Brayton pump housing, and cooling ducts to form a compact energy and Rankine systems, an attractive feature for radioisotope conversion unit. To minimize size and weight, the rotor power systems. shaft turns at very high speeds and the alternator produces high-frequency alternating current. To start such a system, 8. Current state of technology electrical power from a battery would be used to spin up the rotor until the system’s temperature, pressure, and In the years since RTGs were first used, the fuel form mass flow rates permit it to be self-sustaining. A power and heat source technologies have been steadily improved control and conditioning system would also be required to operate at higher temperatures with increasingly larger to regulate operating speed and electrical output. fuel inventories to meet the aerospace nuclear safety goals The two general areas of concern with the use of a tur- consistent with the As Low As Reasonably Achievable bine-driven electrical power system are reliability and (ALARA) approach. As the power levels of the RTGs have spacecraft interactions. The main reliability issues have to increased, improved thermoelectric materials and thermal do with bearings, loss of working fluid and failure of elec- insulation approaches have been developed to increase tronic parts in the control system. Foil bearings are used in long-term power stability and specific power of the RTGs Brayton designs to allow the rotating shaft to ride on a thin to meet the needs of more ambitious space exploration mis- film of working fluid gas during operation. Organic Ran- sions. Advanced technology programs have also been pur- kine units can use either foil bearings or hydrodynamic sued to take advantage of the greater efficiency offered by thrust and journal bearings lubricated by the organic work- dynamic power conversion systems. The remainder of this ing fluid. Both have been shown to be highly stable and section describes the three major radioisotope power sys- reliable once operating conditions are achieved and bearing tems currently being used on active NASA missions or temperatures are controlled. Since a puncture by a micro- being developed for potential near-term use. meteoroid or a crack caused by thermal or mechanical stresses during operation could cause the loss of the work- 9. General purpose heat source-radioisotope thermoelectric ing fluid and total loss of power, this single-point failure generator (GPHS–RTG) mechanism must receive particular attention during the design. Conservative wall thicknesses, expansion joints, The current state-of-the-art in space RTGs is represented and meteoroid armor are usually employed to reduce the by the GPHS–RTG, so named because it was the first to probability of failures of this kind. Totally redundant employ the GPHS modules. The GPHS–RTG, shown in power conversion loops can also be used if the additional Fig. 2, is the largest Pu-238 fueled, long-lived RTG built weight can be tolerated. for use in space missions. Utilizing recently precipitated R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 399 Fig. 2. General purpose heat source radioisotope thermoelectric generator (GPHS-RTG). Pu-238, it produces at least 285 W at launch from a Pu-238 full output voltage. The electrical wiring is also arranged heat source assembly containing a stack of 18 GPHS mod- to minimize the magnetic field of the RTG. ules. The GPHS–RTG operates at a normal voltage output Since 1989, a total of seven GPHS–RTGs have been of 28–30 V-dc. The overall dimensions of the GPHS–RTG launched on four missions. The most recent use of a are 42.2 cm (16.6 in.) diameter by 114 cm (44.9 in.) long. GPHS–RTG was on the New Horizons mission, launched The GPHS–RTG weighs 55.9 kg (123.3 lb) for a specific in January 2006 to encounter Pluto and Charon in 2015. power at launch of 5.1 W /kg (2.3 W /lb). All of these GPHS–RTGs performed, and continue to per- e e The heat source assembly is surrounded by 572 silicon form, as predicted. germanium (SiGe) thermocouples, also known as unicou- ples. The unicouples are individually bolted to and cantile- 10. Multi-mission radioisotope thermoelectric generator vered from the aluminum alloy generator housing and are (MMRTG) surrounded by a thermal insulation package consisting of 60 alternating layers of molybdenum foil and astroquartz The next generation of space RTGs is represented by the fibrous insulation. The silicon molybdenum (SiMo) hot MMRTG shown in Fig. 3. This lower-powered RTG is shoes are radiatively coupled to the heat source. The uni- being developed by DOE for use in missions on the Martian couples are connected in two series-parallel electric wiring surface as well as for potential missions in deep space. This circuits in parallel to enhance reliability and provide the mission flexibility is the primary reason for development of Fig. 3. Multi-mission radioisotope thermoelectric generator (MMRTG). 400 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 the MMRTG, as the GPHS–RTG was only designed for the Stirling generator, is being developed for potential mission use in the vacuum of space. The first planned use flight use in the ASRG, shown in Fig. 4. Like the of the MMRTG is to provide power for the Mars Science MMRTG, the ASRG is being developed for use in poten- Laboratory (MSL) rover scheduled for launch in September tial missions on the Martian surface or in deep space. 2009. An ASRG engineering unit is scheduled to be assembled The MMRTG will produce 110 W minimum (120 W in December 2007 and tested in April 2008. This engineer- e e estimated) at launch from a Pu-238 heat source assembly ing unit will include all system components and will be used containing a stack of 8 Step 2 GPHS modules. The for both reliability and flight environments testing. Once MMRTG operates at a normal output voltage of 28 V- the engineering unit testing is complete, qualification and dc. The overall dimensions of the MMRTG are 64 cm flight systems could be built based on the current design (25 in.) diameter by 66 cm (26 in.) long. The MMRTG or system development could continue, incorporating weighs 44 kg (97 lb) for a specific power at launch of known changes to further increase system specific power. 