View source for Radioisotope thermoelectric generator

{{Short description|Electrical generator that uses heat from radioactive decay}}
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[[File:Cutdrawing of an GPHS-RTG.png|thumb|Diagram of an RTG used on the [[Cassini probe|''Cassini'' probe]]]]

A '''radioisotope thermoelectric generator''' ('''RTG''', '''RITEG'''), sometimes referred to as a radioisotope power system (RPS), is a type of [[nuclear battery]] 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]]. This type of [[Electricity generation|generator]] has no moving parts and is ideal for deployment in remote and harsh environments for extended periods with no risk of parts wearing out or malfunctioning.

RTGs are usually the most desirable power source for unmaintained situations that need a few hundred watts (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 not practical. RTGs have been used as power sources in [[satellite]]s, [[space probe]]s, and uncrewed remote facilities such as a series of [[lighthouse]]s built by the [[Soviet Union]] inside the [[Arctic Circle]].

Safe use of RTGs requires containment of the [[radioisotopes]] long after the productive life of the unit. The expense of RTGs tends to limit their use to niche applications in rare or special situations.

== History ==
[[File:Radioisotope thermoelectric generator plutonium pellet.jpg|thumb|300px|A pellet of [[Plutonium-238|<sup>238</sup>Pu]]O<sub>2</sub> as used in the RTG for the [[Cassini spacecraft|''Cassini'']] and [[Galileo spacecraft|''Galileo'']] missions. This photo was taken after insulating the pellet under a [[graphite]] blanket for several minutes and then removing the blanket. The pellet is [[incandescence|glowing red hot]] because of the heat generated by radioactive decay (primarily α). The initial output is 62 watts.]]

The RTG was invented in 1954 by [[Mound Laboratories]] scientists Kenneth (Ken) C. Jordan (1921-2008) and John Birden (1918-2011).<ref>{{Cite web |url=https://www.invent.org/inductees/kenneth-c-jordan |title=NIHF Inductee Kenneth C. Jordan |access-date=21 January 2023 }}</ref><ref>{{Cite web |url=https://www.invent.org/inductees/john-birden |title=NIHF Inductee John Birden |access-date=21 January 2023 }}</ref> They were inducted into the [[National Inventors Hall of Fame]] in 2013.<ref>{{Cite web |url=http://invent.org/inductee-detail/?IID=479 |title=National Inventors Hall of Fame entry for Ken Jordan |access-date=7 August 2016 |archive-date=17 September 2016 |archive-url=https://web.archive.org/web/20160917020127/http://invent.org/inductee-detail/?IID=479 |url-status=dead }}</ref><ref>{{Cite web |url=http://invent.org/inductee-detail/?IID=473 |title=National Inventors Hall of Fame entry for John Birden |access-date=7 August 2016 |archive-date=17 September 2016 |archive-url=https://web.archive.org/web/20160917034817/http://invent.org/inductee-detail/?IID=473 |url-status=dead }}</ref> Jordan and Birden worked on an Army Signal Corps contract (R-65-8- 998 11-SC-03-91) beginning on 1 January 1957, to conduct research on radioactive materials and thermocouples suitable for the direct conversion of heat to electrical energy using [[polonium-210]] as the heat source. 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 name="BlankeEtAl1960">{{cite book |title=Nuclear Battery-Thermocouple Type Summary Report |url=https://doi.org/10.2172%2F4807049 |publisher=United States Atomic Energy Commission |date=1960 |doi=10.2172/4807049 |doi-access=free |last1=Blanke |first1=B. C. |last2=Birden |first2=J. H. |last3=Jordan |first3=K. C. |last4=Murphy |first4=E. L. }}</ref>

The first RTG launched into space by the United States was [[Systems for Nuclear Auxiliary Power|SNAP 3B]] in 1961 powered by 96 grams of [[plutonium-238]] metal, 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.

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 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 =lecture notes | publisher = Fusion Technology Institute, [[University of Wisconsin–Madison]] | date = Spring 2000 | page = 21 |archive-url=https://web.archive.org/web/20070219054607/https://fti.neep.wisc.edu/neep602/SPRING00/lecture39.pdf |archive-date=19 February 2007 |url-status=dead }}</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]].

In addition to spacecraft, the [[Soviet Union]] built 1,007 RTGs<ref name=bbc20210105/> to power uncrewed lighthouses and navigation beacons on the [[Arctic policy of Russia|Soviet arctic coast]] by the late 1980s.<ref name=bbc20210105/><ref name="Bellona">{{cite web|url=http://bellona.org/news/nuclear-issues/radioactive-waste-and-spent-nuclear-fuel/2005-04-radioisotope-thermoelectric-generators-2|title=Radioisotope Thermoelectric Generators|publisher=[[Bellona Foundation|Bellona]]|date=2 April 2005|access-date=2016-06-13}}</ref> Many different types of RTGs (including [[Beta-M]] type) were built in the Soviet Union for a wide variety of purposes. The lighthouses were not maintained for many years after the [[Dissolution of the Soviet Union|dissolution of the Soviet Union in 1991]]. Some of the RTG units disappeared during this time—either by [[looting]] or by the natural forces of ice/storm/sea.<ref name=bbc20210105/> In 1996, a project was begun by [[government of Russia|Russian]] and international supporters to decommission the RTGs in the lighthouses, and by 2021, all RTGs had been removed.<ref name=bbc20210105>{{cite news |url=https://www.bbc.com/reel/playlist/ultimate-world?vpid=p0931jtm |title=The nuclear lighthouses built by the Soviets in the Arctic |last1=Sudunova|first1=Irina |work=BBC Reel |publisher=[[BBC]] |date=5 January 2021 |accessdate=15 March 2021 |language=en }}</ref>

As of 1992, the [[United States Air Force]] also used RTGs to power remotely-located Arctic equipment, and the US government has used hundreds of such units to power remote stations globally.  Sensing stations for [[Alaskan Air Command#Long Range Radar (LRR) Sites|Top-ROCC]] and [[SEEK IGLOO]] radar systems, predominantly located in [[Alaska]], use RTGs.  The units use [[strontium-90]], and a larger number of such units have been deployed both on the ground and on the [[ocean floor]] than have been used on spacecraft, with public<!-- the US military classified documents on these uses appear not to have been de-classified yet, as of 2021 --> [[regulatory authority|regulatory]] documents suggesting that the US had deployed at least 100–150 during the 1970s and 1980s.<ref name=wido19921016>[http://www.wiseinternational.org/node/701 Alaska fire threatens air force nukes], [[World Information Service on Energy|WISE]], 16 October 1992, accessed 15 March 2021.</ref>{{update after|2021|3|15}}

