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A radioisotope thermoelectric generator (RTG, RITEG) is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.
RTGs have been used as power sources in satellites, space probes and unmanned remote facilities, such as a series of lighthouses built by the former Soviet Union inside the Arctic Circle. RTGs are usually the most desirable power source for robotic or unmaintained situations needing a few hundred watts (or less) of power for durations too long for fuel cells, batteries, or generators to provide economically, and in places where solar cells are not practical. Safe use of RTGs requires containment of the radioisotopes long after the productive life of the unit.
In the same brief letter where he introduced the communications satellite, Arthur C. Clarke suggested that, with respect to spacecraft, "the operating period might be indefinitely prolonged by the use of thermocouples."
RTGs were developed in the US during the late 1950s by Mound Laboratories in Miamisburg, Ohio under contract with the United States Atomic Energy Commission. The project was led by Dr. Bertram C. Blanke.
The first RTG launched into space by the United States was SNAP 3 in 1961, aboard the Navy Transit 4A spacecraft. One of the first terrestrial uses of RTGs was in 1966 by the US Navy at uninhabited Fairway Rock in Alaska. RTGs were used at that site until 1995.
A common RTG application is spacecraft power supply. Systems for Nuclear Auxiliary Power (SNAP) units were used for probes that traveled far from the Sun rendering solar panels impractical. As such, they were used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini, New Horizons and the Mars Science Laboratory. RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17 (SNAP 27s). Because the Apollo 13 moon landing was aborted, its RTG rests in the South Pacific ocean, in the vicinity of the Tonga Trench. RTGs were also used for the Nimbus, Transit and LES satellites. By comparison, only a few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.
In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses and navigation beacons powered by RTGs. Powered by strontium-90 (90Sr), they are very reliable and provide a steady source of power. Critics[who?] argue that they could cause environmental and security problems as leakage or theft of the radioactive material could pass unnoticed for years, particularly as the locations of some of these lighthouses are no longer known due to poor record keeping. In one instance, the radioactive compartments were opened by a thief. In another case, three woodsmen in Georgia came across two ceramic RTG heat sources that had been stripped of their shielding. Two of the three were later hospitalized with severe radiation burns after carrying the sources on their backs. The units were eventually recovered and isolated.
There are approximately 1,000 such RTGs in Russia. All of them have long exhausted their 10-year engineered life spans. They are likely no longer functional, and may be in need of dismantling. Some of them have become the prey of metal hunters, who strip the RTGs' metal casings, regardless of the risk of radioactive contamination.
The design of an RTG is simple by the standards of nuclear technology: the main component is a sturdy container of a radioactive material (the fuel). Thermocouples are placed in the walls of the container, with the outer end of each thermocouple connected to a heat sink. Radioactive decay of the fuel produces heat which flows through the thermocouples to the heat sink, generating electricity in the process.
A thermocouple is a thermoelectric device that converts thermal energy directly into electrical energy using the Seebeck effect. It is made of two kinds of metal (or semiconductors) that can both conduct electricity. They are connected to each other in a closed loop. If the two junctions are at different temperatures, an electric current will flow in the loop.
The radioactive material used in RTGs must have several characteristics:
The first two criteria limit the number of possible fuels to fewer than 30 atomic isotopes within the entire table of nuclides. Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242 and thulium isotopes have also been studied.
Plutonium-238 has the lowest shielding requirements and longest half-life; its energy output is 0.54 kilowatts per kilogram. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25 mm of lead shielding to block the radiation. 238Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is needed in a 238Pu RTG, as the casing itself is adequate. 238Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO2). 238Pu has a half-life of 87.7 years, reasonable power density, and exceptionally low gamma and neutron radiation levels.
Strontium-90 also requires little shielding, as it decays by β emission, with negligible γ emission. While its half life of 28.8 years is much shorter than that of 238Pu, it also has a much lower decay energy. Thus its power density is only 0.46 kilowatts per kilogram. Because the energy output is lower it reaches lower temperatures than 238Pu, which results in lower RTG efficiency. 90Sr is a high yield waste product of nuclear fission and is available in large quantities at a low price.
Some prototype RTGs, first built in 1958 by the US Atomic Energy Commission, have used polonium-210. This isotope provides phenomenal power density because of its high radioactive activity, but has limited use because of its very short half-life of 138 days, again because of its high activity. A kilogram of pure 210Po in the form of a cube would be about 48 mm (about 2 inches) on a side and emit about 140kW. The heat of melting is about 60kJ/kg, the heat of evaporation about 10 times larger. If there is no efficient cooling, the self-heating power is sufficient for melting and then partly vaporizing itself.