2.73 W /kg (1.24 W /lb). The description that follows is based on the current system e e The central heat source cavity is separated from the design. thermoelectric converter cavity by a helium isolation liner. The ASRG is designed to produce at least 147 W at The helium generated within the heat source by alpha beginning of mission (defined as space operation just after decay of the Pu-238 is dumped to the environment by dif- launch). It includes two Stirling generators, each with its fusion through an elastomeric gasket seal. The thermoelec- own GPHS module, for a nominal total heat input of tric converter cavity is hermetically sealed so that it can 500 W . Its overall dimensions are 76 cm (30 in.) by 45 cm operate in an atmospheric environment or in the hard vac- (18 in.) by 39 cm (15 in.). The ASRG flight design mass is uum of space. expected to be 21 kg (46.3 lb) for a specific power (at The thermoelectric converter is composed of 16 modules launch) of 7.0 W /kg (3.17 W /lb). e e of 48 thermocouples each, for a total of 768 thermocouples. The advanced stirling generator is a free-piston heat The thermoelectric materials employed are the same PbTe/ engine that operates on a Stirling thermodynamic cycle. TAGS materials used in the SNAP-19 RTGs for the Pio- Heat is supplied to each generator by a single GPHS mod- neer 10/11 and Viking 1/2 missions. The thermoelectric ele- ule at a hot-end operating temperature of 640 C. Heat is ments are smaller in diameter to increase the voltage rejected from the cold-end of the generator at roughly output of the RTG. The individual thermocouples are 60 C (this temperature varies with environments and fuel spring-loaded between the cold-end module bars and the decay). The closed-cycle system converts the heat from a hot-side graphite heat accumulator block. The thermocou- GPHS module into reciprocating motion, which through ples are connected in a series-parallel electrical circuit to a linear alternator is then converted into an ac electrical enhance reliability. Fibrous bulk thermal insulation is used power output. to minimize bypass heat losses. The thermoelectric con- The ASRG is designed to operate with the Stirling verter operates in an inert cover gas to reduce sublima- generators in synchronous opposed pairs, which will help tion/vaporization of the thermoelectric materials and power degradation during the operating life of the MMRTG. The PbTe/TAGS thermocouples operate between a hot junction temperature of 811 K and a cold junction temperature of about 483 K to produce a thermo- electric efficiency of about 6.8%. Waste heat is radiated from the eight radial fins on the housing. Both the housing and fins are made of aluminum alloys that will readily disintegrate and release the GPHS modules in the case of an inadvertent reentry into the Earth’s atmosphere. The housing and fins are coated with a high-emissivity coating. For the MSL rover mission, the MMRTG is equipped with coolant tubes attached to the fin roots for use in providing waste heat for thermal control of the rover’s equipment. The size of the radiator fins can be tailored to various mission heat sink conditions. 11. Advanced stirling radioisotope generator (ASRG) In addition to continued development of RTGs, the Department of Energy and NASA are also pursuing advanced power conversion technologies that will enable Fig. 4. Advanced stirling radioisotope generator (ASRG) engineering more efficient use of Pu-238. One of these technologies, unit. R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 401 minimize vibration levels under normal operating condi- In order to meet this need, DOE must have a reliable tions. Operation of an earlier generation of generators in and continuing supply of Pu-238. Efforts are underway to this configuration was shown to reduce generator vibration investigate alternative facilities in the United States to pro- levels by a factor of over 100 when compared to an unbal- vide a continuing Pu-238 production and processing capa- anced single generator; however, work remains to demon- bility. Until these Pu-238 supply issues are mitigated by the strate the performance of the ASRG in a flight establishment of a new source, the development and qual- configuration. ification of higher-efficiency systems will be an important The ASRG uses an active power factor control scheme step toward making effective use of the small remaining to convert ac power to dc for the spacecraft bus, while syn- inventory. chronizing the generator motion, maintaining proper hot- end temperature and piston stroke, and providing teleme- Acknowledgements try signals to the spacecraft. Although the ASRG is designed for autonomous operation, the controller may This document describes work performed by and for the also accept commands from the spacecraft as needed for US Government. The authors gratefully acknowledge the specific missions. The controller is being designed for single major contributions made in the preparation of this docu- fault tolerance. Each generator will have its own control ment by Robert T. Carpenter and Stella F. Elder of Orbital board, with a third board included for redundancy. Sciences Corporation, Rebecca Richardson and Robert Wiley of the Department of Energy, and Thierry Caillat 12. Future developments of the Jet Propulsion Laboratory. The preparation of this document was funded, in part, under contract to the US Over the next decade, the US Radioisotope Power Sys- Department of Energy. tems Program faces several programmatic and technologi- cal challenges if it is to continue to provide radioisotope Reference power systems for use in NASA space missions that require [1] Harvesting energy through thermoelectrics: power generation and them. NASA has identified a number of potential missions cooling. Mater Res Soc Bull. 2006;31:3. that will require radioisotope power and/or heat sources. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Energy Conversion and Management Unpaywall

Review of recent advances of radioisotope power systems

Energy Conversion and ManagementMar 1, 2008

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Unpaywall
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0196-8904
DOI
10.1016/j.enconman.2007.10.028
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Abstract