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 ninety were still in use. By the end of 2007, the number was reported to be down to just nine.<ref>{{cite news |title=Nuclear pacemaker still energized after 34 years |newspaper=Reuters |url=https://www.reuters.com/article/health-heart-pacemaker-dc/nuclear-pacemaker-still-energized-after-34-years-idUKN1960427320071219 |archive-url=https://web.archive.org/web/20230227044408/https://www.reuters.com/article/health-heart-pacemaker-dc/nuclear-pacemaker-still-energized-after-34-years-idUKN1960427320071219 |archive-date=2023-02-27 |date=2007-12-19|access-date=2019-03-14|url-status=live}}</ref>  The Mound Laboratory Cardiac Pacemaker program began on 1 June 1966, in conjunction with NUMEC.<ref>{{cite book |url=https://dl.dropboxusercontent.com/u/77675434/Heat%20Source%20Datasheets/CARDIAC%20PACEMAKER.pdf|archive-url=https://web.archive.org/web/20160816084535/https://dl.dropboxusercontent.com/u/77675434/Heat%20Source%20Datasheets/CARDIAC%20PACEMAKER.pdf|url-status=dead|archive-date=2016-08-16<!--Deny Citation Bot--> |title=Cardiac Pacemaker |publisher=Monsanto Research Corporation |place=Miamisburg, Ohio}}</ref>  When it was recognized that the heat source would not remain intact during cremation, the program was cancelled in 1972 because there was no way to completely ensure that the units would not be cremated with their users' bodies.

== Design ==

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. It is the temperature difference between the fuel and the heat sink that allows the thermocouples to generate electricity.

A thermocouple is a [[thermoelectricity|thermoelectric]] device that can convert [[thermal energy]] directly into [[electrical energy]] using the [[Peltier-Seebeck effect|Seebeck effect]]. It is made of two kinds of metal or semiconductor material. If they are connected to each other in a closed loop and the two junctions are at different [[temperature]]s, an electric current will flow in the loop. Typically a large number of thermocouples are connected in series to generate a higher voltage.

RTGs and [[nuclear fission|fission reactors]] use very different nuclear reactions. Nuclear power reactors (including the miniaturized ones used in space) perform controlled nuclear fission in a [[chain reaction]]. The rate of the reaction can be controlled with [[Control rod|neutron absorbing control rods]], so power can be varied with demand or shut off (almost) entirely for maintenance. However, care is needed to avoid uncontrolled operation at dangerously high power levels, or even explosion or [[nuclear meltdown]]. Chain reactions do not occur in RTGs. Heat is produced through spontaneous [[radioactive decay]] at a non-adjustable and steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. In an RTG, heat generation cannot be varied with demand or shut off when not needed and it is not possible to save more energy for later by reducing the power consumption. 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. While spectacular failures like a nuclear meltdown or explosion are impossible with an RTG, there is still a risk of radioactive contamination if the rocket explodes, the device reenters the atmosphere and disintegrates, terrestrial RTGs are damaged by storms or seasonal ice, or are vandalized.

== Developments ==

Due to the shortage of plutonium-238, a new kind of RTG assisted by subcritical reactions has been proposed.<ref>{{cite journal|last=Arias|first=F. J.|date=2011|title=Advanced Subcritical Assistance Radioisotope Thermoelectric Generator: An Imperative Solution for the Future of NASA Exploration|url=http://www.jbis.org.uk/paper.php?p=2011.64.314|journal=[[Journal of the British Interplanetary Society]]|volume=64|pages=314–318|bibcode=2011JBIS...64..314A}}</ref> In this kind of RTG, the alpha decay from the radioisotope is also used in alpha-neutron reactions with a suitable element such as [[beryllium]]. This way a long-lived [[neutron source]] is produced. Because the system is working with a criticality close to but less than 1, i.e. [[Nuclear chain reaction#Effective neutron multiplication factor|K<sub>eff</sub>]] &lt; 1, a [[Nuclear reactor physics#Subcritical multiplication|subcritical multiplication]] is achieved which increases the neutron background and produces energy from fission reactions. Although the number of fissions produced in the RTG is very small (making their gamma radiation negligible), because each fission reaction releases over 30 times more energy than each alpha decay (200&nbsp;[[MeV]] compared to 6&nbsp;MeV), up to a 10% energy gain is attainable, which translates into a reduction of the <sup>238</sup>Pu needed per mission. The idea was proposed to NASA in 2012 for the yearly NASA NSPIRE competition, which translated to Idaho National Laboratory at the Center for Space Nuclear Research (CSNR) in 2013 for studies of feasibility.<ref>Design of a high power (1 kWe), subcritical, power source {{cite web |url=http://csnrstg.usra.edu/public/default.cfm?content=330&child=345 |title=Areas of Research |access-date=2014-10-05 |url-status=dead |archive-url=https://web.archive.org/web/20141006084400/http://csnrstg.usra.edu/public/default.cfm?content=330&child=345 |archive-date=6 October 2014  }}</ref>{{failed verification|date=October 2014}} However the essentials are unmodified.

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]] |access-date=22 October 2010}}</ref>
An RTG using <sup>241</sup>Am was proposed for this type of mission in 2002.<ref name="ip">{{cite book |last1=McNutt |first1=R. L. |display-authors=etal |title=Interstellar Explorer |date=7 May 2002 |publisher=Applied Physics Laboratory, Johns Hopkins University |url=https://www.niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/McNutt_Ralph.pdf |archive-url=https://web.archive.org/web/20130603084149/https://www.niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/McNutt_Ralph.pdf |url-status=dead |archive-date=2013-06-03 <!--Deny Citation Bot-->}}</ref> This could support mission extensions up to 1000 years on the interstellar probe, because the power output would decline more slowly over 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 sources (ARPS).<ref name=ipjpl>{{cite web|url=http://interstellar.jpl.nasa.gov/interstellar/probe/index.html |archive-url=https://web.archive.org/web/20030406073326/http://interstellar.jpl.nasa.gov/interstellar/probe/index.html |url-status=dead |archive-date=6 April 2003 |title=Interstellar Probe |publisher=NASA/JPL |date=5 February 2002 |access-date=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/>

A power enhancement for radioisotope heat sources based on a self-induced electrostatic field has been proposed.<ref>{{cite journal |last1=Arias |first1=F. J. |last2=Parks |first2=G. T. |date=November 2015 |title=Self-induced electrostatic-boosted radioisotope heat sources  |journal=Progress in Nuclear Energy |volume=85 |pages=291–296 |doi=10.1016/j.pnucene.2015.06.016 |issn=0149-1970 |doi-access=free }}</ref>  According to the authors, enhancements of 5-10% could be attainable using beta sources.

