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| [[File:Cutdrawing of an GPHS-RTG.jpg|thumb|450px|Diagram of an RTG used on the [[Cassini probe]]]]
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| A '''radioisotope thermoelectric generator''' ('''RTG''', '''RITEG''') is an [[electrical generator]] that uses an array of [[thermocouple]]s to convert the [[Decay heat|heat released by the decay]] of a suitable [[radioactivity|radioactive]] material into [[electricity]] by the [[Seebeck effect]].
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| RTGs have been used as power sources in [[satellite]]s, [[space probe]]s and such unmanned remote facilities as a series of [[lighthouses]] that the former Soviet Union erected inside the Arctic Circle. RTGs are usually the most desirable power source for [[robotic]] or unmaintained situations that need a few hundred [[watt]]s (or less) of power for durations too long for [[fuel cell]]s, batteries, or generators to provide economically and in places where [[solar cell]]s are impractical. Safely using RTGs requires containing the [[radioisotopes]] long after the productive life of the unit.
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| == History ==
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| [[File:Radioisotope thermoelectric generator plutonium pellet.jpg|thumb|300px|A pellet of [[Plutonium-238|<sup>238</sup>Pu]]O<sub>2</sub> to be used in an RTG for either the [[cassini spacecraft|Cassini]] or [[galileo spacecraft|Galileo]] mission. The initial output is 62 watts. The pellet [[incandescence|glows red hot]] because of the heat generated by the radioactive decay (primarily α). This photo was taken after insulating the pellet under a [[graphite]] blanket for several minutes and then removing the blanket.]]
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| 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."<ref>{{cite journal | url = http://lakdiva.org/clarke/1945ww/1945ww_feb_058.html | title = Peacetime Uses for V2 | publisher = [[Wireless World]]| volume = 2 | number =2 | date = February 1945 | page = 58 }}</ref><ref>{{cite journal | url = http://lakdiva.org/clarke/1945ww/ | title = Peacetime Uses for V2: scanned image of the original Letter to the Editor | publisher = [[Wireless World]]| volume = 2 | number =2 | date = February 1945}}</ref>
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| 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.<ref>{{cite web |url=http://www.osti.gov/bridge/servlets/purl/4807049-6bvOmJ/4807049.pdf |title=Nuclear Battery-Thermocouple Type Summary Report |publisher=[[United States Atomic Energy Commission]] |date=1 October 1960 |publicationdate=15 January 1962}}</ref>
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| The first RTG launched into space by the United States was [[Systems for Nuclear Auxiliary Power|SNAP 3]] in 1961, aboard the Navy [[Transit (satellite)|Transit 4A spacecraft]]. One of the first terrestrial uses of RTGs was in 1966 by the US Navy at uninhabited [[Fairway Rock#The Radioisotope Thermoelectric Generator|Fairway Rock]] in Alaska. RTGs were used at that site until 1995.
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| 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 [[Photovoltaic module|solar panels]] impractical. As such, they were used with [[Pioneer 10]], [[Pioneer 11]], [[Voyager 1]], [[Voyager 2]], [[Galileo probe|Galileo]], [[Ulysses probe|Ulysses]], [[Cassini-Huygens|Cassini]], [[New Horizons]] and the [[Mars Science Laboratory#Power source|Mars Science Laboratory]]. RTGs were used to power the two [[Viking program|Viking]] landers and for the scientific experiments left on the Moon by the crews of [[Apollo program|Apollo]] [[Apollo 12|12]] through [[Apollo 17|17]] (SNAP 27s). Because the [[Apollo 13#Spacecraft location|Apollo 13]] moon landing was aborted, its RTG rests in the [[Pacific Ocean|South Pacific ocean]], in the vicinity of the [[Tonga Trench]].<ref>{{cite web | url = http://fti.neep.wisc.edu/neep602/SPRING00/lecture39.pdf | title = General Safety Considerations | format = pdf lecture notes | publisher = Fusion Technology Institute, [[University of Wisconsin–Madison]] | date = Spring 2000 | page = 21 }}</ref> RTGs were also used for the [[Nimbus program|Nimbus]], [[Transit (satellite)|Transit]] and [[Lincoln Experimental Satellite|LES]] satellites. By comparison, only a few space vehicles have been launched using full-fledged [[nuclear reactor]]s: the Soviet [[US-A|RORSAT]] series and the American [[SNAP-10A]].
