Radioisotope thermoelectric generator

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Diagram of an RTG used on the 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 thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. This type of 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 cells, batteries, or generators to provide economically, and in places where solar cells are not practical. RTGs have been used as power sources in satellites, space probes, and uncrewed remote facilities such as a series of lighthouses 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

A pellet of 238PuO2 as used in the RTG for the Cassini and 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 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>"NIHF Inductee Kenneth C. Jordan". Retrieved 21 January 2023.</ref><ref>"NIHF Inductee John Birden". Retrieved 21 January 2023.</ref> They were inducted into the National Inventors Hall of Fame in 2013.<ref>"National Inventors Hall of Fame entry for Ken Jordan". Archived from the original on 17 September 2016. Retrieved 7 August 2016.</ref><ref>"National Inventors Hall of Fame entry for John Birden". Archived from the original on 17 September 2016. Retrieved 7 August 2016.</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">Blanke, B. C.; Birden, J. H.; Jordan, K. C.; Murphy, E. L. (1960). Nuclear Battery-Thermocouple Type Summary Report. United States Atomic Energy Commission. doi:10.2172/4807049.</ref>

The first RTG launched into space by the United States was SNAP 3B in 1961 powered by 96 grams of plutonium-238 metal, aboard the Navy Transit 4A spacecraft. One of the first terrestrial uses of RTGs was in 1966 by the US Navy at uninhabited Fairway Rock in Alaska. RTGs were used at that site until 1995.

A common RTG application is spacecraft power supply. Systems for Nuclear Auxiliary Power (SNAP) units were used for probes that traveled far from the Sun rendering solar panels impractical. As such, they were used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini, New Horizons, and the Mars Science Laboratory. RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17 (SNAP 27s). Because the Apollo 13 Moon landing was aborted, its RTG rests in the South Pacific Ocean, in the vicinity of the Tonga Trench.<ref>"General Safety Considerations" (PDF). Fusion Technology Institute, University of Wisconsin–Madison. Spring 2000. p. 21. Archived from the original (lecture notes) on 19 February 2007.</ref> RTGs were also used for the Nimbus, Transit and LES satellites. By comparison, only a few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.

In addition to spacecraft, the Soviet Union built 1,007 RTGs<ref name=bbc20210105/> to power uncrewed lighthouses and navigation beacons on the Soviet arctic coast by the late 1980s.<ref name=bbc20210105/><ref name="Bellona">"Radioisotope Thermoelectric Generators". Bellona. 2 April 2005. Retrieved 13 June 2016.</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 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 Russian and international supporters to decommission the RTGs in the lighthouses, and by 2021, all RTGs had been removed.<ref name=bbc20210105>Sudunova, Irina (5 January 2021). "The nuclear lighthouses built by the Soviets in the Arctic". BBC Reel. BBC. Retrieved 15 March 2021.</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 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 regulatory documents suggesting that the US had deployed at least 100–150 during the 1970s and 1980s.<ref name=wido19921016>Alaska fire threatens air force nukes, WISE, 16 October 1992, accessed 15 March 2021.</ref>[needs update]

In the past, small "plutonium cells" (very small 238Pu-powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life".<ref name="Ref_f">Nuclear-Powered Cardiac Pacemakers, 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>"Nuclear pacemaker still energized after 34 years". Reuters. 19 December 2007. Archived from the original on 27 February 2023. Retrieved 14 March 2019.</ref> The Mound Laboratory Cardiac Pacemaker program began on 1 June 1966, in conjunction with NUMEC.<ref>Cardiac Pacemaker (PDF). Miamisburg, Ohio: Monsanto Research Corporation. Archived from the original (PDF) on 16 August 2016.</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). Thermocouples are placed in the walls of the container, with the outer end of each thermocouple connected to a heat sink. Radioactive decay of the fuel produces heat. It is the temperature difference between the fuel and the heat sink that allows the thermocouples to generate electricity.

A thermocouple is a thermoelectric device that can convert thermal energy directly into electrical energy using the 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 temperatures, 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 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 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>Arias, F. J. (2011). "Advanced Subcritical Assistance Radioisotope Thermoelectric Generator: An Imperative Solution for the Future of NASA Exploration". Journal of the British Interplanetary Society. 64: 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. Keff < 1, a 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 MeV compared to 6 MeV), up to a 10% energy gain is attainable, which translates into a reduction of the 238Pu 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 "Areas of Research". Archived from the original on 6 October 2014. Retrieved 5 October 2014.</ref>[failed verification] However the essentials are unmodified.