== 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.

=== {{anchor|Nuclear power systems in space}} Space ===
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{{Main|Nuclear power in space|List of 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 |first=D. M. |last=Harland |title=Apollo 12 – On the Ocean of Storms |year=2011 |publisher=Springer Science & Business Media |place=New York |isbn=978-1-4419-7607-9|page=269 |oclc=1300218719 |doi=10.1007/978-1-4419-7607-9}}</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&nbsp;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 |access-date=28 November 2011}}</ref>

{| class="wikitable sortable"
|-
!rowspan="2"|Name and 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)
!rowspan="2"|Power/total<br />mass(W/kg)
!rowspan="2"|Power/fuel<br />mass (W/kg)
|-
!Electrical ([[watt|W]]) ||Heat (W)
|-
|[[Multi-Mission Radioisotope Thermoelectric Generator|MMRTG]] ||[[Mars Science Laboratory|MSL/''Curiosity'' rover]] and [[Perseverance (rover)|Perseverance]]/[[Mars 2020|Mars 2020 rover]] || data-sort-value="110" | c. 110 || data-sort-value="2000" | c. 2,000 || <sup>238</sup>Pu || ~data-sort-value="4" | c. 4 || data-sort-value="45" | <45 || 2.4
|c. 30
|-
|[[GPHS-RTG]] || [[Cassini-Huygens|''Cassini'' (3)]], [[New Horizons|''New Horizons'' (1)]], [[Galileo (spacecraft)|''Galileo'' (2)]], [[Ulysses probe|''Ulysses'' (1)]] || 300 || 4,400 || <sup>238</sup>Pu || 7.8 || 55.9–57.8<ref name="GLB"/> || 5.2–5.4
|38
|-
|[[MHW-RTG]] || [[Lincoln Experimental Satellite|LES-8/9]], [[Voyager 1|''Voyager&nbsp;1'' (3)]], [[Voyager 2|''Voyager&nbsp;2'' (3)]] || 160<ref name="GLB"/> || 2,400<ref name="tose">{{cite web |url=http://www.totse.com/en/technology/space_astronomy_nasa/spacnuke.html |title=Totse.com &#124; Nuclear Power in Space |access-date=2012-10-19 |url-status=dead |archive-url=https://web.archive.org/web/20080619143130/http://www.totse.com/en/technology/space_astronomy_nasa/spacnuke.html |archive-date=19 June 2008  }}</ref> || <sup>238</sup>Pu || data-sort-value="4.5" | c. 4.5 || 37.7<ref name="GLB"/> || 4.2
|c. 36
|-
|[[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"/> || 1.3
|?
|-
|[[Systems_for_Nuclear_Auxiliary_Power#SNAP-9|SNAP-9A]] || [[Transit (satellite)|Transit 5BN1/2]] (1) || 25<ref name="GLB"/>|| 525<ref name="tose"/> || <sup>238</sup>Pu || data-sort-value="1" | c. 1 || 12.3<ref name="GLB"/> || 2.0
|c. 30
|-
|SNAP-19 || [[Nimbus program|Nimbus-3]] (2), [[Pioneer 10|''Pioneer&nbsp;10'' (4)]]'', ''[[Pioneer 11|''Pioneer&nbsp;11'' (4)]] || 40.3<ref name="GLB">[https://fas.org/nuke/space/bennett0706.pdf "Space Nuclear Power"] G.L.Bennett 2006</ref>|| 525 || <sup>238</sup>Pu || data-sort-value="1" | c. 1 || 13.6<ref name="GLB"/> || 2.9
|c. 40
|-
|modified SNAP-19 || [[Viking program|Viking 1 (2), Viking&nbsp;2 (2)]] || 42.7<ref name="GLB"/>|| 525 || <sup>238</sup>Pu || data-sort-value="1" | c. 1 || 15.2<ref name="GLB"/> || 2.8
|c. 40
|-
|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]]|access-date=13 September 2011|url-status=dead|archive-url=https://web.archive.org/web/20120124141108/http://www.nasm.si.edu/exhibitions/attm/la.s27.1.html|archive-date=24 January 2012}}</ref>|| 3.8 || 20 || 3.65
|19
|-
| (fission reactor) [[BES-5|Buk (BES-5)]]** || [[US-A]]s (1) || 3,000 || 100,000 || highly enriched <sup>235</sup>U || 30 || data-sort-value="1000" | 1,000 || 3.0
|100
|-
| (fission reactor) 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|access-date=4 April 2010|url-status=dead|archive-url=https://web.archive.org/web/20100504162240/http://www.etec.energy.gov/History/Major-Operations/SNAP-Overview.html|archive-date=4 May 2010}}</ref> || 30,000 || highly enriched <sup>235</sup>U ||   || 431 || 1.4
|?
|-
| [[Advanced Stirling Radioisotope Generator|ASRG]]**** || prototype design (not launched), [[Discovery Program]] || data-sort-value="140" | c. 140 (2x70) || data-sort-value="500" | c. 500 || [[Plutonium-238|<sup>238</sup>Pu]] || data-sort-value="1" | 1 || data-sort-value="34" | 34 || 4.1
|c. 100
|}

<nowiki>**</nowiki> not really an RTG, the BES-5 Buk ([[:ru:Ядерные реакторы на космических аппаратах#Бук|БЭС-5]]) reactor was a fast 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)|access-date=14 September 2011|author=Chitaykin, V.I|author2=Meleta, Ye.A. | author3=Yarygin, V.I. |author4=Mikheyev, A.S. |author5= Tulin, S.M. | pages=178–185}}</ref><ref>{{cite web|url=http://world-nuclear.org/info/inf82.html|title=Nuclear Reactors for Space | access-date=14 September 2011}}</ref>