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| In addition to spacecraft, the [[Soviet Union]] constructed many unmanned lighthouses and navigation beacons powered by RTGs.<ref name="Bellona">{{cite web |url=http://bellona.no/bellona.org/english_import_area/international/russia/navy/northern_fleet/incidents/37598 |title=Radioisotope Thermoelectric Generators |publisher=[[Bellona Foundation|Bellona]] |date=2 April 2005 |accessdate=2013-05-07}}</ref> Powered by [[Strontium-90|strontium-90 (<sup>90</sup>Sr)]], they are very reliable and provide a steady source of power. Critics{{Who|date=August 2011}} 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.<ref name="Bellona" /> In another case, three woodsmen in [[Georgia (country)|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.<ref name="Ref_c">{{cite web|title=IAEA Bulletin Volume 48, No.1 – Remote Control: Decommissioning RTGs|publisher=Malgorzata K. Sneve|url=http://www.iaea.org/Publications/Magazines/Bulletin/Bull481/pdfs/rtg.pdf|accessdate=11 July 2009}}</ref>
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| 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.<ref name="Ref_d">{{cite web|title=Report by Minister of Atomic Energy Alexander Rumyantsev at the IAEA conference "Security of Radioactive Sources," Vienna, Austria. March 11th 2003 (Internet Archive copy)| url=http://www.iaea.org/worldatom/Press/Focus/RadSources/statement_rus.pdf| archiveurl=http://web.archive.org/web/20030806043406/http://www.iaea.org/worldatom/Press/Focus/RadSources/statement_rus.pdf| archivedate=6 August 2003| accessdate=10 October 2009}}</ref>
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| The [[United States Air Force]] uses RTGs to power remote sensing stations for ''Top-ROCC'' and ''Save-Igloo'' radar systems predominantly located in [[Alaska]].<ref name="Ref_e">[http://www10.antenna.nl/wise/379-80/3724.html Alaska fire threatens air force nukes], [[World Information Service on Energy|WISE]]</ref>
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| In the past, small "plutonium cells" (very small <sup>238</sup>Pu-powered RTGs) were used in implanted [[Artificial pacemaker|heart pacemakers]] to ensure a very long "battery life".<ref name="Ref_f">[http://osrp.lanl.gov/pacemakers.shtml Nuclear-Powered Cardiac Pacemakers], [[Los Alamos National Laboratory|LANL]]</ref> {{As of|2004}}, about 90 were still in use.
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| == Design ==
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| 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). [[Thermocouple]]s 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.
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| A thermocouple is a [[thermoelectricity|thermoelectric]] device that converts [[thermal energy]] directly into [[electrical energy]] using the [[Peltier-Seebeck effect|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 [[temperature]]s, an electric current will flow in the loop.
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| == Fuels ==
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| [[File:RTG radiation measurement.jpg|thumb|right|300px|Inspection of [[Cassini-Huygens|Cassini spacecraft]] RTGs before launch]]
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| [[File:New Horizons 1.jpg|thumb| 300px|''[[New Horizons]]'' in [http://maps.google.com/maps?q=28.5100,+-80.6475+(Payload+Hazardous+Servicing+Facility)&t=k&iwloc=A&hl=en assembly hall] ]]
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| === Criteria ===
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| The radioactive material used in RTGs must have several characteristics:
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| # It should produce high energy radiation. Energy release per decay is proportional to power production per [[Mole (unit)|mole]]. [[Alpha decay]]s in general release about 10 times as much energy as the [[beta decay]] of strontium-90 or cesium-137.
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| # Radiation must be of a type easily absorbed and transformed into thermal radiation, preferably [[Alpha particle|alpha radiation]]. [[Beta particle|Beta radiation]] can emit considerable [[gamma radiation|gamma]]/[[X-rays|X-ray radiation]] through [[bremsstrahlung]] secondary radiation production and therefore requires heavy shielding. Isotopes must not produce significant amounts of gamma, [[neutron radiation]] or penetrating radiation in general through other [[decay mode]]s or [[decay chain]] products.
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| # Its [[half-life]] must be so long that it will release energy at a relatively continuous rate for a reasonable amount of time. The amount of energy released per time ([[Power (physics)|power]]) of a given quantity is inversely proportional to half-life. An isotope with twice the half-life and the same energy per decay will release power at half the rate per [[Mole (unit)|mole]]. Typical half-lives for [[radioisotopes]] used in RTGs are therefore several decades, although [[isotopes]] with shorter half-lives could be used for specialized applications.
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| # For spaceflight use, the fuel must produce a large amount of power per [[mass]] and [[volume]] ([[density]]). Density and weight are not as important for terrestrial use unless size is also restricted. The [[decay energy]] can be calculated if the energy of radioactive radiation or the mass loss before and after radioactive decay is known.
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| === Selection of isotopes ===
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| The first two criteria limit the number of possible fuels to fewer than 30 atomic isotopes<ref name=NPE3>[https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Radioisotopes%20Power%20Production.pdf NPE chapter 3 Radioisotope Power Generation]</ref> within the entire [[table of nuclides]]. [[Plutonium-238]], [[Curium|curium-244]] and [[strontium-90]] are the most often cited candidate isotopes, but other such isotopes as [[polonium-210]], [[promethium-147]], [[caesium-137]], [[cerium]]-144, [[ruthenium-106]], [[cobalt-60]], [[curium]]-242, [[americium]]-241 and [[thulium]] isotopes have also been studied.
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| === <sup>238</sup>Pu, <sup>90</sup>Sr ===
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| [[Plutonium-238]] has the lowest shielding requirements and longest half-life; its power 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. <sup>238</sup>Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is needed in a <sup>238</sup>Pu RTG, as the casing itself is adequate. <sup>238</sup>Pu has become the most widely used fuel for RTGs, in the form of [[plutonium(IV) oxide]] (PuO<sub>2</sub>). <sup>238</sup>Pu has a half-life of 87.7 years, reasonable power density, and exceptionally low gamma and neutron radiation levels.
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| [[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 <sup>238</sup>Pu, 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 <sup>238</sup>Pu, which results in lower RTG efficiency. <sup>90</sup>Sr is a high yield waste product of nuclear fission and is available in large quantities at a low price.<ref>Rod Adams, [http://atomicinsights.com/1996/09/rtg-heat-sources-two-proven-materials.html RTG Heat Sources: Two Proven Materials], 1 Sep 1996, Retrieved 20 Jan 2012.</ref>
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| === <sup>210</sup>Po ===
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| 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 decay#Radioactive decay rates|radioactive activity]], but has limited use because of its very short half-life of 138 days. A kilogram of pure <sup>210</sup>Po in the form of a cube would be about 48 mm (about 2 inches) on a side and emit about [[Decay energy|140 kW]].