Americium-241 is a potential candidate isotope with a longer half-life than 238Pu: 241Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the power density of 241Am is only 1/4 that of 238Pu, and 241Am produces more penetrating radiation through decay chain products than 238Pu and needs about 18 mm of lead shielding. Even so, its shielding requirements in an RTG are the second lowest of all possible isotopes: only 238Pu requires less. With a current global shortage of 238Pu, a closer look is being given to 241Am.
Most RTGs use 238Pu, which decays with a half-life of 87.7 years. RTGs using this material will therefore diminish in power output by 0.787% of their capacity per year. 23 years after production, such an RTG will have decreased in power by 16.6%, i.e. providing 83.4% of its initial output. Thus, with a starting capacity of 470 W, after 23 years it would have a capacity of 392 W. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well; at the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for Voyager 2. Therefore in early 2001, the RTGs were working at about 67% of their original capacity instead of the expected 83.4%.
This life span was of particular importance during the Galileo mission. Originally intended to launch in 1986, it was delayed by the Space Shuttle Challenger accident. Because of this unforeseen event, the probe had to sit in storage for 4 years before launching in 1989. Subsequently, its RTGs had decayed somewhat, necessitating replanning the power budget for the mission.
RTGs use thermoelectric couples or "thermocouples" to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%. Thermoelectric materials in space missions to date have included silicon germanium alloys, lead telluride and tellurides of antimony, germanium and silver (TAGS). Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.
A thermionic converter – an energy conversion device which relies on the principle of thermionic emission—can achieve efficiencies between 10–20%, but requires higher temperatures than those at which standard RTGs run. Some prototype 210Po RTGs have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these unfeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes.
Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light emitted by a hot surface rather than visible light into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and can be overlaid on top of thermocouples, potentially doubling efficiency. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%, but have not been tested with actual radioisotopes. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells and silicon thermocouples degrade faster than thermocouples, especially in the presence of ionizing radiation.
Dynamic generators can provide power at more than 4 times the conversion efficiency of RTGs. NASA and DOE have been developing a next-generation radioisotope-fueled power source called the Stirling Radioisotope Generator (SRG) that uses free-piston Stirling engines coupled to linear alternators to convert heat to electricity. SRG prototypes demonstrated an average efficiency of 23%. Greater efficiency can be achieved by increasing the temperature ratio between the hot and cold ends of the generator. The use of non-contacting moving parts, non-degrading flexural bearings, and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibration can be eliminated as a concern by implementation of dynamic balancing or use of dual-opposed piston movement. Potential applications of a Stirling radioisotope power system include exploration and science missions to deep-space, Mars, and the Moon.
RTGs pose a risk of radioactive contamination: if the container holding the fuel leaks, the radioactive material may contaminate the environment.
For spacecraft, the main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy.
However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the Cassini-Huygens probe launched in 1997 estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its 3 RTGs (or from its 129 radioisotope heater units) during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million. If an accident which had the potential to cause contamination occurred during the launch phases (such as the spacecraft failing to reach orbit), the probability of contamination actually being caused by the RTGs was estimated at about 1 in 10. In any event, the launch was successful and Cassini-Huygens reached Saturn.
The plutonium 238 used in these RTGs has a half-life of 87.74 years, in contrast to the 24,110 year half-life of plutonium 239 used in nuclear weapons and reactors. A consequence of the shorter half-life is that plutonium 238 is about 275 times more radioactive than plutonium 239 (i.e. 17.3 Ci/g compared to 0.063 Ci/g). For instance, 3.6 kg of plutonium 238 undergoes the same number of radioactive decays per second as 1 tonne of plutonium 239. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same, plutonium 238 is around 275 times more toxic by weight than plutonium 239.
The alpha radiation emitted by either isotope will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk is the skeleton, the surface of which is likely to absorb the isotope, and the liver, where the isotope will collect and become concentrated.
There have been several known accidents involving RTG-powered spacecraft:
To minimize the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion- and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the Earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble.
The most recent accident involving a spacecraft RTG was the failure of the Russian Mars 96 probe launch on 16 November 1996. The two RTGs onboard carried in total 200 g of plutonium and are assumed to have survived reentry as they were designed to do. They are thought to now lie somewhere in a northeast-southwest running oval 320 km long by 80 km wide which is centred 32 km east of Iquique, Chile.