RTG have been proposed for use on realistic interstellar precursor missions and interstellar probes.<ref name=ip/> An example of this is the Innovative Interstellar Explorer (2003–current) proposal from NASA.<ref name="Ref_t">"Innovative Interstellar Probe". JHU/APL. Retrieved 22 October 2010.</ref> An RTG using 241Am was proposed for this type of mission in 2002.<ref name="ip">McNutt, R. L.; et al. (7 May 2002). Interstellar Explorer (PDF). Applied Physics Laboratory, Johns Hopkins University. Archived from the original (PDF) on 3 June 2013.</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>"Interstellar Probe". NASA/JPL. 5 February 2002. Archived from the original on 6 April 2003. Retrieved 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>Arias, F. J.; Parks, G. T. (November 2015). "Self-induced electrostatic-boosted radioisotope heat sources". Progress in Nuclear Energy. 85: 291–296. doi:10.1016/j.pnucene.2015.06.016. ISSN 0149-1970.</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.

Space

Main articles: Nuclear power in space and 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>Harland, D. M. (2011). Apollo 12 – On the Ocean of Storms. New York: Springer Science & Business Media. p. 269. doi:10.1007/978-1-4419-7607-9. ISBN 978-1-4419-7607-9. OCLC 1300218719.</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-238 dioxide.<ref name="LaunchNuclearSafety">"Mars Science Laboratory Launch Nuclear Safety" (PDF). NASA/JPL/DoE. 2 March 2011. Retrieved 28 November 2011.</ref>

Name and model Used on (# of RTGs per user) Maximum output Radio-
isotope 		Max fuel
used (kg) 		Mass (kg) Power/total
mass(W/kg) 		Power/fuel
mass (W/kg)
Electrical (W) Heat (W)
MMRTG MSL/Curiosity rover and Perseverance/Mars 2020 rover c. 110 c. 2,000 238Pu c. 4 <45 2.4 c. 30
GPHS-RTG Cassini (3), New Horizons (1), Galileo (2), Ulysses (1) 300 4,400 238Pu 7.8 55.9–57.8<ref name="GLB"/> 5.2–5.4 38
MHW-RTG LES-8/9, Voyager 1 (3), Voyager 2 (3) 160<ref name="GLB"/> 2,400<ref name="tose">"Totse.com | Nuclear Power in Space". Archived from the original on 19 June 2008. Retrieved 19 October 2012.</ref> 238Pu c. 4.5 37.7<ref name="GLB"/> 4.2 c. 36
SNAP-3B Transit-4A (1) 2.7<ref name="GLB"/> 52.5 238Pu ? 2.1<ref name="GLB"/> 1.3 ?
SNAP-9A Transit 5BN1/2 (1) 25<ref name="GLB"/> 525<ref name="tose"/> 238Pu c. 1 12.3<ref name="GLB"/> 2.0 c. 30
SNAP-19 Nimbus-3 (2), Pioneer 10 (4), Pioneer 11 (4) 40.3<ref name="GLB">"Space Nuclear Power" G.L.Bennett 2006</ref> 525 238Pu c. 1 13.6<ref name="GLB"/> 2.9 c. 40
modified SNAP-19 Viking 1 (2), Viking 2 (2) 42.7<ref name="GLB"/> 525 238Pu c. 1 15.2<ref name="GLB"/> 2.8 c. 40
SNAP-27 Apollo 12–17 ALSEP (1) 73 1,480 238Pu<ref name=NASM>"SNAP-27". Smithsonian National Air and Space Museum. Archived from the original on 24 January 2012. Retrieved 13 September 2011.</ref> 3.8 20 3.65 19
(fission reactor) Buk (BES-5)** US-As (1) 3,000 100,000 highly enriched 235U 30 1,000 3.0 100
(fission reactor) SNAP-10A*** SNAP-10A (1) 600<ref name=doe1>"SNAP Overview". USDOE ETEC. Archived from the original on 4 May 2010. Retrieved 4 April 2010.</ref> 30,000 highly enriched 235U 431 1.4 ?
ASRG**** prototype design (not launched), Discovery Program c. 140 (2x70) c. 500 238Pu 1 34 4.1 c. 100