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| === <sup>242</sup>Cm, <sup>244</sup>Cm, <sup>241</sup>Am ===
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| [[Curium-242]] and curium-244 have also been studied as well, but require heavy shielding for gamma and neutron radiation produced from [[spontaneous fission]].
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| [[Americium-241]] is a potential candidate isotope with a longer half-life than <sup>238</sup>Pu: <sup>241</sup>Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the power density of <sup>241</sup>Am is only 1/4 that of <sup>238</sup>Pu, and <sup>241</sup>Am produces more penetrating radiation through decay chain products than <sup>238</sup>Pu 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 <sup>238</sup>Pu requires less. With a current global shortage<ref name="Ref_f">Nell Greenfield-Boyce, [http://www.npr.org/templates/story/story.php?storyId=113223613 Plutonium Shortage Could Stall Space Exploration], [[NPR]], 28 Sep 2009, retrieved 2 Nov 2010.</ref> of <sup>238</sup>Pu, a closer look is being given to <sup>241</sup>Am.
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| == Life span ==
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| [[File:Soviet RTG.jpg|thumb| 300px|right|[[Strontium-90|<sup>90</sup>Sr]]-powered Soviet RTGs in dilapidated and vandalized condition.]]
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| Most RTGs use <sup>238</sup>Pu, which decays with a half-life of 87.7 years. RTGs using this material will therefore diminish in power output by 1−0.5{1/87.74} = 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%.<ref>{{cite web|url=http://voyager.jpl.nasa.gov/mission/weekly-reports/index.htm |title=Voyager Mission Operations Status Reports |publisher=Voyager.jpl.nasa.gov web|accessdate=24 July 2011}}</ref>
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| This life span was of particular importance during the [[Galileo (spacecraft)|Galileo]] mission. Originally intended to launch in 1986, it was delayed by the [[Space Shuttle Challenger]] [[STS-51L|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.{{Citation needed|date=December 2011}}
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| == Efficiency ==
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| RTGs use thermoelectric couples or "[[thermocouple]]s" 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.
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| {{Thermoelectric effect|cTopic=Applications}}
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| 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 <sup>210</sup>Po 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. | |
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| [[Thermophotovoltaic cell]]s 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%,<ref name="Ref_g">[http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213980.pdf An Overview and Status of NASA's Radioisotope Power Conversion Technology NRA], NASA, November 2005</ref> 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.
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| 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 engine]]s 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 [[Flexure bearing|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.
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| The increased efficiency of the SRG may be demonstrated by a theoretical comparison of thermodynamic properties, as follows. These calculations are simplified and do not account for the decay of thermal power input due to the long half-life of the radioisotopes used in these generators. The assumptions for this analysis include that both systems are operating at steady state under the conditions observed in experimental procedures (see table below for values used). Both generators can be simplified to heat engines to be able to compare their current efficiencies to their corresponding Carnot efficiencies. The system is assumed to be the components, apart from the heat source and heat sink.,<ref>{{cite web|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38757/1/06-0429.pdf |title=New Thermoelectric Materials and Devices for Terrestrial Power Generators |format=PDF |date= |accessdate=2013-05-07}}</ref><ref>http://large.stanford.edu/courses/2011/ph241/chenw1/docs/TM-2005-213981.pdf</ref><ref>http://solarsystem.nasa.gov/rps/docs/ASRGfacts2_10rev3_21.pdf</ref>
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| The thermal efficiency, denoted η<sub>th</sub>, is given by:
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| :<math>\eta_{th} = \frac{\text{Desired Output}}{\text{Required Input}} = \frac{W'_{out}}{Q'_{in}}</math>
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| Where primes ( ' ) denote the time derivative.
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| From a general form of the First Law of Thermodynamics, in rate form:
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| <math>\Delta E'^{\mathrm{sys}}=Q'_{in}+ W'_{in} - Q'_{out} - W'_{out}\,</math>
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| Assuming the system is operating at steady state and <math>W'_{in}=0 \,</math>,
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| <math> W'_{out} = Q'_{in} - Q'_{out} \,</math>
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| η<sub>th</sub>, then, can be calculated to be 110 W / 2000 W = 5.5% (or 140 W / 500 W = 28% for the SRG). Additionally, the Second Law efficiency, denoted η<sub>II</sub>, is given by:
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| :<math>\eta_{II} = \frac{\eta_{th}}{\eta_{th,rev}}</math>
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| <br /> | |
| Where η<sub>th,rev</sub> is the Carnot efficiency, given by:
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| :<math>\eta_{th} = 1 - \frac{T_{heat sink}}{T_{heat source}}</math>
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| <br /> | |
| In which T<sub>heat sink</sub> is the external temperature (which has been measured to be 510 K for the MMRTG (Multi-Mission RTG){{which|date=May 2013}} and 363 K for the SRG) and T<sub>heat source</sub> is the temperature of the MMRTG{{which|date=May 2013}}, assumed 823 K (1123 K for the SRG). This yields a Second Law efficiency of 14.46% for the MMRTG{{which|date=May 2013}} (or 41.37% for the SRG).
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| == Safety ==
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| [[File:RTGmodule.png|thumb|right|350px|Diagram of a stack of [[General Purpose Heat Source|general purpose heat source]] modules as used in RTGs]]
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| === Radioactive contamination ===
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| RTGs pose a risk of [[radioactive contamination]]: if the container holding the fuel leaks, the radioactive material may contaminate the environment.