Many Beta-M RTGs produced by the Soviet Union to power lighthouses and beacons have become orphaned sources of radiation. Several of these units have been illegally dismantled for scrap metal resulting in the complete exposure of the Sr-90 source, fallen into the ocean, or have defective shielding due to poor design or physical damage. The US Department of Defense cooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used by terrorists to construct a dirty bomb.
28 U.S. space missions have safely flown radioisotope energy sources since 1961.
RTGs and nuclear power reactors use very different nuclear reactions. Nuclear power reactors use controlled nuclear fission. When an atom of U-235 or Pu-239 fuel fissions, neutrons are released that trigger additional fissions in a chain reaction at a rate that can be controlled with neutron absorbers. This is an advantage in that power can be varied with demand or shut off entirely for maintenance. It is also a disadvantage in that care is needed to avoid uncontrolled operation at dangerously high power levels.
Chain reactions do not occur in RTGs, so heat is produced at a fully predictable and steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. An accidental power excursion is impossible. On the other hand, heat generation cannot be varied with demand or shut off when not needed. Auxiliary power supplies (such as rechargeable batteries) may be needed to meet peak demand, and adequate cooling must be provided at all times including the prelaunch and early flight phases of a space mission.
There are no nuclear proliferation risks associated with plutonium-238. The same properties, primarily its high specific power, that make it a desirable RTG fuel make it useless in nuclear weapons. Pu-238 is fissionable, not fissile. It will occasionally spontaneously fission instead of undergoing alpha decay or it can be induced to fission with an external source of fast neutrons produced by various fusion reactions, but it cannot sustain the chain-reaction needed in a nuclear weapon fission primary. Because of its relatively high spontaneous fission rate compared with that of the fissile bomb fuel isotope Pu-239, its presence even as a contaminant would degrade performance by increasing the likelihood of a fizzle, a low yield caused by premature initiation of the chain reaction before optimum conditions have been reached. Any significant amounts of Pu-238 would also generate heat that would have to be continually dissipated until the bomb was used.
Pu-238 could in principle be used as the tertiary stage to boost the yield of a fission-fusion-fission (thermonuclear) weapon, but there is no reason to use it in this way. Natural or even depleted uranium will also fission with fast fusion neutrons, is far more readily available, and generates essentially no heat in storage.
RTG have been proposed for use on realistic interstellar precursor missions and interstellar probes. An example of this is the Innovative Interstellar Explorer (2003–current) proposal from NASA. A RTG using 241Am was proposed for this type of mission in 2002. This could support mission extensions up to 1000 years on the interstellar probe, because the power output would be more stable in the long-term than plutonium. Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products. An interstellar probe proposal from 1999 suggested using three advanced radioisotope power source (ARPS).
The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes. One mission proposed using the electricity to power ion engines, calling this method radioisotope electric propulsion (REP).
A typical RTG is powered by radioactive decay and features electricity from thermoelectric conversion, but for the sake of knowledge, some systems with some variations on that concept are included here:
MHW = Multi-Hundred Watt
|Name & Model||Used On (# of RTGs per User)||Maximum output||Radio-|
|Electrical (W)||Heat (W)|
|ASRG*||prototype design (not launched), Discovery Program||~140 (2x70)||~500||238Pu||~1||~34|
|GPHS-RTG||Cassini (3), New Horizons (1), Galileo (2), Ulysses (1)||300||4400||238Pu||7.8||55.9–57.8|
|MHW-RTG||LES-8/9, Voyager 1 (3), Voyager 2 (3)||160||2400||238Pu||~4.5||37.7|
|SNAP-9A||Transit 5BN1/2 (1)||25||525||238Pu||~1||12.3|
|SNAP-19||Nimbus-3 (2), Pioneer 10 (4), Pioneer 11 (4)||40.3||525||238Pu||~1||13.6|
|modified SNAP-19||Viking 1 (2), Viking 2 (2)||42.7||525||238Pu||~1||15.2|
|SNAP-27||Apollo 12–17 ALSEP (1)||73||1,480||238Pu||3.8||20|
|Buk (BES-5)**||US-As (1)||3000||100,000||235U||30||~1000|
|SNAP-10A***||SNAP-10A (1)||600||30,000||Enriched uranium||431|
*** The SNAP-10A used enriched uranium fuel, zirconium hydride as a moderator, liquid sodium potassium alloy coolant, and was activated or deactivated with beryllium reflectors. Reactor heat fed a thermoelectric conversion system for electrical production.