Radioisotope power systems have demonstrated numerous advantages over other types of power supplies for long-lived, unattended applications in space and in remote terrestrial locations. Many especially challenging power applications can be satisfied by proper selec- tion, design, and integration of the radioisotope heat source and the power conversion technologies that are now available or that can be developed. This paper provides a brief review of the factors influencing selection of radioisotopes and design of power systems, and dis- cusses the current state of practice and future programmatic and technical challenges to continued use of radioisotope power systems in space. 2007 Published by Elsevier Ltd. Keywords: Radioisotope power systems; Radioisotope thermoelectric generator (RTG); Plutonium-238; Space power systems; General purpose heat source (GPHS) 1. Introduction has launched 45 radioisotope thermoelectric generators (RTGs) on 26 spacecraft for various NASA and Depart- Radioisotope power systems are nuclear power systems ment of Defense missions in high and low Earth orbit, on that derive their energy from the spontaneous decay of the surfaces of the moon and Mars, and fly-bys to and radionuclides, as distinguished from nuclear fission energy beyond the outer planets (see Table 1). created in reactor power systems. The two major compo- nents of any radioisotope power system, or generator, are 2. Radioisotope fuels a radioisotope heat source and an energy conversion sys- tem. Heat is produced during the decay process within Selection of a suitable radioisotope, commonly referred the heat source. This heat is partially transformed into elec- to as fuel, for use in space radioisotope power systems is tricity and the waste heat is transferred to space or the envi- the key to their acceptance and use. The characteristics of ronment surrounding the generator. Such power systems an acceptable fuel include a long half-life, low radiation are rugged, compact and highly reliable, and can be safely emissions, high power density and specific power and a sta- produced and used with minimal risk to operating person- ble fuel form with a high melting point. The fuel must be nel, the general public, and the Earth’s environment. safely producible in useful quantities and at a reasonable Since before the first Sputnik was launched into space, cost and must be capable of being used safely in all normal the Department of Energy (DOE) and its predecessor agen- and potential accident environments. cies have been developing the technology to fabricate and The size and weight of a heat source are directly related deliver radioisotope power systems for use on US military to the half-life of the fuel. If the half-life is too long, the and civilian space missions. Since 1961, the United States radioactive decay rate is slower and associated heat pro- duction rate is low. This results in a fuel loading that is too large and too heavy for space missions. If the half-life is too short, a great deal of heat may be produced initially, Corresponding author. Tel.: +1 301 903 3456; fax: +1 301 903 1510. E-mail address: Robert.Lange@nuclear.energy.gov (R.G. Lange). but the heat production rate will decay quickly. Because of 0196-8904/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.enconman.2007.10.028 394 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 Table 1 US spacecraft using nuclear power systems Power source # RTGs Spacecraft Mission type Launch Status SNAP-3 1 Transit 4A Navigational 1961 Currently in orbit SNAP-3 1 Transit 4B Navigational 1961 Currently in orbit SNAP-9A 1 Transit 5BN-1 Navigational 1963 Currently in orbit SNAP-9A 1 Transit 5BN-2 Navigational 1963 Currently in orbit SNAP-9A 1 Transit 5BN-3 Navigational 1964 Mission aborted; heat source burned up as designed SNAP-10A Reactor Snapshot Experimental 1965 Achieved orbit; was shut down after 43 days SNAP-19 2 Nimbus B-1 Meteorological 1968 Mission aborted; heat sources retrieved SNAP-19 2 Nimbus III Meteorological 1969 Currently in orbit ALRHU Heater Apollo 11 Lunar 1969 On lunar surface SNAP-27 1 Apollo 12 Lunar/ALSEP 1969 On lunar surface; station shut down SNAP-27 1 Apollo 13 Lunar/ALSEP 1970 Mission aborted; heat source jettisoned into ocean SNAP-27 1 Apollo 14 Lunar/ALSEP 1971 On lunar surface; station shut down SNAP-27 1 Apollo 15 Lunar/ALSEP 1971 On lunar surface; station shut down SNAP-19 4 Pioneer 10 Planetary 3/2/72 Operated to Jupiter and beyond solar system SNAP-27 1 Apollo 16 Lunar/ALSEP 1972 On lunar surface; station shut down Transit-RTG 1 Triad-01-1X Navigational 1972 Currently in orbit SNAP-27 1 Apollo 17 Lunar/ALSEP 1972 On lunar surface; station shut down SNAP-19 4 Pioneer 11 Planetary 1973 Operated to Jupiter, Saturn, beyond solar system SNAP-19 2 ea. Viking 1,2 Mars landers 1975 On Martian surface; landers shut down MHW–RTG 2 ea. LES 8, LES 9 Communication 1976 Currently in earth orbit MHW–RTG 3 ea. Voyager 1,2 Planetary 1977 Still operating; extended interstellar mission GPHS–RTG 2 Galileo Planetary 1987 Deliberately crashed into Jupiter at end-of-mission GPHS–RTG 1 Ulysses Planetary 1990 Successful ongoing mission to the Sun’s polar regions LWRHU Heaters Mars pathfinder Mars rover 1996 Mission successfully completed GPHS–RTG 3 Cassini Planetary 1997 Successful ongoing mission to Saturn and its moons LWRHU Heaters MER Mars rovers (2) 2003 Extended mission ongoing GPHS–RTG 1 New horizons Planetary 2006 Successfully operating en route to Pluto this, excess fuel must be added to maintain the amount of reactor or a very high-powered accelerator facility. Both of heat required at the end-of-mission (EOM). The half-life of these approaches require major investments in nuclear the radioisotope fuel should be at least as long as or longer facilities capable of processing highly radioactive spent than the mission lifetime to reduce the heat variation over reactor fuel or irradiated targets. Chemical processing tech- the mission. nology to produce the proper fuel compound with the nec- The levels of penetrating radiation (gamma, X-ray, and essary purity must also be available, along with fabrication neutron) emissions must be inherently low for any radio- processes and facilities to produce the final fuel form. These radioisotope fuel facilities must be operated under isotope fuel used in space applications. This will reduce the burden required to protect workers and the spacecraft the strictest safety and environmental standards and take from the potential damaging effects of radiation. This is into account the ultimate disposal of any radioactive also important for protection of the public and the environ- wastes generated. ment in the event of a launch accident. The radioisotope When it is determined that a proposed fuel form can be fuel must also be useable in a form with a high melting produced in adequate quantities and with the desired point that remains stable during postulated launch acci- nuclear characteristics, the production quality fuel form, dents. The fuel form must be chemically compatible with along with its encapsulation and other heat source compo- its containment material over