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| 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.<ref name="Ref_h">[http://www.cnn.com/TECH/space/9908/16/cassini.flyby/ Nuclear-powered NASA craft to zoom by Earth on Tuesday], CNN news report, 16 August 1999</ref><ref name="Ref_i">[http://www.mtexpress.com/index2.php?issue_date=07-22-2005&ID=2005104284 Valley says pee-eww to plutonium plan], Idaho Mountain Express and Guide, 22 July 2005</ref>
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| 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 unit]]s) 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.<ref name="Ref_j">[http://saturn.jpl.nasa.gov/spacecraft/safety/fseis4.pdf Cassini Final Supplemental Environmental Impact Statement], Chapter 4, NASA, September 1997 ([http://saturn.jpl.nasa.gov/spacecraft/safety-eis.cfm Links to other chapters and associated documents])</ref> 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.<ref name="Ref_k">[http://saturn.jpl.nasa.gov/spacecraft/safety/fseisd.pdf Cassini Final Supplemental Environmental Impact Statement], Appendix D, Summary of tables of safety analysis results, Table D-1 on page D-4, see conditional probability column for GPHS-RTG</ref> In any event, the launch was successful and Cassini–Huygens reached [[Saturn]].
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| 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 [[Nuclear reactor|reactors]]. A consequence of the shorter half-life is that plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. {{convert|17.3|Ci|GBq|lk=on}}/[[gram|g]] compared to {{convert|0.063|Ci}}/g<ref name="Ref_l">[http://www.ieer.org/fctsheet/pu-props.html Physical, Nuclear, and Chemical, Properties of Plutonium], IEER Factsheet</ref>). For instance, 3.6 [[kilogram|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,<ref name="Ref_m">[http://www.ead.anl.gov/pub/doc/tbl1-rad-rc.pdf Mortality and Morbidity Risk Coefficients for Selected Radionuclides], Argonne National Laboratory{{dead link|date=May 2013}}</ref> plutonium-238 is around 275 times more toxic by weight than plutonium-239.
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| 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.
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| There have been several known accidents involving RTG-powered spacecraft:
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| # The first one was a launch failure on 21 April 1964 in which the U.S. [[Transit (satellite)|Transit-5BN-3]] navigation satellite failed to achieve orbit and burnt up on re-entry north of [[Madagascar]].<ref name="Transit">{{cite web|url=http://www.astronautix.com/craft/transit.htm |title=Transit |publisher=Encyclopedia Astronautica |accessdate=2013-05-07}}</ref> The {{convert|17000|Ci|TBq|abbr=on}} plutonium metal fuel in its [[Systems Nuclear Auxiliary Power Program|SNAP]]-9a RTG was injected into the atmosphere over the Southern Hemisphere where it burnt up, and traces of plutonium-238 were detected in the area a few months later.
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| # The second was the Nimbus B-1 weather satellite whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Launched from the [[Vandenberg Air Force Base]], its SNAP-19 RTG containing relatively inert [[plutonium dioxide]] was recovered intact from the seabed in the [[Santa Barbara Channel]] five months later and no environmental contamination was detected.<ref>
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| {{cite book|title = Space Nuclear Power|author = A. Angelo Jr. and D. Buden|year = 1985|publisher = Krieger Publishing Company|isbn = 0-89464-000-3 }}</ref>
| |
| # In 1969 the launch of the first [[Lunokhod]] lunar rover mission failed, spreading [[polonium 210]] over a large area of Russia <ref name="Energy_Resources_for_Space_Missions">{{cite web|url=http://www.spacesafetymagazine.com/2014/01/16/energy-resources-space-missions/ |title=Energy Resources for Space Missions |publisher=Space Safety Magazine |accessdate=2014-01-18}}</ref>
| |
| # The failure of the [[Apollo 13]] mission in April 1970 meant that the [[Lunar Module]] reentered the atmosphere carrying an RTG and burnt up over [[Fiji]]. It carried a SNAP-27 RTG containing {{convert|44,500|Ci|TBq|abbr=on}} of plutonium dioxide which survived reentry into the Earth's atmosphere intact, as it was designed to do, the trajectory being arranged so that it would plunge into 6–9 kilometers of water in the [[Tonga trench]] in the [[Pacific Ocean]]. The absence of plutonium-238 contamination in atmospheric and seawater sampling confirmed the assumption that the cask is intact on the seabed. The cask is expected to contain the fuel for at least 10 half-lives (i.e. 870 years). The US Department of Energy has conducted seawater tests and determined that the graphite casing, which was designed to withstand reentry, is stable and no release of plutonium should occur. Subsequent investigations have found no increase in the natural background radiation in the area. The Apollo 13 accident represents an extreme scenario because of the high re-entry velocities of the craft returning from [[Geospace|cis-lunar space]] (the region between Earth's atmosphere and the Moon). This accident has served to validate the design of later-generation RTGs as highly safe.