** not really an RTG, the BES-5 Buk (БЭС-5) reactor was a fast reactor which used thermocouples based on semiconductors to convert heat directly into electricity<ref>Chitaykin, V.I; Meleta, Ye.A.; Yarygin, V.I.; Mikheyev, A.S.; Tulin, S.M. "Use of nuclear space technology of direct energy conversion for terrestrial application". International Atomic Energy Agency, Vienna (Austria). pp. 178–185. Retrieved 14 September 2011.</ref><ref>"Nuclear Reactors for Space". Retrieved 14 September 2011.</ref>

*** 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/>

**** not really an RTG, the ASRG uses a Stirling power device that runs on radioisotope (see Stirling radioisotope generator)

Terrestrial

Name and model Use Maximum output Radioisotope Max fuel used
(kg) 		Mass (kg)
Electrical (W) Heat (W)
Beta-M Obsolete Soviet uncrewed
lighthouses and beacons		 10 230 90SrTiO3<ref>"Radioisotope Thermoelectric Generators - Bellona". bellona.no. Archived from the original on 20 December 2010. Retrieved 14 March 2022.</ref> 0.26 560
Efir-MA 30 720 ? ? 1,250
IEU-1 80 2,200 90Sr ? 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) 90Sr ? 2(3) × 1,050
Sentinel 25<ref name=OTA>"Power Sources for Remote Arctic Applications" (PDF). Washington, DC: U.S. Congress, Office of Technology Assessment. June 1994. OTA-BP-ETI-129.</ref> Remote U.S. arctic monitoring sites 9–20 SrTiO3 0.54 907–1,814
Sentinel 100F<ref name=OTA/> 53 Sr2TiO4 1.77 1,234
RIPPLE X<ref>RIPPLE I – X and Large Source</ref> Buoys, Lighthouses 33<ref>Irish Lights- Rathlin O'Birne</ref> SrTiO3 1,500
Milliwatt RTG<ref>"The Actinide Research Quarterly: winter 94-95". www.lanl.gov. Retrieved 2 November 2023.</ref> Permissive Action Link power source 4–4.5 238Pu ? ?

Fuels

Inspection of Cassini spacecraft RTGs before launch
New Horizons in assembly hall

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

  1. 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) 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. Typical half-lives for radioisotopes used in RTGs are therefore several decades, although isotopes with shorter half-lives could be used for specialized applications.
  2. 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. Alpha decays in general release about ten times as much energy as the beta decay of strontium-90 or caesium-137.[citation needed]
  3. Radiation must be of a type easily absorbed and transformed into thermal radiation, preferably alpha radiation. Beta radiation can emit considerable gamma/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 modes 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>NPE chapter 3 Radioisotope Power Generation Archived 18 December 2012 at the Wayback Machine</ref> within the entire table of nuclides.

Plutonium-238, 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 actinides 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 metals and alkaline earth metals 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 products 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 Megawattthermal range of power. However, for such applications actinides are less suitable than lighter radioisotopes as the 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 reactors, microreactors or non-nuclear power sources.

Material Shielding requirement Power density (W/g) Half-life (years)
238Pu Low 0.54 0.54
 