<nowiki>***</nowiki> not really an RTG, 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/>

<nowiki>****</nowiki> not really an RTG, the ASRG uses a [[Stirling engine|Stirling]] power device that runs on radioisotope (see [[Stirling radioisotope generator]])

=== Terrestrial ===
{| class="wikitable sortable"
|-
!rowspan="2"|Name and 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 uncrewed <br />lighthouses and beacons || 10 || 230 || [[Strontium-90|<sup>90</sup>Sr]]TiO<sub>3</sub><ref>{{cite web |url=http://bellona.no/bellona.org/english_import_area/international/russia/navy/northern_fleet/incidents/31772 |title=Radioisotope Thermoelectric Generators - Bellona |website=bellona.no |access-date=14 March 2022 |archive-url=https://web.archive.org/web/20101220003227/http://bellona.no/bellona.org/english_import_area/international/russia/navy/northern_fleet/incidents/31772 |archive-date=20 December 2010 |url-status=dead}}</ref> || 0.26 || 560
|-
|Efir-MA || 30 || 720 || ? || ? || 1,250
|-
|IEU-1   || 80 || 2,200 || <sup>90</sup>Sr || ? || 2,500
|-
|IEU-2   || 14 || 580 || ? || ? || 600
|-
|Gong || 18 || 315 || ? || ? || 600
|-
|Gorn || 60 || 1,100 || ? || ? || 1,050
|-
|IEU-2M || 20 || 690 || ? || ? || 600
|-
|IEU-1M || 120 (180) || 2,200 (3,300) || <sup>90</sup>Sr || ? || 2(3) × 1,050
|-
|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 || 907–1,814
|-
|Sentinel 100F<ref name=OTA/> || 53 || || Sr<sub>2</sub>TiO<sub>4</sub> || 1.77 || 1,234
|-
|RIPPLE X<ref>[https://rwm.nda.gov.uk/publication/summary-of-assessment-report-for-ripple-i-x-and-large-sources-final-stage/?download RIPPLE I – X and Large Source]</ref>|| Buoys, Lighthouses || 33<ref>[http://www.irishlights.ie/tourism/our-lighthouses/rathlin-obirne.aspx Irish Lights- Rathlin O'Birne]</ref> || || SrTiO<sub>3</sub> || ||1,500
|-
|Milliwatt RTG<ref>{{Cite web |title=The Actinide Research Quarterly: winter 94-95 |url=https://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/94-95winter/surveillance.html |access-date=2023-11-02 |website=www.lanl.gov}}</ref>
|[[Permissive action link|Permissive Action Link]] power source
|4–4.5
|
|<sup>238</sup>Pu
|?
|?
|}

== Fuels ==
[[file:RTG radiation measurement.jpg|thumb|Inspection of [[Cassini-Huygens|''Cassini'' spacecraft]] RTGs before launch]]
[[file:New Horizons 1.jpg|thumb|''[[New Horizons]]'' in assembly hall]]

The radioactive material used in RTGs must have several characteristics:<ref name=NPE3/>

# Its [[half-life]] must be long enough so that it will release energy at a relatively constant 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.
# 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 there are size restrictions. The [[decay energy]] can be calculated if the energy of radioactive radiation or the mass loss before and after radioactive decay is known. Energy release per decay is proportional to power production per [[Mole (unit)|mole]]. [[Alpha decay]]s in general release about ten times as much energy as the [[beta decay]] of strontium-90 or caesium-137.{{Citation needed|date=April 2016}}
# 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.<ref name="BlankeEtAl1960"/>

The first two criteria limit the number of possible fuels to fewer than thirty 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] {{webarchive|url=https://web.archive.org/web/20121218194925/https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Radioisotopes%20Power%20Production.pdf |date=18 December 2012 }}</ref> within the entire [[table of nuclides]].

[[Plutonium-238]], [[Curium|curium-244]], [[strontium-90]], and nowadays [[americium-241]] are the most often cited candidate isotopes, but 43 more isotopes out of approximately 1300  were considered at the beginning in the 1950s.<ref name="BlankeEtAl1960"/>

The table below does not necessarily give power densities for the pure material but for a [[chemically inert]] form. For [[actinide]]s this is of little concern as their oxides are usually inert enough (and can be transformed into [[ceramics]] further increasing their stability), but for [[alkali metal]]s and [[alkaline earth metal]]s like caesium or strontium respectively, relatively complex (and heavy) chemical compounds have to be used. For example, strontium is commonly used as [[strontium titanate]] in RTGs, which increases [[molar mass]] by about a factor of 2. Furthermore, depending on the source, isotopic purity may not be obtainable. Plutonium extracted from [[spent nuclear fuel]] has a low share of Pu-238, so plutonium-238 for use in RTGs is usually purpose-made by [[neutron irradiation]] of [[neptunium-237]], further raising costs. Caesium in [[fission product]]s is almost equal parts Cs-135 and Cs-137, plus significant amounts of stable Cs-133 and, in "young" spent fuel, short lived Cs-134. If [[isotope separation]], a costly and time-consuming process, is to be avoided, this has to be factored in, too. While historically RTGs have been rather small, there is in theory nothing preventing RTGs from reaching into the Megawatt<sub>thermal</sub> range of power. However, for such applications actinides are less suitable than lighter radioisotopes as the [[critical mass (nuclear physics)|critical mass]] is orders of magnitude below the mass needed to produce such amounts of power. As Sr-90, Cs-137 and other lighter radionuclides ''cannot'' maintain a [[nuclear chain reaction]] under any circumstances, RTGs of arbitrary size and power could be assembled from them if enough material can be produced. In general, however, potential applications for such large-scale RTGs are more the domain of [[small modular reactor]]s, microreactors or non-nuclear power sources.