the operating life of the heat nents, must be extensively tested to qualify it for space use source. It is also highly desirable that the fuel form have a and to support the necessary launch safety reviews and low solubility rate in the human body and in the natural approvals. Development and qualification of a new heat environment. Radioactive decay products must not source for flight use requires a large effort in terms of costs adversely affect the integrity of the fuel form, and the decay and schedule. In addition, there are only a limited number process should not degrade its properties. of radioisotope fuels that meet the requirements of half- Any radioisotope fuel selected for space power applica- life, radiation, power density, fuel form, and availability tions must be producible in sufficient quantities and on a for use in space power system applications. schedule to meet mission requirements. There are only two methods for obtaining radioisotopes in the quantities 3. Historical fuels perspective needed for space power applications. The first involves pro- cessing spent fuel from a nuclear reactor to isolate by-prod- Development of radioisotope power systems began in ucts of interest. The other is the deliberate production of the early 1950s and since then a variety of radioisotopes radioisotopes by irradiation of target materials in a nuclear has been evaluated for space and terrestrial applications. R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 395 The isotope initially selected for development was to Pu-238. However, this source has failed to materialize. cerium-144 (Ce-144) because it was one of the most plenti- The short-lived isotope curium-242 (Cm-242) was also ful fission products available from reprocessing defense initially selected to fuel an isotope power system for the nuclear reactor fuel. Its short half-life (290 days) made 90-day Surveyor mission to the Moon, but the Surveyor Ce-144 compatible with the 6-month military reconnais- program later decided not to use isotope systems. None of sance satellite mission envisioned as its primary application these alternative fuels has been used in space by the United at that time. The cerium oxide fuel form and its heavy fuel States. capsule met all safety tests for intact containment of the In the final analysis, Pu-238 is clearly superior to other fuel during potential launch abort fires, explosions, and ter- radioisotope fuels for use in long-duration space missions minal impacts. However, the high radiation field associated especially for deep space exploration. The technology for with the beta/gamma emission of Ce-144 caused handling producing and processing Pu-238 fuel forms has been and payload interaction problems as well as safety prob- clearly demonstrated over more than 40 years. Pu-238 lems upon random reentry from orbit. The Ce-144 fueled fueled heat sources have been through rigorous flight qual- SNAP-1 (SNAP stands for Systems for Nuclear Auxiliary ification testing and have performed safely and reliably in Power) power system was never used in space. all of the radioisotope power systems employed in the US By the late 1950s, large amounts of polonium-210 space program to date. The availability of Pu-238 fuel for (Po-210) became available, also as a byproduct of the future space missions is a continuing concern. For nearly nuclear weapons program. Po-210 is an alpha emitter with 30 years the production and processing of Pu-238 fuel a very high power density and low radiation emissions. Po- was accomplished as a byproduct of the production of 210 metal was used to fuel the small SNAP-3 technology materials for nuclear weapons. Changes in the Nation’s demonstration RTG that was first displayed at the White nuclear weapons program eliminated this traditional capa- House in January 1959. Several SNAP-3 RTGs were fueled bility to produce Pu-238 in 1986. Therefore, for Pu-238 fuel with Po-210 and used in various exhibits. However, the to continue to be available for use in the space program, a short half-life of Po-210 (138 days) makes it suitable for reliable source or sources must be established. Since 1996, only limited duration space power applications and no Pu-238 has been purchased from Russia for use in US SNAP-3 RTGs were deployed in space. space missions. Alternative domestic production capabili- In order to provide a longer-lived radioisotope fuel, ties for Pu-238 are currently being investigated by DOE. strontium-90 (Sr-90), an abundant fission product with a 28.6-year half-life, was recovered from defense wastes at 4. General purpose heat source (GPHS) Hanford. A very stable and insoluble fuel form, strontium titanate, was developed and widely used in terrestrial Once the appropriate isotope is selected for a space mis- power systems. Sr-90 and its daughter Yttrium-90 give off sion, it must be combined with other components to create significant radiation that requires heavy shielding, but a heat source. The heat source must efficiently and reliably shield weights are not as critical in most terrestrial power transfer the isotope heat to the conversion system while systems as for space power systems. withstanding routine mission environments and postulated By 1960, plutonium-238 (Pu-238) had been identified as accident scenarios. Numerous heat source designs have an attractive radioisotope fuel that could be made by irra- been used for terrestrial and space applications. However, diating neptunium-237 (Np-237) targets in the defense pro- since the launch of the Galileo spacecraft in 1989, all duction reactors. Its availability was extremely limited due NASA missions powered by radioisotopes have used some to shortage of Np-237 target material that must be recov- version of the general purpose heat source (GPHS). The ered from recycling high burnup, enriched uranium fuel. GPHS module has undergone an extensive safety analysis However, Pu-238 has all of the necessary nuclear character- and test program and was the only radioisotope heat istics for a space power system fuel: long half-life (87.74 source to be qualified for and used in a launch on the Space years), low radiation emissions, high power density, and Shuttle. All current-generation radioisotope power systems useful fuel forms (metal or oxide). After flight qualification designed for use on US space missions incorporate the of its heat source, a Pu-238 fueled SNAP-3 RTG was GPHS. launched on the Transit 4A Navy navigation satellite in The GPHS module is shown in Fig. 1. Each module is June 1961 – the first use of nuclear power in space. designed to deliver up to 250 thermal watts (W )atBOM Because of the limited availability of Pu-238, several and has a mass of 1.43 kg (3.16 lb). The module size and other radioisotopes have been thoroughly evaluated for shape were selected to survive orbital reentry through the space use. Sr-90 and Po-210 fuels were considered for use atmosphere and impact the Earth at a modest terminal in higher powered military satellite constellations for which velocity of 50 m/s (164 ft/s). Each module is a cuboid with there were insufficient quantities of Pu-238 available, but the overall dimensions of 9.72 cm  9.32 cm  5.31 cm these projects were all cancelled before launch. Curium- (3.83 in.  3.67 in.  2.09 in.). 