Aradioisotope thermoelectricgenerator (RTG,RITEG), orradioisotope power system (RPS), is a type ofnuclear battery that uses an array ofthermocouples to convert theheat released by the decay of a suitableradioactive material intoelectricity by theSeebeck effect. This type ofgenerator 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 hundredwatts (or less) of power for durations too long forfuel cells, batteries, or generators to provide economically, and in places wheresolar cells are not practical. RTGs have been used as power sources insatellites,space probes, and uncrewed remote facilities such as a series oflighthouses built by theSoviet Union inside theArctic Circle.[1][unreliable source?][2]
Safe use of RTGs requires containment of theradioisotopes 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.
The RTG was invented in 1954 byMound Laboratories scientists Kenneth (Ken) C. Jordan (1921–2008) and John Birden (1918–2011).[3][4] They were inducted into theNational Inventors Hall of Fame in 2013.[5][6] Jordan and Birden worked on anArmy 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 usingpolonium-210 as the heat source. RTGs were developed in the US during the late 1950s byMound Laboratories inMiamisburg, Ohio, under contract with theUnited States Atomic Energy Commission. The project was led by Dr. Bertram C. Blanke.[7]
The first RTG launched into space by the United States wasSNAP 3B in 1961 powered by 96 grams ofplutonium-238 metal, aboard the NavyTransit 4A spacecraft. One of the first terrestrial uses of RTGs was in 1966 by the US Navy at uninhabitedFairway Rock inAlaska. RTGs were used at that site until 1995.
A common RTG application isspacecraft power supply.Several generations of RTG design have been used for probes that traveled far from the Sun, renderingsolar panels impractical. As such, they have been used forPioneer 10 and11;Voyager 1 and2;Galileo;Ulysses;Cassini;New Horizons; and are planned for theDragonfly mission to Titan. RTGs were also used instead of solar panels to power the twoViking landers, and for the scientific experiments left on the Moon by the crews ofApollo 12 through17 (SNAP 27s). Because theApollo 13 Moon landing was aborted, its RTG rests in theSouth Pacific Ocean, in the vicinity of theTonga Trench.[8] TheCuriosity andPerseverance Mars rover designs selected RTGs to allow greater flexibility in landing sites and longer lifespan than the solar-powered option,[9][10] as used inprior generations of rovers. RTGs were also used for theNimbus,Transit andLES satellites. By comparison, only a few space vehicles have been launched using full-fledgednuclear reactors: the SovietRORSAT series and the AmericanSNAP-10A.
In addition to spacecraft, theSoviet Union built 1,007 RTGs[11] to power uncrewed lighthouses and navigation beacons on theSoviet Arctic coast by the late 1980s.[11][12] Many different types of RTGs (includingBeta-M type) were built in the Soviet Union for a wide variety of purposes. The lighthouses were not maintained for many years after thedissolution of the Soviet Union in 1991. Some of the RTG units disappeared during this time—either bylooting or by the natural forces of ice/storm/sea.[11] In 1996, a project was begun byRussian and international supporters to decommission the RTGs in the lighthouses, and by 2021, all RTGs had been removed.[11]
As of 1992, theUnited 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 forTop-ROCC andSEEK IGLOO radar systems, predominantly located inAlaska, use RTGs. The units usestrontium-90, and a larger number of such units have been deployed both on the ground and on theocean floor than have been used on spacecraft, with publicregulatory documents suggesting that the US had deployed at least 100–150 during the 1970s and 1980s.[13][needs update]
In the past, small "plutonium cells" (very small238Pu-powered RTGs) were used in implantedheart pacemakers to ensure a very long "battery life".[14] As of 2004[update], about ninety were still in use. By the end of 2007, the number was reported to be down to just nine.[15] TheMound Laboratory Cardiac Pacemaker program began on 1 June 1966, in conjunction with NUMEC.[16] The program was cancelled in 1972, when it was recognized that the heat source would not remain intact during cremation and there was no way to completely ensure that the units would not be cremated with their users' bodies.