 87.7 87.7
 
90Sr High 0.46 0.46
 
 28.8 28.8
 
210Po Low 140 140
 
 0.378 0.378
 
241Am Medium 0.114 0.114
 
 432 432
 

238Pu

Plutonium-238 has a half-life of 87.7 years, reasonable power density of 0.57 watts per gram,<ref>Miotla, Dennis (21 April 2008). "Assessment of Plutonium-238 Production Alternatives: Briefing for Nuclear Energy Advisory Committee" (PDF).</ref> and exceptionally low gamma and neutron radiation levels. 238Pu has the lowest shielding requirements. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25 mm of lead shielding to block the radiation. 238Pu (the best of these three) needs less than 2.5 mm, and in many cases, no shielding is needed in a 238Pu RTG, as the casing itself is adequate. 238Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO2).[citation needed] However, plutonium(IV) oxide containing a natural abundance of oxygen emits neutrons at the rate of ~2.3x103 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 ~2.8x103 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 16O2 exchange method. Regular production batches of 238PuO2 particles precipitated as a hydroxide were used to show that large production batches could be effectively 16O2-exchanged on a routine basis. High-fired 238PuO2 microspheres were successfully 16O2-exchanged showing that an exchange will take place regardless of the previous heat treatment history of the 238PuO2.<ref name = "Chadwell-Mound-1971-09-24" >Chadwell, C. B.; Elswick, T. C. (24 September 1971). "Neutron Emission Rate Reduction in PuO2 by Oxygen Exchange". Mound Laboratory Document MLM-1844. doi:10.2172/4747800. OSTI 4747800.</ref> This lowering of the neutron emission rate of PuO2 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: Craig, Carol. "RTG: A Source of Power; A History of the Radioisotopic Thermoelectric Generators Fueled at Mound" (PDF). Mound Laboratory Document MLM-MU-82-72-0006. Archived from the original (PDF) on 16 August 2016. </ref>

Unlike the other three isotopes discussed in this section, 238Pu 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 50 g (1.8 oz) were produced in total between 2013 and 2018.<ref name='Forbes Dec 2018'>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 300 to 400 grams (11 to 14 oz) 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 1.5 kg (3.3 lb) per year by 2025.<ref>Plutonium supply for NASA missions faces long-term challenges. Jeff Foust. Space News, 10 October 2017.</ref><ref name='Forbes Dec 2018'/>

90Sr

Strontium-90 has been used by the Soviet Union in terrestrial RTGs. 90Sr decays by β emission, with minor γ emission. While its half life of 28.8 years is much shorter than that of 238Pu, 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 238Pu, which results in lower RTG efficiency. 90Sr has a high fission product yield in the fission of both 235
U and 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, RTG Heat Sources: Two Proven Materials Archived 7 February 2012 at the Wayback Machine, 1 September 1996, Retrieved 20 January 2012.</ref> As 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 strontium titanate (SrTiO3) 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 SrTiO3 instead of the native metal is that its production requires energy. It also reduces power density, as the TiO3 part of the material does not produce any decay heat. Starting from the oxide or the native metal, one pathway to obtaining SrTiO3 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.

210Po

Some prototype RTGs, first built in 1958 by the US Atomic Energy Commission, have used polonium-210. This isotope provides phenomenal power density (pure 210Po emits 140 W/g) because of its high decay rate, but has limited use because of its very short half-life of 138 days. A half-gram sample of 210Po reaches temperatures of over 500 °C (900 °F).<ref>"Polonium" (PDF). Argonne National Laboratory. Archived from the original (PDF) on 10 March 2012.</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 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 CANDUs.

241Am

Americium-241 is a candidate isotope with much greater availability than 238Pu. Although 241Am has a half-life of 432 years which is more than 238Pu 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">Ambrosi; Williams (2019). "European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration". Space Science Reviews. 215 (8): 55. Bibcode:2019SSRv..215...55A. doi:10.1007/s11214-019-0623-9. S2CID 213765953.</ref> The power density of 241Am is only 1/4 that of 238Pu, and 241Am produces more penetrating radiation through decay chain products than 238Pu and needs more shielding. Its shielding requirements in a RTG are the third lowest: only 238Pu and 210Po require less. With a current global shortage<ref name="Ref_ff">Nell Greenfield-Boyce, Plutonium Shortage Could Stall Space Exploration, NPR, 28 September 2009, retrieved 2 November 2010</ref> of 238Pu, 241Am is being studied as RTG fuel by ESA<ref name="AmbrosiEtAl2019"/><ref name="ESAPP">Dr Major S. Chahal, [1], 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>"UK scientists generate electricity from rare element to power future space missions". National Nuclear Laboratory. Retrieved 6 May 2019.</ref> An advantage over 238Pu is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of 241Am RTGs expect 2-2.2 We/kg for 5–50 We RTGs design but practical testing shows that only 1.3-1.9 We 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.

250Cm

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 reflecting most of them), a different shielding material would have to be added in applications where neutrons are a concern.

Life span

90Sr-powered Soviet RTGs in dilapidated condition.