{| class="wikitable"
! Material !! Shielding requirement !! colspan="2"| Power density (W/g) !! colspan="2"| Half-life (years)
|-
| <sup>238</sup>Pu || Low    || {{bartable|0.54||2}}  || {{bartable|87.7||0.5}}
|-
| <sup>90</sup>Sr  || High   || {{bartable|0.46||2}}  || {{bartable|28.8||0.5}}
|-
| <sup>210</sup>Po || Low   || {{bartable|140||2}}   || {{bartable|0.378||0.5}}
|-
| <sup>241</sup>Am || Medium || {{bartable|0.114||2}} || {{bartable|432||0.5}} 
|}

=== <sup>238</sup>Pu ===

[[Plutonium-238]] has a half-life of 87.7 years, reasonable [[power density]] of 0.57 watts per gram,<ref>{{cite web | url = http://energy.gov/sites/prod/files/NEGTN0NEAC_PU-238_042108.pdf  | title = Assessment of Plutonium-238 Production Alternatives: Briefing for Nuclear Energy Advisory Committee | date = 21 April 2008 | first = Dennis |last=Miotla }}</ref> and exceptionally low gamma and neutron radiation levels. <sup>238</sup>Pu has the lowest shielding requirements. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25&nbsp;mm of [[lead shielding]] to block the radiation. <sup>238</sup>Pu (the best of these three) needs less than 2.5&nbsp;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>).{{Citation needed|date = April 2016}} 
However, plutonium(IV) oxide containing a natural abundance of oxygen emits neutrons at the rate of ~2.3x10<sup>3</sup> n/sec/g of plutonium-238.  This emission rate is relatively high compared to the neutron emission rate of plutonium-238 metal.  The metal containing no light element impurities emits ~2.8x10<sup>3</sup> n/sec/g of plutonium-238.  These neutrons are produced by the spontaneous fission of plutonium-238.

The difference in the emission rates of the metal and the oxide is due mainly to the alpha, neutron reaction with the oxygen-18 and oxygen-17 present in the oxide.  The normal amount of oxygen-18 present in the natural form is 0.204% while that of oxygen-17 is 0.037%.  The reduction of the oxygen-17 and oxygen-18 present in the plutonium dioxide will result in a much lower neutron emission rate for the oxide; this can be accomplished by a gas phase <sup>16</sup>O<sub>2</sub> exchange method.  Regular production batches of <sup>238</sup>PuO<sub>2</sub> particles precipitated as a hydroxide were used to show that large production batches could be effectively <sup>16</sup>O<sub>2</sub>-exchanged on a routine basis.  High-fired <sup>238</sup>PuO<sub>2</sub> microspheres were successfully <sup>16</sup>O<sub>2</sub>-exchanged showing that an exchange will take place regardless of the previous heat treatment history of the <sup>238</sup>PuO<sub>2</sub>.<ref name = "Chadwell-Mound-1971-09-24" >{{cite journal | title = Neutron Emission Rate Reduction in PuO<sub>2</sub> by Oxygen Exchange | first1=C. B. |last1=Chadwell|first2=T. C. |last2=Elswick | journal = Mound Laboratory Document MLM-1844 | date =  24 September 1971 | doi = 10.2172/4747800 | osti = 4747800 | url = https://digital.library.unt.edu/ark:/67531/metadc1034174/ }}</ref> This lowering of the neutron emission rate of PuO<sub>2</sub> containing normal oxygen by a factor of five was discovered during the cardiac pacemaker research at Mound Laboratory in 1966, due in part to the Mound Laboratory's experience with production of stable isotopes beginning in 1960. For production of the large heat sources the shielding required would have been prohibitive without this process.<ref name="Craig-Mound-nodate">See the Pu-238 heat sources fabricated at Mound, revised table:  {{cite web | title = RTG: A Source of Power; A History of the Radioisotopic Thermoelectric Generators Fueled at Mound | first = Carol | last = Craig | work = Mound Laboratory Document MLM-MU-82-72-0006 | url = https://dl.dropboxusercontent.com/u/77675434/Mound%20Heat%20Sources.pdf | url-status = dead | archive-url = https://web.archive.org/web/20160816092738/https://dl.dropboxusercontent.com/u/77675434/Mound%20Heat%20Sources.pdf | archive-date = 16 August 2016 | df = dmy-all }}
</ref>

Unlike the other three isotopes discussed in this section, <sup>238</sup>Pu must be specifically synthesized and is not abundant as a nuclear waste product. At present only Russia has maintained high-volume production, while in the US, no more than {{cvt|50|g|abbr=on}} were produced in total between 2013 and 2018.<ref name='Forbes Dec 2018'>[https://www.forbes.com/sites/startswithabang/2018/12/13/nasa-doesnt-have-enough-nuclear-fuel-for-its-deep-space-missions/#47a2787d1c18 NASA Doesn't Have Enough Nuclear Fuel For Its Deep Space Missions]. Ethan Siegel, ''Forbes''. 13 December 2018.</ref> The US agencies involved desire to begin the production of the material at a rate of {{convert|300|to|400|g}} per year. If this plan is funded, the goal would be to set up automation and scale-up processes in order to produce an average of {{convert|1.5|kg|abbr=on}} per year by 2025.<ref>[http://spacenews.com/plutonium-supply-for-nasa-missions-faces-long-term-challenges/ Plutonium supply for NASA missions faces long-term challenges]. Jeff Foust. ''Space News'', 10 October 2017.</ref><ref name='Forbes Dec 2018'/>