| |
| [[File:ALSEP Apollo 14 RTG.jpg|thumb|right|350px|A [[Systems Nuclear Auxiliary Power Program|SNAP]]-27 RTG deployed by the astronauts of [[Apollo 14]] identical to the one lost in the reentry of [[Apollo 13]]]]
| |
| | |
| There were also five failures involving Soviet or Russian spacecraft which were carrying nuclear reactors rather than RTGs between 1973 and 1993 (see [[RORSAT]]).<ref name="Ref_p">[http://www.iaass.org/pdf/FINAL%20DRAFT1_IAASS%20Stragtegic%20Plan%20Complete_Rev4.pdf IASS Strategic Plan 2005–2009], page 39{{dead link|date=May 2013}}</ref> In 1978, [[Cosmos 954]] accidentally reentered Earth's atmosphere, strewing radioactive uranium 235 over 124,000 kilometers in northern Canada, and exposing several people to harmful radiation. This was the only time the [[Liability Convention|1972 UN Liability Convention]] has been invoked.<ref name="Energy_Resources_for_Space_Missions">{{cite web|url=http://www.spacesafetymagazine.com/2014/01/16/energy-resources-space-missions/ |title=Energy Resources for Space Missions |publisher=Space Safety Magazine |accessdate=2014-01-18}}</ref>
| |
| | |
| 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 [[solubility|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]].<ref name="Ref_q">[http://nssdc.gsfc.nasa.gov/planetary/text/mars96_timeline.txt Mars 96 timeline], NASA</ref>
| |
| | |
| Many [[Beta-M]] RTGs produced by the Soviet Union to power [[lighthouse]]s and [[beacon]]s have become [[orphan source|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 [[terrorist]]s to construct a [[dirty bomb]].<ref name="Bellona" />
| |
| | |
| 28 U.S. space missions have safely flown radioisotope energy sources since 1961.<ref name="Ref_s">{{cite web|url=http://sse.jpl.nasa.gov/scitech/display.cfm?ST_ID=2149 |title=NASA: Enabling Exploration: Small Radioisotope Power Systems |publisher=Sse.jpl.nasa.gov |accessdate=2013-05-07}}{{dead link|date=May 2013}}</ref>
| |
| | |
| === Nuclear fission ===
| |
| {{original research|section|date=August 2013}}
| |
| 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.
| |
| | |
| While running, nuclear reactors create high levels of particularly dangerous radiation, like high-energy neutrons. After shutdown of a reactor, power levels drop quickly to a few percent of the rated power, and drop further to around one per mille within one year. If a reactor is still off ("cold") at launch, even, if is destroyed in a launch accident, the amounts of radiation released will be rather low, as only unused fuel will be set free. Even, if a space reactor is destroyed after having operated for some time on orbit in a reentry accident, the amount of long-term radiation released is much less compared to an equal power rating RTG, due to the aforementioned quick power drop.{{citation needed|date=August 2013}}
| |
| | |
| Chain reactions do not occur in RTGs, so heat is produced at an unchangeable, though steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. An accidental power excursion is impossible. However, if a launch or re-entry accident occurs and the fuel is dispersed, the combined power output of the now [[radionuclides]] set free does not drop. In an RTG, heat generation cannot be varied with demand or shut off when not needed. Therefore, 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 pre-launch and early flight phases of a space mission.
| |
| | |
| Plutonium-238 is a [[Fissile#Fissile vs fissionable|fissionable]] material. While it cannot be used in a conventional nuclear reactor, as plutonium-238 is not [[Fissile#Fissile vs fissionable|fissile]] with thermal (or slow) neutrons, it is fissionable with fast neutrons, as they occur during the chain reaction of a nuclear bomb, or inside proposed "fast" neutron reactors. The [[critical mass]] of plutonium-238 is similar to that of plutonium-239,<ref name=WSRC-MS-99-00313>{{cite web|last=A. Blanchard et al.|title=Updated Critical Mass Estimates for Plutonium-238|url=http://sti.srs.gov/fulltext/ms9900313/ms9900313.html|accessdate=29 August 2013}}</ref> the fuel of the Nagasaki nuclear bomb. Some properties of plutonium-238, namely its high decay heat and its (as compared to plutonium-239) high neutron production rate, make building a Pu-238-bomb rather complex. Nonetheless, even a low-yield Pu-238 bomb would release much more intense mid-term (with half-lives between one year and one hundred years) radiation, as even a high-yield Pu-239 bomb with the same amount of Plutonium would do.{{citation needed|date=August 2013}}
| |
| | |
| While a Pu-238 bomb would likely be a [[Fizzle (nuclear test)|Fizzle]] with respect to its equivalent TNT yield, it would likely be a very effective [[dirty bomb]].{{citation needed|date=August 2013}} While plutonium-238 is quite safe outside of the human body due to the short reach of the α radiation, it becomes very unsafe when it enters the human body, for example by inhalation of particulates: Due to the short reach of the α-rays, radiation damage to the tissue surrounding such particulates is very high, increasing the risk of cancer.