Most RTGs use 238Pu, which decays with a half-life of 87.7 years. RTGs using this material will therefore diminish in power output by a factor of 1 – (1/2)1/87.7, 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 W, after this length of time it would have a capacity of only 392 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 W for Voyager 1 and to 319 W for Voyager 2.<ref>"Voyager Mission Operations Status Reports". Voyager.jpl.nasa.gov web. Retrieved 24 July 2011.</ref> By 2022, these numbers had dropped to around 220 W.<ref>Jansen, Kristin. "NASA Celebrates 45 Years of Voyager 1, Enabled by Radioisotope Power". NASA News. NASA. Retrieved 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 (CoAs3), 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 mission.<ref>"Spacecraft 'Nuclear Batteries' Could Get a Boost from New Materials". JPL News. Jet Propulsion Laboratory. 13 October 2016. Retrieved 19 October 2016.</ref>

Safety

Diagram of a stack of 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". The Soviet Union constructed many uncrewed lighthouses and navigation beacons powered by RTGs using strontium-90 (90Sr). 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" /> In another case, three woodsmen in Tsalendzhikha Region, Georgia found two ceramic RTG orphan sources 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">"IAEA Bulletin Volume 48, No.1 – Remote Control: Decommissioning RTGs" (PDF). Malgorzata K. Sneve. Retrieved 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">"Report by Minister of Atomic Energy Alexander Rumyantsev at the IAEA conference "Security of Radioactive Sources," Vienna, Austria. March 11th 2003 (Internet Archive copy)" (PDF). Archived from the original (PDF) on 6 August 2003. Retrieved 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">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">"Top 10 Space Age Radiation Incidents". listverse.com. 20 January 2012. Retrieved 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 units) during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million.<ref name="Ref_j">Cassini Final Supplemental Environmental Impact Statement Archived 29 September 2006 at the Wayback Machine, Chapter 4, NASA, September 1997 (Links to other chapters and associated documents Archived 7 September 2006 at the Wayback Machine)</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">Cassini Final Supplemental Environmental Impact Statement Archived 29 September 2006 at the Wayback Machine, 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 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 reactors. A consequence of the shorter half-life is that plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. 17.3 curies (640 GBq)/g compared to 0.063 curies (2.3 GBq)/g<ref name="Ref_l">Physical, Nuclear, and Chemical, Properties of Plutonium, IEER Factsheet</ref>). For instance, 3.6 kg of plutonium-238 undergoes the same number of radioactive decays per second as 1 tonne of plutonium-239. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same,<ref name="Ref_m">Mortality and Morbidity Risk Coefficients for Selected Radionuclides, Argonne National Laboratory Archived 10 July 2007 at the Wayback Machine</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, 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 Lia [ka] 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

A 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:

  1. A launch failure on 21 April 1964 in which the U.S. Transit-5BN-3 navigation satellite failed to achieve orbit and burned up on re-entry north of Madagascar.<ref name="Transit">"Transit". Encyclopedia Astronautica. Archived from the original on 24 June 2002. Retrieved 7 May 2013.</ref> The 17,000 Ci (630 TBq) plutonium metal fuel in its 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.
  2. 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 238PuO2 microsphere fuel recovered and reused.

A. Angelo Jr. and D. Buden (1985). Space Nuclear Power. Krieger Publishing Company. ISBN 0-89464-000-3.</ref>

  1. 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">"Energy Resources for Space Missions". Space Safety Magazine. Retrieved 18 January 2014.</ref>
  2. 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 44,500 Ci (1,650 TBq) 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 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.
  3. 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 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">Mars 96 timeline, NASA</ref>

One RTG, the 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 238plutonium zirconium alloy fuel oxidized soil particles that are moving in a plume under the glacier.<ref>Kohli, M. S.; Conboy, K. J. (2002). Spies in the Himalayas: Secret Missions and Perilous Climbs. Lawrence, Kansas: University Press of Kansas. doi:10.2307/j.ctvj7wp9g. ISBN 978-0-7006-1223-9. OCLC 50803689.</ref>[page needed]

Many Beta-M RTGs produced by the Soviet Union to power lighthouses and beacons have become orphaned sources of radiation. Several of these units have been illegally dismantled for scrap metal (resulting in the complete exposure of the Sr-90 source), fallen into the ocean, or have defective shielding due to poor design or physical damage. The US Department of Defense cooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used by terrorists to construct a dirty bomb.<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 SrTiO3 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

Notes

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References

External links

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