=== <sup>90</sup>Sr ===

[[Strontium-90]] has been used by the Soviet Union in terrestrial RTGs. <sup>90</sup>Sr decays by β emission, with minor γ emission.  While its half life of 28.8 years is much shorter than that of <sup>238</sup>Pu, it also has a lower decay energy with a power density of 0.46 watts per gram.<ref name=Adams/>  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 has a high [[fission product yield]] in the fission of both {{chem|235|U}} and {{chem|239|Pu}} and is thus available in large quantities at a relatively low price if extracted from [[spent nuclear fuel]].<ref name=Adams>Rod Adams, [http://atomicinsights.com/1996/09/rtg-heat-sources-two-proven-materials.html RTG Heat Sources: Two Proven Materials] {{webarchive|url=https://web.archive.org/web/20120207133936/http://atomicinsights.com/1996/09/rtg-heat-sources-two-proven-materials.html |date=7 February 2012 }}, 1 September 1996, Retrieved 20 January 2012.</ref><!--the article on Sr90 claims a different decay energy for the pure metal--> As {{chem|90|Sr}} is a very reactive [[alkaline earth metal]] and a so-called "bone seeker" that accumulates in bone-tissue due to its chemical similarity to [[calcium]] (once in the bones it can significantly damage the [[bone marrow]], a rapidly dividing tissue), it is usually not employed in pure form in RTGs. The most common form is the [[perovskite (structure)|perovskite]] strontium titanate (SrTiO<sub>3</sub>) which is chemically nigh-inert and has a high melting point. While its [[Mohs hardness]] of 5.5 has made it ill-suited as a [[diamond simulant]], it is of sufficient hardness to withstand some forms of accidental release from its shielding without too fine dispersal of dust. The downside to using SrTiO<sub>3</sub> instead of the native metal is that its production requires energy. It also reduces power density, as the TiO<sub>3</sub> part of the material does not produce any decay heat. Starting from the oxide or the native metal, one pathway to obtaining SrTiO<sub>3</sub> is to let it transform to [[strontium hydroxide]] in aqueous solution, which absorbs [[carbon dioxide]] from air to become less soluble [[strontium carbonate]]. Reaction of strontium carbonate with [[titanium dioxide]] at high temperature produces the desired strontium titanate plus [[carbon dioxide]]. If desired, the strontium titanate product can then be formed into a ceramic-like aggregate via [[sintering]].

=== <sup>210</sup>Po ===

Some prototype RTGs, first built in 1958 by the US Atomic Energy Commission, have used [[polonium-210]]. This isotope provides phenomenal power density (pure <sup>210</sup>Po emits [[Decay energy|140&nbsp;W]]/g) because of its high [[Radioactive decay#Rates|decay rate]], but has limited use because of its very short half-life of 138 days. A half-gram sample of <sup>210</sup>Po reaches temperatures of over {{convert|500|C|F|sigfig=1}}.<ref>{{cite web|title=Polonium|url=http://www.ead.anl.gov/pub/doc/polonium.pdf|publisher=Argonne National Laboratory|archive-url=https://web.archive.org/web/20120310145431/http://www.ead.anl.gov/pub/doc/polonium.pdf|archive-date=2012-03-10}}</ref> As Po-210 is a pure alpha-emitter and does not emit significant gamma or X-ray radiation, the shielding requirements are as low as Pu-238. While the short half-life also reduces the time during which accidental release to the environment is a concern, Polonium-210 is extremely radiotoxic if ingested and can cause significant harm even in chemically inert forms, which pass through the digestive tract as a "foreign object". A common route of production (whether accidental or deliberate) is neutron irradiation of {{chem|209|Bi}}, the only naturally occurring isotope of [[Bismuth]]. It is this accidental production that is cited as an argument against the use of [[lead-bismuth eutectic]] as a coolant in liquid metal reactors. However, if a sufficient demand for Polonium-210 exists, its extraction could be worthwhile similar to how [[tritium]] is economically recovered from the heavy water moderator in [[CANDU]]s.

=== <sup>241</sup>Am ===

[[Americium-241]] is a candidate isotope with much greater availability than <sup>238</sup>Pu. Although <sup>241</sup>Am has a half-life of 432 years which is more than <sup>238</sup>Pu and could hypothetically power a device for centuries, missions with more than 10 years were not the subject of the research until 2019.<ref name="AmbrosiEtAl2019">{{Cite journal |title=European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration |journal=Space Science Reviews |last1=Ambrosi |last2=Williams |doi=10.1007/s11214-019-0623-9 |year=2019|volume=215 |issue=8 |page=55 |bibcode=2019SSRv..215...55A |s2cid=213765953 |doi-access=free }}</ref> 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 more shielding.  Its shielding requirements in a RTG are the third lowest: only <sup>238</sup>Pu and <sup>210</sup>Po require less.  With a current global shortage<ref name="Ref_ff">Nell Greenfield-Boyce, [https://www.npr.org/templates/story/story.php?storyId=113223613 Plutonium Shortage Could Stall Space Exploration], [[NPR]], 28 September 2009, retrieved 2 November 2010</ref> of <sup>238</sup>Pu, <sup>241</sup>Am is being studied as RTG fuel by [[ESA]]<ref name="AmbrosiEtAl2019"/><ref name="ESAPP">Dr Major S. Chahal, [http://www.oosa.unvienna.org/pdf/pres/stsc2012/tech-18E.pdf], [[UK Space Agency]], 9 February 2012, retrieved 13 November 2014.</ref> and in 2019, UK's [[National Nuclear Laboratory]] announced the generation of usable electricity.<ref>{{cite web |title=UK scientists generate electricity from rare element to power future space missions |url=https://www.nnl.co.uk/2019/05/uk-scientists-generate-electricity-from-rare-element-to-power-future-space-missions/ |website=[[National Nuclear Laboratory]] |access-date=6 May 2019}}</ref> An advantage over <sup>238</sup>Pu is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of <sup>241</sup>Am RTGs expect 2-2.2&nbsp;W<sub>e</sub>/kg for 5–50&nbsp;W<sub>e</sub> RTGs design but practical testing shows that only 1.3-1.9&nbsp;W<sub>e</sub> can be achieved.<ref name="AmbrosiEtAl2019"/>
Americium-241 is currently used in small quantities in household smoke detectors and thus its handling and properties are well established. However, it decays to [[Neptunium-237]], the most chemically mobile among the Actinides.

=== <sup>250</sup>Cm ===

[[Curium-250]] is the smallest transuranic isotope that primarily decays by spontaneous fission, a process that releases many times more energy than alpha decay. Compared to plutonium-238, curium-250 provides about a quarter of the power density, but 95 times the half-life (~8300 years vs. ~87 years). As it is a neutron emitter (weaker than [[californium-252]] but not entirely negligible) some applications require a further shielding against [[neutron radiation]]. As lead, which is an excellent shielding material against gamma rays and beta ray induced [[Bremsstrahlung]], is not a good neutron shield (instead [[neutron reflector|reflecting]] most of them), a different shielding material would have to be added in applications where neutrons are a concern.

== Life span ==
[[File:Soviet RTG.jpg|thumb|[[Strontium-90|<sup>90</sup>Sr]]-powered Soviet RTGs in dilapidated condition.]]

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 a factor of 1 – (1/2)<sup>1/87.7</sup>, which is 0.787%, per year.