| |
| | |
| == RTG for interstellar probes ==
| |
| RTG have been proposed for use on realistic interstellar precursor missions and [[interstellar probe]]s.<ref name=ip/> An example of this is the [[Innovative Interstellar Explorer]] (2003–current) proposal from NASA.<ref name="Ref_t">{{cite web|url=http://interstellarexplorer.jhuapl.edu/index.php |title=Innovative Interstellar Probe |publisher=[[Applied Physics Laboratory|JHU/APL]] |accessdate=22 October 2010}}</ref>
| |
| A RTG using <sup>241</sup>Am was proposed for this type of mission in 2002.<ref name=ip>[http://www.niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/McNutt_Ralph.pdf Ralph L. McNutt, et all – '''Interstellar Explorer''' (2002) – Johns Hopkins University] (.pdf)</ref> 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.<ref name=ip/> Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products.<ref name=ip/> An interstellar probe proposal from 1999 suggested using three [[advanced radioisotope power source]] (ARPS).<ref name=ipjpl>{{cite web|url=http://interstellar.jpl.nasa.gov/interstellar/probe/index.html |title=Interstellar Probe |publisher=NASA/JPL |date=5 February 2002 |accessdate=22 October 2010}}</ref>
| |
| | |
| The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.<ref name=ip/> One mission proposed using the electricity to power [[ion engines]], calling this method [[radioisotope electric propulsion]] (REP).<ref name=ip/>
| |
| | |
| == Models ==
| |
| 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:
| |
| | |
| === Space ===
| |
| | |
| {| class="wikitable"
| |
| |-
| |
| !rowspan="2"|Name & Model
| |
| !rowspan="2"|Used On (# of RTGs per User)
| |
| !colspan="2"| Maximum output
| |
| !rowspan="2"|Radio-<br />isotope
| |
| !rowspan="2"|Max fuel<br />used (kg)
| |
| !rowspan="2"|Mass (kg)
| |
| |-
| |
| !Electrical ([[watt|W]]) ||Heat (W)
| |
| |-
| |
| |[[Advanced Stirling Radioisotope Generator|ASRG]]* ||prototype design (not launched), [[Discovery Program]] || ~140 (2x70) || ~500 || [[Plutonium-238|<sup>238</sup>Pu]] || ~1 || ~34
| |
| |-
| |
| |[[Multi-Mission Radioisotope Thermoelectric Generator|MMRTG]] ||[[Mars Science Laboratory|MSL/Curiosity rover]] || ~110 || ~2000 || <sup>238</sup>Pu || ~4 || <45
| |
| |-
| |
| |[[GPHS-RTG]] || [[Cassini-Huygens|Cassini (3)]], [[New Horizons|New Horizons (1)]], [[Galileo probe|Galileo (2)]], [[Ulysses probe|Ulysses (1)]] || 300 || 4400 || <sup>238</sup>Pu || 7.8 || 55.9–57.8<ref name="GLB"/>
| |
| |-
| |
| |[[MHW-RTG]] || [[Lincoln Experimental Satellite|LES-8/9]], [[Voyager 1|Voyager 1 (3)]], [[Voyager 2|Voyager 2 (3)]] || 160<ref name="GLB"/> || 2400<ref name="tose">http://www.totse.com/en/technology/space_astronomy_nasa/spacnuke.html</ref> || <sup>238</sup>Pu || ~4.5 || 37.7<ref name="GLB"/>
| |
| |-
| |
| |[[Systems Nuclear Auxiliary Power Program|SNAP-3B]] || [[Transit (satellite)|Transit-4A]] (1) || 2.7<ref name="GLB"/>|| 52.5 || <sup>238</sup>Pu || ? || 2.1<ref name="GLB"/>
| |
| |-
| |
| |SNAP-9A || [[Transit (satellite)|Transit 5BN1/2]] (1) || 25<ref name="GLB"/>|| 525<ref name="tose"/> || <sup>238</sup>Pu || ~1 || 12.3<ref name="GLB"/>
| |
| |-
| |
| |SNAP-19 || [[Nimbus program|Nimbus-3]] (2), [[Pioneer 10|Pioneer 10 (4)]], [[Pioneer 11|Pioneer 11 (4)]] || 40.3<ref name="GLB">[http://www.fas.org/nuke/space/bennett0706.pdf "Space Nuclear Power"] G.L.Bennett 2006</ref>|| 525 || <sup>238</sup>Pu || ~1 || 13.6<ref name="GLB"/>
| |
| |-
| |
| |modified SNAP-19 || [[Viking program|Viking 1 (2), Viking 2 (2)]] || 42.7<ref name="GLB"/>|| 525 || <sup>238</sup>Pu || ~1 || 15.2<ref name="GLB"/>
| |
| |-
| |
| |SNAP-27 || [[Project Apollo|Apollo 12–17]] [[ALSEP]] (1) || 73 || 1,480 || <sup>238</sup>Pu<ref name=NASM>{{cite web|title=SNAP-27|url=http://www.nasm.si.edu/exhibitions/attm/la.s27.1.html|publisher=[[National Air and Space Museum{{!}}Smithsonian National Air and Space Museum]]|accessdate=13 September 2011}}</ref>|| 3.8 || 20
| |
| |-
| |
| |[[BES-5|Buk (BES-5)]]** || [[US-A]]s (1) || 3000 || 100,000 || <sup>235</sup>U || 30 || ~1000
| |
| |-
| |
| |SNAP-10A*** || [[SNAP-10A]] (1) || 600<ref name=doe1>{{cite web|url=http://www.etec.energy.gov/History/Major-Operations/SNAP-Overview.html|title=SNAP Overview|publisher=USDOE ETEC|accessdate=4 April 2010}}</ref> || 30,000 || Enriched uranium || || 431
| |
| |}
| |
| | |
| <nowiki>*</nowiki> The ASRG is not really a RTG, it uses a [[Stirling engine|stirling]] power device that runs on radioisotope (see [[stirling radioisotope generator]])
| |
| | |