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The design of an RTG is simple by the standards ofnuclear 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 aheat 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[17][18].
A thermocouple is athermoelectric device that can convertthermal energy directly intoelectrical energy using theSeebeck 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 differenttemperatures, an electric current will flow in the loop. Typically a large number of thermocouples are connected in series to generate a higher voltage.
RTGs andfission reactors use very different nuclear reactions. Nuclear power reactors (including the miniaturized ones used in space) perform controlled nuclear fission in achain reaction. The rate of the reaction can be controlled with neutron absorbingcontrol 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 evennuclear accident. Chain reactions do not occur in RTGs. Heat is produced through spontaneousradioactive decay at a non-adjustable and steadily decreasing rate that depends only on the amount of fuel isotope and itshalf-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.
Due to the shortage of plutonium-238, a new kind of RTG assisted bysubcritical reactions has been proposed.[19] In this kind of RTG, the alpha decay from the radioisotope is also used in alpha-neutron reactions with a suitable element such asberyllium. This way a long-livedneutron source is produced. Because the system has a criticality close to but less than 1, i.e.Keff < 1, asubcritical multiplication is achieved which increases the neutron background and produces energy from fission reactions. Though the number of fissions produced in the RTG is very small (making their gamma radiation negligible), because each fission 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 the238Pu 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.[20][failed verification] However the essentials are unmodified.
RTG have been proposed for use on realistic interstellar precursor missions andinterstellar probes.[21] An example of this is theInnovative Interstellar Explorer (2003–current) proposal from NASA.[22]An RTG using241Am was proposed for this type of mission in 2002.[21] This could support mission extensions up to 1000 years on the interstellar probe, because241Am decays more slowly than238Pu.[21] Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products.[21] An interstellar probe proposal from 1999 suggested using three advanced radioisotope power sources (ARPS).[23] The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.[21] One mission proposed using the electricity to powerion engines, calling this method radioisotope electric propulsion (REP).[21]
A power enhancement for radioisotope heat sources based on a self-induced electrostatic field has been proposed.[24] According to the authors, enhancements of 5-10% could be attainable using beta sources.
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.
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.[25] 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 MSLCuriosity has 4.8 kg ofplutonium-238 dioxide.[26]
| 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,Perseverance/Mars 2020 rover andDragonfly (Not launched) | 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[27] | 5.2–5.4 | 38 |
| MHW-RTG | LES-8/9,Voyager 1 (3),Voyager 2 (3) | 160[27] | 2,400[28] | 238Pu | c. 4.5 | 37.7[27] | 4.2 | c. 36 |
| SNAP-3B | Transit-4A (1) | 2.7[27] | 52.5 | 238Pu | ? | 2.1[27] | 1.3 | ? |
| SNAP-9A | Transit 5BN1/2 (1) | 25[27] | 525[28] | 238Pu | c. 1 | 12.3[27] | 2.0 | c. 30 |
| SNAP-19 | Nimbus-3 (2),Pioneer 10 (4),Pioneer 11 (4) | 40.3[27] | 525 | 238Pu | c. 1 | 13.6[27] | 2.9 | c. 40 |
| modified SNAP-19 | Viking 1 (2), Viking 2 (2) | 42.7[27] | 525 | 238Pu | c. 1 | 15.2[27] | 2.8 | c. 40 |
| SNAP-27 | Apollo 12–17ALSEP (1) | 73 | 1,480 | 238Pu[29] | 3.8 | 20 | 3.65 | 19 |
| (fission reactor)Buk (BES-5)** | US-As (1) | 3,000 | 100,000 | highly enriched235U | 30 | 1,000 | 3.0 | 100 |
| (fission reactor) SNAP-10A*** | SNAP-10A (1) | 600[30] | 30,000 | highly enriched235U | 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[31][32]
*** 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[30] Reactor heat fed a thermoelectric conversion system for electrical production.[30]
**** not really an RTG, the ASRG uses aStirling power device that runs on radioisotope (seeStirling radioisotope generator)
| 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[33] | 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[34] | Remote U.S. arctic monitoring sites | 9–20 | SrTiO3 | 0.54 | 907–1,814 | |
| Sentinel 100F[34] | 53 | Sr2TiO4 | 1.77 | 1,234 | ||
| RIPPLE X[35] | Buoys, Lighthouses | 33[36] | SrTiO3 | 1,500 | ||
| Milliwatt RTG[37] | Permissive Action Link power source | 4–4.5 | 238Pu | ? | ? | |
The radioactive material used in RTGs must have several characteristics:[38]
The first two criteria limit the number of possible fuels to fewer than thirty nuclides[38] within the entiretable of nuclides.