|Name & Model||Use||Maximum output||Radioisotope||Max fuel used|
|Electrical (W)||Heat (W)|
|Beta-M||Obsolete Soviet unmanned|
lighthouses & beacons
|IEU-1M||120 (180)||2200 (3300)||?||?||2(3) × 1050|
|Sentinel 25||Remote U.S. arctic monitoring sites||9–20||SrTiO3||0.54||907–1814|
Known spacecraft/nuclear power systems and their fate. Systems face a variety of fates, for example, Apollo's SNAP-27 were left on the Moon. Some other spacecraft also have small radioisotope heaters, for example each of the Mars Exploration Rovers have a 1 watt radioisotope heater. Spacecraft use different amounts of material, for example MSL Curiosity has 4.8 kg of plutonium-238 dioxide, while the Cassini spacecraft has 32.7 kg.
|Name and/or model||Launched||Fate/Location|
|MSL/Curiosity rover MMRTG (1)||2011||Mars Surface|
|Apollo 12 SNAP-27 ALSEP||1969||Lunar surface (Ocean of Storms)|
|Apollo 13 SNAP-27 ALSEP||1970||Earth re-entry (over Pacific nr Fiji)|
|Apollo 14 SNAP-27 ALSEP||1971||Lunar surface (Fra Mauro)|
|Apollo 15 SNAP-27 ALSEP||1971||Lunar surface (Hadley–Apennine)|
|Apollo 16 SNAP-27 ALSEP||1972||Lunar surface|
|Apollo 17 SNAP-27 ALSEP||1972||Lunar surface|
|Transit-4A SNAP-3B? (1)||1961||Earth orbit?|
|Transit 5A3 SNAP-3 (1)||1963||Earth orbit?|
|Transit 5BN-1 SNAP-3 (1)||1963||Earth orbit?|
|Transit 5BN-2 SNAP-9A (1)||1963||Earth orbit?|
|Transit 9 ?||1964||Earth orbit?|
|Transit 5B4 ?||1964||Earth orbit?|
|Transit 5B6 ?||1965||Earth orbit?|
|Transit 5B7 ?||1965||Earth orbit?|
|Transit 5BN-3 SNAP-9A (1)||1964||Failed to reach orbit|
|Nimbus-B SNAP-19 (2)||1968||Recovered after crash|
|Nimbus-3 SNAP-19 (2)||1969||Earth re-entry 1972|
|Pioneer 10 SNAP-19 (4)||1972||Ejected from Solar System|
|Pioneer 11 SNAP-19 (4)||1973||Ejected from Solar System|
|Viking 1 lander modified SNAP-19||1976||Mars surface (Chryse Planitia)|
|Viking 2 lander modified SNAP-19||1976||Mars surface|
|Cassini GPHS-RTG (3)||1997||Orbiting Saturn|
|New Horizons GPHS-RTG (1)||2006||Leaving the Solar System|
|Galileo GPHS-RTG (2),||1989||Jupiter atmospheric entry|
|Ulysses GPHS-RTG (1)||1990||Heliocentric orbit|
|LES-8 MHW-RTG||1976||Near geostationary orbit|
|LES-9 MHW-RTG||1976||Near geostationary orbit|
|Voyager 1 MHW-RTG(3)||1977||Ejected from Solar System|
|Voyager 2 MHW-RTG(3)||1977||Ejected from Solar System|
For comparison, some of various space-flown fission reactor systems:
|Name or model||Fate/Location|
|SNAP-10A||Earth orbit (900+ km altitude)|
|Cosmos 469 BES-5 Buk|
|Cosmos 516||High orbited 1972|
|Cosmos 626||Earth orbit|
|Cosmos 954||Earth re-entry 1978 (over Canada)|
|Cosmos 1176||11788/11971 Earth orbit 870–970 km|
|Cosmos 1402||Earth re-entry 1983 (South Atlantic)|
|Cosmos 1607||High orbited 1985|
|Cosmos 1670||High orbited 1985|
|Cosmos 1677||High orbited 1985|
|Cosmos 1682||High orbited 1986|
|Cosmos 1736||High orbited 1986|
|Cosmos 1771||High orbited 1986|
|Cosmos 1818 TEU-5 Topol||Earth orbit|
|Cosmos 1867 TEU-5 Topol|
|Cosmos 1900||High orbited 1987|
|Cosmos 1932||Earth orbit 800–900 km|
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