| <nowiki>**</nowiki> The BES-5 Buk ([[:ru:Ядерные реакторы на космических аппаратах#Бук|БЭС-5]]) reactor was a fast breeder reactor which used thermocouples based on semiconductors to convert heat directly into electricity.<ref>{{cite web|title=Use of nuclear space technology of direct energy conversion for terrestrial application|url=http://www.iaea.org/inisnkm/nkm/aws/fnss/abstracts/abst_1172_13.html|publisher=International Atomic Energy Agency, Vienna (Austria)|accessdate=14 September 2011|author=Chitaykin, V.I|coauthors=Meleta, Ye.A.; Yarygin, V.I.; Mikheyev, A.S.; Tulin, S.M.|pages=178–185}}</ref><ref>{{cite web|url=http://world-nuclear.org/info/inf82.html|title=Nuclear Reactors for Space|accessdate=14 September 2011}}</ref>
| |
| | |
| <nowiki>***</nowiki> 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.<ref name=doe1/> Reactor heat fed a thermoelectric conversion system for electrical production.<ref name=doe1/>
| |
| | |
| === Terrestrial ===
| |
| {| class="wikitable"
| |
| |-
| |
| !rowspan="2"|Name & Model
| |
| !rowspan="2"|Use
| |
| !colspan="2"| Maximum output
| |
| !rowspan="2"|Radioisotope
| |
| !rowspan="2"|Max fuel used<br /> (kg)
| |
| !rowspan="2"|Mass (kg)
| |
| |-
| |
| !Electrical (W) ||Heat (W)
| |
| |-
| |
| |[[Beta-M]] || rowspan=8|Obsolete Soviet unmanned <br>lighthouses & beacons || 10 || 230 || [[Strontium-90|<sup>90</sup>Sr]] || 0.26 || 560
| |
| |-
| |
| |Efir-MA || 30 || 720 || ? || ? || 1250
| |
| |-
| |
| |IEU-1 || 80 || 2200 || ? || ? || 2500
| |
| |-
| |
| |IEU-2 || 14 || 580 || ? || ? || 600
| |
| |-
| |
| |Gong || 18 || 315 || ? || ? || 600
| |
| |-
| |
| |Gorn || 60 || 1100 || <sup>90</sup>Sr || ? || 1050
| |
| |-
| |
| |IEU-2M || 20 || 690 || ? || ? || 600
| |
| |-
| |
| |IEU-1M || 120 (180) || 2200 (3300) || ? || ? || 2(3) × 1050
| |
| |-
| |
| |Sentinel 25<ref name=OTA>{{cite web
| |
| |title = Power Sources for Remote Arctic Applications
| |
| |date = June 1994
| |
| |location = Washington, DC
| |
| |publisher = U.S. Congress, Office of Technology Assessment
| |
| |url = http://govinfo.library.unt.edu/ota/Ota_1/DATA/1994/9423.PDF
| |
| |id = OTA-BP-ETI-129 }}</ref> || rowspan=2|Remote U.S. arctic monitoring sites || 9–20 || || [[strontium titanate|SrTiO<sub>3</sub>]] || 0.54 || 907–1814
| |
| |-
| |
| |Sentinel 100F<ref name=OTA/> || 53 || || Sr<sub>2</sub>TiO<sub>4</sub> || 1.77 || 1234
| |
| |}
| |
| | |
| ===Nuclear power systems in space===
| |
| 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.<ref name=storms>{{cite book|url=http://books.google.com/books?id=vgrGPWSy4PgC&pg=PA269&lpg=PA269&dq=Transit+4A+re-entry&source=bl&ots=rMGkzgAd6N&sig=WF3AjIm2YdiL-4oobuY2VhIiYUc&hl=en&ei=OUm5TovSJ63-2QXKuYCnBw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCQQ6AEwAQ#v=onepage&q=Transit%204A%20re-entry&f=false |title=David Harland – '''Apollo 12 – On the Ocean of Storms''' (2010) – Page 269 of 522 (Google books link) |publisher=Springer |accessdate=2013-05-07}}</ref> 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 dioxide|plutonium-238 dioxide]],<ref name="LaunchNuclearSafety">{{cite web | url=http://www.nasa.gov/pdf/604332main_APP%20MSL%20Launch%20Nuclear%20Safety%20FS%203-2-11.pdf | title=Mars Science Laboratory Launch Nuclear Safety |publisher=NASA/JPL/DoE |date=2 March 2011 |accessdate=28 November 2011}}</ref> while the [[Cassini-Huygens|Cassini]] spacecraft has 32.7 kg.<ref name="Krivobok">Ruslan Krivobok: [http://en.rian.ru/analysis/20091111/156797969.html Russia to develop nuclear-powered spacecraft for Mars mission]. Ria Novosti, 11 November 2009, retrieved 2 January 2011</ref>
| |
| | |
| {| class="wikitable sortable"
| |
| |-
| |
| ! Name and/or model
| |
| ! Launched
| |
| ! class="unsortable" | Fate/location
| |
| |-
| |
| | [[Mars Science Laboratory|MSL/Curiosity rover]] [[Multi-Mission Radioisotope Thermoelectric Generator|MMRTG]] (1) || 2011 || Mars surface
| |
| |-
| |
| | [[Apollo 12]] [[SNAP-27]] [[ALSEP]] || 1969 || Lunar surface ([[Oceanus Procellarum|Ocean of Storms]])<ref name=storms/>
| |
| |-
| |
| | [[Apollo 13]] SNAP-27 ALSEP || 1970 || Earth re-entry (over Pacific nr Fiji)
| |
| |-
| |
| | [[Apollo 14]] SNAP-27 ALSEP || 1971 || Lunar surface ([[Fra Mauro (crater)|Fra Mauro]])
| |
| |-
| |
| | [[Apollo 15]] SNAP-27 ALSEP || 1971 || Lunar surface ([[Hadley–Apennine (lunar region)|Hadley–Apennine)]]
| |
| |-
| |
| | [[Apollo 16]] SNAP-27 ALSEP || 1972 || Lunar surface ([[Descartes Highlands]])
| |
| |-
| |
| | [[Apollo 17]] SNAP-27 ALSEP || 1972 || Lunar surface ([[Taurus–Littrow (lunar valley)|Taurus–Littrow]])
| |
| |-
| |
| | [[Transit (satellite)|Transit-4A]] [[SNAP-3B]]? (1) || 1961 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5A3]] [[SNAP-3]] (1) || 1963 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5BN-1]] [[SNAP-3]] (1) || 1963 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5BN-2]] SNAP-9A (1) || 1963 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 9]] || 1964 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5B4]] || 1964 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5B6]] || 1965 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5B7]] || 1965 || Earth orbit