244 (Cm-244) was expected to become available in signifi- Each GPHS module contains four pressed and sintered cant quantities from the US breeder reactor fuel cycle and Pu-238 oxide fuel pellets, each with nominal output of was investigated as a potential alternative long-lived fuel 62.5 W . The cylindrical fuel pellet is 2.75 cm (1.08 in.) in t 396 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 Fig. 1. General purpose heat source (GPHS). diameter and 2.75 cm (1.08 in.) in length. Each fuel pellet is aeroshell between the two GISs. This modification, known individually encapsulated in a welded iridium alloy con- as a Step 1 GPHS module, was used on the New Horizons tainment shell or cladding with a minimum wall thickness mission to Pluto. Systems currently being designed will of 0.05 cm (0.02 in.). The iridium alloy is capable of resist- incorporate another improvement to the aeroshell, which ing oxidation in the post-impact environment while it also makes the broad ablation faces thicker. This variation of provides chemical compatibility with the fuel and graphite the GPHS is known as Step 2. The Step 2 GPHS module is components during high-temperature operation and postu- 9.96 cm  9.32 cm  5.82 cm (3.92 in.  3.67 in.  2.29 in.) lated accident conditions. The iridium fuel cladding is and weighs 1.60 kg (3.53 lbs.). The more robust GPHS aero- equipped with a frit vent that allows release of helium pro- shell provides additional protection for the fueled clads dur- duced by the decay of the Pu-238 without releasing pluto- ing potential launch pad accidents and higher-velocity nia particles. The combination of fuel pellet and cladding is reentries into the atmosphere. called a fueled clad. Two fueled clads are encased in a Graphite Impact Shell 5. Energy conversion systems (GIS) made of Fine Weave Pierced Fabric (FWPF) car- bon–carbon composite material. The GIS is designed to The radioisotope heat source delivers its heat to some limit the damage to the iridium clads during free-fall type of energy conversion system that converts part of impacts. Two of these GISs are inserted into an aeroshell the heat into useful electrical power. There are two general that is also made of FWPF graphite. A thermal insulation classes of energy conversion systems, static and dynamic. layer of Carbon Bonded Carbon Fiber graphite surrounds Static systems include thermoelectric, thermionic, and ther- each GIS to limit the peak temperature of the iridium clad- mophotovoltaic conversion devices that can convert heat ding during atmospheric reentry heating and to maintain to electricity directly with no moving parts. Dynamic sys- its ductility during the subsequent impact. The aeroshell tems involve heat engines with working fluids that trans- is designed to contain the two GISs under severe reentry form heat to mechanical energy that in turn is used to conditions and to provide additional impact protection generate electricity. Both rotating and reciprocating against hard surfaces at its terminal velocity. It also pro- engines may be used. Potential dynamic systems include vides protection for the fueled clads against overpressures Rankine, Brayton, and Stirling engines that operate on var- and fragment impacts during postulated missile explosion ious types of working fluids. events. The aeroshell serves as the primary structural mem- Efficiency is an important consideration in selecting an ber to maintain the integrity and position of a stack of energy conversion system because of its effect on the radio- GPHS modules within a power system during normal oper- isotope inventory and its impact on cost, availability, size ations including testing, transportation, and launch. and weight. System reliability is also important. Since mis- The original GPHS module (shown in Fig. 1) was used on sion success depends on having sufficient electrical power the Galileo, Ulysses, and Cassini missions. It has been over the life of the mission, the selection of an energy con- improved since then by adding an internal web in the version system must be consistent with mission power R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 397 levels and lifetimes. Graceful power degradation over the the cover gas. Helium gas buildup within the converter life of a mission is acceptable as long as it is within predict- must be controlled by using a separate container around able limits. Other characteristics important in selecting an the heat source or permeable seals in the generator design. energy conversion unit for a radioisotope space power sys- Gas management considerations in the generator housing tem are weight, size, ruggedness to withstand shock and design and the use of bulk insulation materials increase vibration loads, survivability in hostile particle and radia- the size and weight of the generator. However, this type tion environments, scalability in power levels, flexibility of RTG is equally useful for space vacuum or for planetary in integration with various types of spacecraft and launch atmospheric applications. vehicles, and versatility to operate in the vacuum of deep SiGe materials can be operated at hot junction temper- space or on planetary surfaces with or without solar energy atures up to 1000 C. Their sublimation rates and oxida- inputs. tion effects, even at these higher temperatures, can be controlled by use of sublimation barriers around the ele- 6. Radioisotope thermoelectric generators ments and an inert cover gas within the generator during ground operation. The advantage of SiGe RTGs is clearest Of the various static conversion technologies, thermo- in space vacuum applications. In this case, a pressure electric energy conversion has received the most interest, release device opens on reaching altitude to vent the cover both in development and use, for radioisotope power sys- gas to