One example is the [[MHW-RTG]] used by the [[Voyager probes]]. In the year 2000, 23 years after production, the radioactive material inside the RTG had decreased in power by 16.6%, i.e. providing 83.4% of its initial output; starting with a capacity of 470&nbsp;W, after this length of time it would have a capacity of only 392&nbsp;W. A related loss of power in the Voyager RTGs is the degrading properties of the bi-metallic thermocouples used to convert [[thermal energy]] into [[electrical energy]]; the RTGs were working at about 67% of their total original capacity instead of the expected 83.4%. By the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315&nbsp;W for ''Voyager 1'' and to 319&nbsp;W for ''Voyager 2''.<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|access-date=24 July 2011}}</ref> By 2022, these numbers had dropped to around 220&nbsp;W.<ref>{{cite web |last1=Jansen |first1=Kristin |title=NASA Celebrates 45 Years of Voyager 1, Enabled by Radioisotope Power |url=https://rps.nasa.gov/news/63/nasa-celebrates-45-years-of-voyager-1-enabled-by-radioisotope-power/ |website=NASA News |publisher=NASA |access-date=12 November 2023}}</ref>

NASA has developed a [[multi-mission radioisotope thermoelectric generator]] (MMRTG) in which the thermocouples would be made of [[skutterudite]], a [[cobalt]] [[arsenide mineral|arsenide]] (CoAs<sub>3</sub>), which can function with a smaller temperature difference than the current [[tellurium]]-based designs. This would mean that an otherwise similar RTG would generate 25% more power at the beginning of a mission and at least 50% more after seventeen years. NASA hopes to use the design on the next [[New Frontiers program|New Frontiers]] mission.<ref>{{cite web | url =http://www.jpl.nasa.gov/news/news.php?feature=6646 | title =Spacecraft 'Nuclear Batteries' Could Get a Boost from New Materials | date =13 October 2016 | website =JPL News | publisher =Jet Propulsion Laboratory | access-date =19 October 2016 }}</ref>

== Safety ==
[[File:RTGmodule.png|thumb|Diagram of a stack of [[General Purpose Heat Source|general purpose heat source]] modules as used in RTGs]]

=== Theft ===

Radioactive materials contained in RTGs are dangerous and can even be used for malicious purposes. They are not useful for a genuine [[nuclear weapon]], but still can serve in a "[[Dirty bomb#Constructing and obtaining material for a dirty bomb|dirty bomb]]". The [[Soviet Union]] constructed many uncrewed lighthouses and navigation beacons powered by RTGs using [[strontium-90]] (<sup>90</sup>Sr). They are very reliable and provide a steady source of power. Most have no protection, not even fences or warning signs, and the locations of some of these facilities are no longer known due to poor record keeping. In one instance, the radioactive compartments were opened by a thief.<ref name="Bellona" /> [[Lia radiological accident|In another case]], three woodsmen in [[Georgia (country)|Tsalendzhikha Region, Georgia]] found two ceramic RTG [[orphan source]]s that had been stripped of their shielding; two of the woodsmen 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=https://www.iaea.org/sites/default/files/publications/magazines/bulletin/bull48-1/48105994247.pdf|access-date=30 March 2015}}</ref> There are approximately 1,000 such RTGs in Russia, all of which have long since exceeded their designed operational lives of ten years. Most of these RTGs likely no longer function, and may need to be dismantled. Some of their metal casings have been stripped by metal hunters, despite 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| archive-url=https://web.archive.org/web/20030806043406/http://www.iaea.org/worldatom/Press/Focus/RadSources/statement_rus.pdf| archive-date=6 August 2003| access-date=10 October 2009}}</ref> Transforming the radioactive material into an inert form reduces the danger of theft by people unaware of the radiation hazard  (such as happened in the [[Goiânia accident]] in an abandoned Cs-137 source where the caesium was present in easily water-soluble [[caesium chloride]] form). However, a sufficiently chemically skilled malicious actor could extract a volatile species from inert material and/or achieve a similar effect of dispersion by physically grinding the inert matrix into a fine dust.

=== Radioactive contamination ===

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.<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="Top 10 Space Age Radiation Incidents">{{cite web|url= https://listverse.com/2012/01/20/top-10-space-age-radiation-incidents | title= Top 10 Space Age Radiation Incidents | work=listverse.com | date= 20 January 2012 | access-date=30 January 2018 }}</ref>

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 three 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] {{webarchive|url=https://web.archive.org/web/20060929000952/http://saturn.jpl.nasa.gov/spacecraft/safety/fseis4.pdf |date=29 September 2006 }}, Chapter 4, NASA, September 1997 ([http://saturn.jpl.nasa.gov/spacecraft/safety-eis.cfm Links to other chapters and associated documents]  {{webarchive|url=https://web.archive.org/web/20060907222902/http://saturn.jpl.nasa.gov/spacecraft/safety-eis.cfm |date=7 September 2006 }})</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 1 in 10.<ref name="Ref_k">[http://saturn.jpl.nasa.gov/spacecraft/safety/fseisd.pdf Cassini Final Supplemental Environmental Impact Statement] {{webarchive|url=https://web.archive.org/web/20060929001253/http://saturn.jpl.nasa.gov/spacecraft/safety/fseisd.pdf |date=29 September 2006 }}, Appendix D, Summary of tables of safety analysis results, Table D-1 on page D-4, see conditional probability column for GPHS-RTG</ref> The launch was successful and ''Cassini–Huygens'' reached [[Saturn]].

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 [[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&nbsp;[[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 {{webarchive |url=https://web.archive.org/web/20070710105841/http://www.ead.anl.gov/pub/doc/tbl1-rad-rc.pdf |date=10 July 2007 }}</ref> 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.

A case of RTG-related irradiation is the [[Lia radiological accident]] in [[Georgia (country)|Georgia]], December 2001. [[Strontium-90]] RTG cores were dumped behind, unlabeled and improperly dismanteled, near the Soviet-built [[Enguri Dam]]. Three villagers from the nearby village of {{ill|Lia, Georgia|ka|ლია|lt=Lia}} were unknowingly exposed to it and injured; one of them died in May 2004 from the injuries sustained. The [[International Atomic Energy Agency]] led recovery operations and organized medical care. Two remaining RTG cores are yet to be found as of 2022.