| |
| |-
| |
| | [[Transit (satellite)|Transit 5BN-3]] SNAP-9A (1) || 1964 || Failed to reach orbit<ref>{{cite web|url=http://www.astronautix.com/project/transit.htm |title=Transit |publisher=Encyclopedia Astronautica |date= |accessdate=2013-05-07}}</ref>
| |
| |-
| |
| | [[Nimbus program|Nimbus-B]] [[SNAP-19]] (2) || 1968 || Recovered after crash
| |
| |-
| |
| | [[Nimbus program|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–Huygens#Plutonium power source|Cassini]] [[GPHS-RTG]] (3) || 1997 || Orbiting [[Saturn]]
| |
| |-
| |
| | [[New Horizons]] GPHS-RTG (1) || 2006 || Leaving the Solar System
| |
| |-
| |
| | [[Galileo (spacecraft)|Galileo]] GPHS-RTG (2), || 1989 || Jupiter atmospheric entry
| |
| |-
| |
| | [[Ulysses (spacecraft)|Ulysses]] GPHS-RTG (1) || 1990 || Heliocentric orbit
| |
| |-
| |
| |[[Lincoln Experimental Satellite|LES-8]] [[MHW-RTG]] || 1976 || Near [[geostationary orbit]]
| |
| |-
| |
| |[[Lincoln Experimental Satellite|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
| |
| |}
| |
| | |
| == See also ==
| |
| {{Portal|Sustainable development}}
| |
| {{cmn|2|
| |
| * [[Advanced Stirling Radioisotope Generator]]
| |
| * [[Alkali-metal thermal to electric converter]]
| |
| * [[Atomic battery]]
| |
| * [[Betavoltaics]]
| |
| * [[Optoelectric nuclear battery]]
| |
| * [[Radioisotope heater units]]
| |
| * [[Radioactive isotope]]
| |
| * [[Thermionic converter]]
| |
| }}
| |
| | |
| == References ==
| |
| {{Reflist|2}}
| |
| ;Notes
| |
| {{Refbegin|2}}
| |
| * [http://saturn.jpl.nasa.gov/spacecraft/safety.cfm Safety discussion of the RTGs used on the Cassini-Huygens mission.]
| |
| * [http://www.ne.doe.gov/pubs/npspace.pdf Nuclear Power in Space (PDF)]
| |
| * [http://saturn.jpl.nasa.gov/spacecraft/safety/eisss2.pdf Detailed report on Cassini RTG (PDF)]
| |
| * [http://fti.neep.wisc.edu/neep602/SPRING00/lecture5.pdf Detailed lecture on RTG fuels (PDF)]
| |
| * [http://atom.kaeri.re.kr/ton/nuc1.html Detailed chart of all radioisotopes]
| |
| * [http://www.grc.nasa.gov/WWW/RT2002/5000/5490thieme.html Stirling Thermoelectic Generator]
| |
| * [http://www.atsdr.cdc.gov/toxprofiles/tp143.pdf Toxicity profile for plutonium], Agency for Toxic substances and Disease Registry, U.S. Public Health Service, December 1990
| |
| * [http://saturn.jpl.nasa.gov/spacecraft/safety/fseis4.pdf Environmental Impact of Cassini-Huygens Mission.]
| |
| * [http://solarsystem.nasa.gov/multimedia/downloads/Standard_RPS_Report_Final_011205.pdf Expanding Frontiers with Radioisotope Power Systems (PDF)]
| |
| {{Refend}}
| |
| | |
| == External links ==
| |
| {{Commons category|Radioisotope thermoelectric generators}}
| |
| * [http://solarsystem.nasa.gov/rps/rtg.cfm NASA Radioisotope Power Systems website – RTG page]
| |
| * [http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=705 NASA JPL briefing, Expanding Frontiers with Radioisotope Power Systems] – gives RTG information and a link to a longer presentation
| |
| * [http://www.seds.org/spaceviews/cassini/rtgpages.html SpaceViews: The Cassini RTG Debate]
| |
| * [http://www.grc.nasa.gov/WWW/RT/2004/RP/RPT-shah.html Stirling Radioisotope Generator]
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| * [http://www.osti.gov/accomplishments/rtg.html DOE contributions – good links ]
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| * [https://inlportal.inl.gov/portal/server.pt?open=514&objID=1482&parentname=CommunityPage&parentid=17&mode=2&in_hi_userid=200&cached=true Idaho National Laboratory – Producer of RTGs]
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| * [http://www.inl.gov/research/mars-science-laboratory/ Idaho National Laboratory MMRTG page with photo-based "virtual tour"]
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| {{Nuclear technology}}
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| {{Footer energy}}
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| {{Voyager program}}
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| {{Use dmy dates|date=February 2011}}
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| {{DEFAULTSORT:Radioisotope Thermoelectric Generator}}
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| [[Category:Nuclear power in space]]
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| [[Category:Nuclear technology]]
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| [[Category:Electrical generators]]
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| [[Category:Battery (electricity)]]
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