space. This allows the use of lightweight multifoil tems. Thermoelectric converters are highly reliable over thermal insulation and operation of the unicouples without extended operating lifetimes, compact, rugged, radiation a cover gas, eliminating significant mass when compared to resistant, easily adapted to various applications, and pro- telluride RTGs. duce no noise, vibration, or torque during operation. Ther- The efficiency of current RTGs using either PbTe/TAGS moelectric converters require no start-up devices to or SiGe thermoelectric elements can be improved by using operate. They start producing electrical power as soon as more efficient thermoelectric materials capable of operating the heat source is installed. Power output is easily regulated up to 1273 K. Since the 1990s, investigation of materials at design level by maintaining a matched resistive load on with low thermal conductivity has been a key aspect in the converter. A limiting feature of thermoelectric conver- the research and development of advanced thermoelectric sion is its relatively low conversion efficiency, typically less materials at the NASA Jet Propulsion Laboratory (JPL). than 10%. A comprehensive review of recent development in thermo- Thermoelectric materials, when operating over a tem- electric materials has been provided in a recent volume of perature gradient, produce a voltage called the Seebeck the Materials Research Bulletin [1]. Exciting results have voltage. When connected in series with a load, the inter- been obtained on thin-films, superlattices and quantum nally generated voltage causes an electron current to flow dot materials. These materials may, however, be more through the load producing useful power. Power is pro- appropriate for small-scale electronics applications where duced in a thermoelectric element (thermocouple) placed spot cooling or low levels of power generation may be between a heat source and a heat sink. Typically, thermo- required. Half-Heuslers intermetallic alloys have also couples are low voltage devices so a number of them must received increasing attention as potential high-temperature be connected in series to produce normal load voltages. thermoelectric materials. Thermocouples can be connected in a series-parallel Efforts at NASA JPL have primarily focused on skutter- arrangement to enhance reliability by minimizing the effect udites, zintl, and zinc antimonide materials. Filled and on total power due to failure of a single thermocouple. unfilled skutterudites have been intensively investigated The most widely used thermoelectric materials, in order mostly because of the possibility of independently manipu- of increasing temperature capability, are bismuth telluride lating the thermal and electronic transport properties. (BiTe); lead telluride (PbTe); tellurides of antimony, ger- While much development work remains before any of these manium, and silver (TAGS); lead tin telluride (PbSnTe); materials could be successfully integrated into a power gen- and silicon germanium (SiGe). All of these except BiTe erator either for space or terrestrial applications, recent have been used in RTGs that have been flown on space results clearly indicate that the figure of merit (ZT)  1 limit missions. Many more materials have been, and are still observed for most of the thermoelectric materials used to being, investigated in hopes of producing higher-efficiency, date in practical thermoelectric generators can be broken, lower weight power systems with stable performance over offering future prospects for higher efficiency generators. long operating lifetimes. The telluride materials are limited to a maximum hot 7. Dynamic conversion systems junction temperature of 550 C. Due to the deleterious effects of oxygen on these materials and their high vapor While RTGs have a long history of providing reliable pressures, the tellurides must be operated in a sealed gener- power for long-duration space missions, the higher conver- ator with an inert cover gas to retard sublimation and sion efficiencies of dynamic systems would allow for better vapor phase transport within the converter. Bulk-type, use of limited radioisotope fuel and would offer higher sys- fibrous thermal insulation is used due to the presence of tem power per unit mass. System efficiencies of 25% or 398 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 more are achievable, reducing the radioisotope inventory Stirling cycle systems differ from Rankine and Brayton required to less than one-third of what would be required systems in that they do not include turbines, pumps or for a comparable RTG. However, a great deal of testing compressors. The Stirling cycle uses a working gas that will be required in order to demonstrate a reliability level expands by absorption of heat on the hot-side and con- comparable to that of an RTG. Dynamic conversion radio- tracts by rejection of heat on the cold side causing rapidly isotope power systems have not yet been used in space. changing pressure cycles across a piston, forcing it to move The difference between static and dynamic systems is the in a reciprocating fashion. The most promising type of Stir- thermal-to-electric power conversion mechanism. For ling engine for radioisotope generator applications is the dynamic systems, the thermal energy in the working fluid free-piston Stirling engine, which requires no lubricating is partially transformed into mechanical work to drive an fluids and produces electricity by means of a linear alterna- alternator to produce electricity. The three different tor within a hermetically sealed engine housing. The piston dynamic conversion concepts that have received the most moves back and forth at a resonant frequency on a cushion attention for use in radioisotope power systems employ of working gas between it and the surrounding cylinder Rankine, Brayton or Stirling cycles. The Rankine cycle is wall. A permanent magnet is attached to the power piston based on a two-phase working fluid that requires special to produce electrical currents in surrounding alternator vapor–liquid boiler and condenser designs for use in the coils as it oscillates back and forth. Since the reciprocating microgravity of space. Development programs have con- motion of the piston would cause unbalanced vibration sidered Rankine systems using liquid metal and organic loads, Stirling engines for space applications are generally working fluids. Brayton and Stirling cycles use a single designed in pairs with dynamically opposed pistons to min- gas working fluid such