=== Accidents ===
[[File:ALSEP Apollo 14 RTG.jpg|thumb|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 have been several known accidents involving RTG-powered spacecraft:

# A launch failure on 21 April 1964 in which the U.S. [[Transit (satellite)|Transit-5BN-3]] navigation satellite failed to achieve orbit and burned up on re-entry north of [[Madagascar]].<ref name="Transit">{{cite web|url=http://www.astronautix.com/craft/transit.htm |archive-url=https://web.archive.org/web/20020624022922/http://www.astronautix.com/craft/transit.htm |url-status=dead |archive-date=24 June 2002 |title=Transit |publisher=Encyclopedia Astronautica |access-date=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 ejected into the atmosphere over the Southern Hemisphere where it burned up, and traces of plutonium-238 were detected in the area a few months later. This incident resulted in the NASA Safety Committee requiring intact reentry in future RTG launches, which in turn impacted the design of RTGs in the pipeline. 
# 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>The RTGs were returned to Mound for disassembly and the <sup>238</sup>PuO<sub>2</sub> microsphere fuel recovered and reused.
{{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 |work=Space Safety Magazine |access-date=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 burned up over [[Fiji]]. It carried a SNAP-27 RTG containing {{convert|44,500|Ci|TBq|abbr=on}} of plutonium dioxide in a graphite cask on the lander leg 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.
# [[Mars 96]] was launched by Russia in 1996, but failed to leave Earth orbit, and re-entered the atmosphere a few hours later. The two RTGs onboard carried in total 200&nbsp;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&nbsp;km long by 80&nbsp;km wide which is centred 32&nbsp;km east of [[Iquique]], [[Chile]].<ref name="Ref_q">[http://nssdc.gsfc.nasa.gov/planetary/text/mars96_timeline.txt Mars 96 timeline], NASA</ref>

One RTG, the [[SNAP 19|SNAP-19C]], was lost near the top of [[Nanda Devi]] mountain in India in 1965 when it was stored in a rock formation near the top of the mountain in the face of a snowstorm before it could be installed to power a CIA remote automated station collecting telemetry from the Chinese rocket testing facility. The seven capsules<ref>[[:File:SNAP-19C Mound Data Sheet.pdf]]</ref> were carried down the mountain onto a glacier by an avalanche and never recovered. It is most likely that they melted through the glacier and were pulverized, whereupon the <sup>238</sup>plutonium zirconium alloy fuel oxidized soil particles that are moving in a plume under the glacier.<ref>{{cite book |title=Spies in the Himalayas: Secret Missions and Perilous Climbs |year=2002 |first1= M. S. |last1=Kohli |first2=K. J. |last2=Conboy |publisher=University Press of Kansas |place=Lawrence, Kansas |isbn=978-0-7006-1223-9 |oclc=50803689 |doi=10.2307/j.ctvj7wp9g}}</ref>{{page needed|date=May 2023}}

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" /> However, the Strontium titanate perovskite used is resistant to all likely forms of environmental degradation and cannot melt or dissolve in water. [[Bioaccumulation]] is unlikely as SrTiO<sub>3</sub> passes through the digestive tract of humans or other animals unchanged, but the animal or human who ingested it would still receive a significant radiation dose to the sensitive [[intestinal lining]] during passage. Mechanical degradation of "pebbles" or larger objects into fine dust is more likely and could disperse the material over a wider area, however this would also reduce the risk of any single exposure event resulting in a high dose.

== See also ==

* {{annotated link|Alkali-metal thermal to electric converter}}
* {{annotated link|Atomic battery}}
* {{annotated link|Betavoltaics}}
* {{annotated link|Kilopower Reactor Using Stirling Technology}}
* {{annotated link|Optoelectric nuclear battery}}
* {{annotated link|Radioisotope heater unit}}
* {{annotated link|Radioactive isotope}}
* {{annotated link|Stirling Radioisotope Generator}}
* {{annotated link|Thermionic converter}}

== Notes ==
{{reflist}}

== References ==

{{refbegin|30em}}
* [https://web.archive.org/web/20040603204919/http://saturn.jpl.nasa.gov/spacecraft/safety.cfm Safety discussion of the RTGs used on the ''Cassini-Huygens'' mission.]
* [https://web.archive.org/web/20060320153643/http://www.ne.doe.gov/pubs/npspace.pdf Nuclear Power in Space (PDF)]
* [https://web.archive.org/web/20080725123446/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)] {{Webarchive|url=https://web.archive.org/web/20130215003518/http://fti.neep.wisc.edu/neep602/SPRING00/lecture5.pdf |date=15 February 2013 }}
* [http://atom.kaeri.re.kr/ton/nuc1.html Detailed chart of all radioisotopes]
* [https://web.archive.org/web/20030827015208/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
* [https://web.archive.org/web/20060929000952/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)] {{Webarchive|url=https://web.archive.org/web/20060930142116/http://solarsystem.nasa.gov/multimedia/downloads/Standard_RPS_Report_Final_011205.pdf |date=30 September 2006 }}
* {{ cite web | url = http://energy.gov/sites/prod/files/NEGTN0NEAC_PU-238_042108.pdf | title = Assessment of Plutonium-238 Production Alternatives: Briefing for Nuclear Energy Advisory Committee | date = April 21, 2008 | first = Dennis (Deputy Assistant Secretary for Nuclear Power Deployment) | last = Miotla | publisher = [[United States Department of Energy]] }}
{{refend}}

== External links ==
{{commons category|Radioisotope thermoelectric generators}}

* [https://web.archive.org/web/20120807005925/http://solarsystem.nasa.gov/rps/rtg.cfm NASA Radioisotope Power Systems website – RTG page]
* [https://web.archive.org/web/20050320084117/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
* [https://web.archive.org/web/20040803042552/http://www.seds.org/spaceviews/cassini/rtgpages.html SpaceViews: The ''Cassini'' RTG Debate]
* [https://web.archive.org/web/20060614021723/http://www.grc.nasa.gov/WWW/RT/2004/RP/RPT-shah.html Stirling Radioisotope Generator]
* [http://www.osti.gov/accomplishments/rtg.html DOE contributions – good links ]
* [https://web.archive.org/web/20110718063021/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]
* [http://www.inl.gov/research/mars-science-laboratory/ Idaho National Laboratory MMRTG page with photo-based "virtual tour"]

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