as a helium–xenon mixture for imize the net load transmitted to the spacecraft. Brayton and pure helium for Stirling. The Stirling cycle provides higher conversion efficiencies In a Brayton or Rankine system, the gaseous working than the Rankine and Brayton cycles at the same cycle tem- fluid from the heat source heat exchanger turns a turbine peratures. System efficiencies of 30% or more are possible rotor that is mounted on a common shaft with the alterna- at operating temperatures achievable with isotope heat tor rotor and the compressor or liquid pump rotor. The sources and oxidation-resistant superalloy structural mate- remainder of the power conversion system includes the tur- rials. The Stirling cycle also retains its high performance bine housing, turbine nozzles, bearings, alternator coils, characteristics at lower power levels than the Brayton pump housing, and cooling ducts to form a compact energy and Rankine systems, an attractive feature for radioisotope conversion unit. To minimize size and weight, the rotor power systems. shaft turns at very high speeds and the alternator produces high-frequency alternating current. To start such a system, 8. Current state of technology electrical power from a battery would be used to spin up the rotor until the system’s temperature, pressure, and In the years since RTGs were first used, the fuel form mass flow rates permit it to be self-sustaining. A power and heat source technologies have been steadily improved control and conditioning system would also be required to operate at higher temperatures with increasingly larger to regulate operating speed and electrical output. fuel inventories to meet the aerospace nuclear safety goals The two general areas of concern with the use of a tur- consistent with the As Low As Reasonably Achievable bine-driven electrical power system are reliability and (ALARA) approach. As the power levels of the RTGs have spacecraft interactions. The main reliability issues have to increased, improved thermoelectric materials and thermal do with bearings, loss of working fluid and failure of elec- insulation approaches have been developed to increase tronic parts in the control system. Foil bearings are used in long-term power stability and specific power of the RTGs Brayton designs to allow the rotating shaft to ride on a thin to meet the needs of more ambitious space exploration mis- film of working fluid gas during operation. Organic Ran- sions. Advanced technology programs have also been pur- kine units can use either foil bearings or hydrodynamic sued to take advantage of the greater efficiency offered by thrust and journal bearings lubricated by the organic work- dynamic power conversion systems. The remainder of this ing fluid. Both have been shown to be highly stable and section describes the three major radioisotope power sys- reliable once operating conditions are achieved and bearing tems currently being used on active NASA missions or temperatures are controlled. Since a puncture by a micro- being developed for potential near-term use. meteoroid or a crack caused by thermal or mechanical stresses during operation could cause the loss of the work- 9. General purpose heat source-radioisotope thermoelectric ing fluid and total loss of power, this single-point failure generator (GPHS–RTG) mechanism must receive particular attention during the design. Conservative wall thicknesses, expansion joints, The current state-of-the-art in space RTGs is represented and meteoroid armor are usually employed to reduce the by the GPHS–RTG, so named because it was the first to probability of failures of this kind. Totally redundant employ the GPHS modules. The GPHS–RTG, shown in power conversion loops can also be used if the additional Fig. 2, is the largest Pu-238 fueled, long-lived RTG built weight can be tolerated. for use in space missions. Utilizing recently precipitated R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 399 Fig. 2. General purpose heat source radioisotope thermoelectric generator (GPHS-RTG). Pu-238, it produces at least 285 W at launch from a Pu-238 full output voltage. The electrical wiring is also arranged heat source assembly containing a stack of 18 GPHS mod- to minimize the magnetic field of the RTG. ules. The GPHS–RTG operates at a normal voltage output Since 1989, a total of seven GPHS–RTGs have been of 28–30 V-dc. The overall dimensions of the GPHS–RTG launched on four missions. The most recent use of a are 42.2 cm (16.6 in.) diameter by 114 cm (44.9 in.) long. GPHS–RTG was on the New Horizons mission, launched The GPHS–RTG weighs 55.9 kg (123.3 lb) for a specific in January 2006 to encounter Pluto and Charon in 2015. power at launch of 5.1 W /kg (2.3 W /lb). All of these GPHS–RTGs performed, and continue to per- e e The heat source assembly is surrounded by 572 silicon form, as predicted. germanium (SiGe) thermocouples, also known as unicou- ples. The unicouples are individually bolted to and cantile- 10. Multi-mission radioisotope thermoelectric generator vered from the aluminum alloy generator housing and are (MMRTG) surrounded by a thermal insulation package consisting of 60 alternating layers of molybdenum foil and astroquartz The next generation of space RTGs is represented by the fibrous insulation. The silicon molybdenum (SiMo) hot MMRTG shown in Fig. 3. This lower-powered RTG is shoes are radiatively coupled to the heat source. The uni- being developed by DOE for use in missions on the Martian couples are connected in two series-parallel electric wiring surface as well as for potential missions in deep space. This circuits in parallel to enhance reliability and provide the mission flexibility is the primary reason for development of Fig. 3. Multi-mission radioisotope thermoelectric generator (MMRTG). 400 R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 the MMRTG, as the GPHS–RTG was only designed for the Stirling generator, is being developed for potential mission use in the vacuum of space. The first planned use flight use in the ASRG, shown in Fig. 4. Like the of the MMRTG is to provide power for the Mars Science MMRTG, the ASRG is being developed for use in poten- Laboratory (MSL) rover scheduled for launch in September tial missions on the Martian surface or in deep space. 2009. An ASRG engineering unit is scheduled to be assembled The MMRTG will produce 110 W minimum (120 W in December 2007 and tested in April 2008. This engineer- e e estimated) at launch from a Pu-238 heat source assembly ing unit will include all system components and will be used containing a stack of 8 Step 2 GPHS modules. The for both reliability and flight environments testing. Once MMRTG operates at a normal output voltage of 28 V- the engineering unit testing is complete, qualification and dc. The overall dimensions of the MMRTG are 64 cm flight systems could be built based on the current design (25 in.) diameter by 66 cm (26 in.) long. The MMRTG or system development could continue, incorporating weighs 44 kg (97 lb) for a specific power at launch of known changes to further increase system specific power. 2.73 W /kg (1.24 W /lb). The description that follows is based on the current system e e The central heat source cavity is separated from the design. thermoelectric converter cavity by a helium isolation liner. The ASRG is designed to produce at least 147 W at The helium generated within the heat source by alpha beginning of mission (defined as space operation just after decay of the Pu-238 is dumped to the environment by dif- launch). It includes two Stirling generators, each with its fusion through an elastomeric gasket seal. The thermoelec- own GPHS module, for a nominal total heat input of tric converter cavity is hermetically sealed so that it can 500 W . Its overall dimensions are 76 cm (30 in.) by 45 cm operate in an atmospheric environment or in the hard vac- (18 in.) by 39 cm (15 in.). The ASRG flight design mass is uum of space. expected to be 21 kg (46.3 lb) for a specific power (at The thermoelectric converter is composed of 16 modules launch) of 7.0 W /kg (3.17 W /lb). e e of 48 thermocouples each, for a total of 768 thermocouples. The advanced stirling generator is a free-piston heat The thermoelectric materials employed are the same PbTe/ engine that operates on a Stirling thermodynamic cycle. TAGS materials used in the SNAP-19 RTGs for the Pio- Heat is supplied to each generator by a single GPHS mod- neer 10/11 and Viking 1/2 missions. The thermoelectric ele- ule at a hot-end operating temperature of 640 C. Heat is ments are smaller in diameter to increase the voltage rejected from the cold-end of the generator at roughly output of the RTG. The individual thermocouples are 60 C (this temperature varies with environments and fuel spring-loaded between the cold-end module bars and the decay). The closed-cycle system converts the heat from a hot-side graphite heat accumulator block. The thermocou- GPHS module into reciprocating motion, which through ples are connected in a series-parallel electrical circuit to a linear alternator is then converted into an ac electrical enhance reliability. Fibrous bulk thermal insulation is used power output. to minimize bypass heat losses. The thermoelectric con- The ASRG is designed to operate with the Stirling verter operates in an inert cover gas to reduce sublima- generators in synchronous opposed pairs, which will help tion/vaporization of the thermoelectric materials and power degradation during the operating life of the MMRTG. The PbTe/TAGS thermocouples operate between a hot junction temperature of 811 K and a cold junction temperature of about 483 K to produce a thermo- electric efficiency of about 6.8%. Waste heat is radiated from the eight radial fins on the housing. Both the housing and fins are made of aluminum alloys that will readily disintegrate and release the GPHS modules in the case of an inadvertent reentry into the Earth’s atmosphere. The housing and fins are coated with a high-emissivity coating. For the MSL rover mission, the MMRTG is equipped with coolant tubes attached to the fin roots for use in providing waste heat for thermal control of the rover’s equipment. The size of the radiator fins can be tailored to various mission heat sink conditions. 11. Advanced stirling radioisotope generator (ASRG) In addition to continued development of RTGs, the Department of Energy and NASA are also pursuing advanced power conversion technologies that will enable Fig. 4. Advanced stirling radioisotope generator (ASRG) engineering more efficient use of Pu-238. One of these technologies, unit. R.G. Lange, W.P. Carroll / Energy Conversion and Management 49 (2008) 393–401 401 minimize vibration levels under normal operating condi- In order to meet this need, DOE must have a reliable tions. Operation of an earlier generation of generators in and continuing supply of Pu-238. Efforts are underway to this configuration was shown to reduce generator vibration investigate alternative facilities in the United States to pro- levels by a factor of over 100 when compared to an unbal- vide a continuing Pu-238 production and processing capa- anced single generator; however, work remains to demon- bility. Until these Pu-238 supply issues are mitigated by the strate the performance of the ASRG in a flight establishment of a new source, the development and qual- configuration. ification of higher-efficiency systems will be an important The ASRG uses an active power factor control scheme step toward making effective use of the small remaining to convert ac power to dc for the spacecraft bus, while syn- inventory. chronizing the generator motion, maintaining proper hot- end temperature and piston stroke, and providing teleme- Acknowledgements try signals to the spacecraft. Although the ASRG is designed for autonomous operation, the controller may This document describes work performed by and for the also accept commands from the spacecraft as needed for US Government. The authors gratefully acknowledge the specific missions. The controller is being designed for single major contributions made in the preparation of this docu- fault tolerance. Each generator will have its own control ment by Robert T. Carpenter and Stella F. Elder of Orbital board, with a third board included for redundancy. Sciences Corporation, Rebecca Richardson and Robert Wiley of the Department of Energy, and Thierry Caillat 12. Future developments of the Jet Propulsion Laboratory. The preparation of this document was funded, in part, under contract to the US Over the next decade, the US Radioisotope Power Sys- Department of Energy. tems Program faces several programmatic and technologi- cal challenges if it is to continue to provide radioisotope Reference power systems for use in NASA space missions that require [1] Harvesting energy through thermoelectrics: power generation and them. NASA has identified a number of potential missions cooling. Mater Res Soc Bull. 2006;31:3. that will require radioisotope power and/or heat sources.

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Published: Mar 1, 2008

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