Plutonium-238,curium-244,strontium-90, and most recentlyamericium-241 are the most often cited candidate isotopes, but 43 more isotopes out of approximately 1,300 were considered at the beginning in the 1950s.[7]
The table below does not necessarily give power densities for the pure material but for achemically inert form. Foractinides this is of little concern as their oxides are usually inert enough (and can be transformed intoceramics further increasing their stability), but foralkali metals andalkaline earth metals like caesium or strontium respectively, relatively complex (and heavy) chemical compounds have to be used. For example, strontium is commonly used asstrontium titanate in RTGs, which increasesmolar mass by about a factor of 2. Furthermore, depending on the source, isotopic purity may not be obtainable. Plutonium extracted fromspent nuclear fuel has a low share of Pu-238, so plutonium-238 for use in RTGs is usually purpose-made byneutron irradiation ofneptunium-237, further raising costs. Caesium infission 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. Ifisotope 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 thecritical mass is orders of magnitude below the mass needed to produce such amounts of power. As Sr-90, Cs-137 and other lighter radionuclidescannot maintain anuclear 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 ofsmall modular reactors, microreactors or non-nuclear power sources.
| Material | Shielding requirement | Power density (W/g) | Half-life (years) | ||
|---|---|---|---|---|---|
| 238Pu | Low | 0.54 | 87.7 | ||
| 90Sr | High | 0.95 | 28.8 | ||
| 210Po | Low | 140 | 0.378 | ||
| 241Am | Medium | 0.114 | 432 | ||
Plutonium-238 has a half-life of 87.7 years, reasonablepower density of 0.57 watts per gram,[39] 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 oflead 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 a238Pu RTG, as the casing itself is adequate.238Pu has become the most widely used fuel for RTGs, in the form ofplutonium(IV) oxide (PuO2).[40]However, plutonium(IV) oxide containing a natural abundance of oxygen emits neutrons at the rate of roughly2.3×103 n/sec/g of238Pu. This emission rate is relatively high compared to the neutron emission rate of plutonium-238 metal. The metal containing no light element impurities emits roughly2.8×103 n/sec/g of238Pu. These neutrons are produced by the spontaneous fission of238Pu.
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 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 phase16O2 exchange method. Regular production batches of238PuO2 particles precipitated as a hydroxide were used to show that large production batches could be effectively16O2-exchanged on a routine basis. High-fired238PuO2 microspheres were successfully16O2-exchanged showing that an exchange will take place regardless of the previous heat treatment history of the238PuO2.[41] 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.[42]
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.[43] 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.[44][43]
Strontium-90 has been used by the Soviet Union in terrestrial RTGs.90Sr decays byβ− decay into90Y, which quickly β-decays again. It has a lower decay energy than238Pu, but its shorter half-life of 28.8 years and lower atomic weight yield a power density for pure metal of 0.95 watts per gram.[45] As90
Sr is a very reactivealkaline earth metal and abone seeker that accumulates in bone-tissue due to its chemical similarity tocalcium (once in the bones it can significantly damage thebone marrow, a rapidly dividing tissue), it is usually not used in pure form in RTGs. The most common form is theperovskite strontium titanate (SrTiO3) which is chemically nigh-inert and has a high melting point. While itsMohs hardness of 5.5 has made it ill-suited as adiamond 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 it reduces power density, as the TiO3 part of the material does not produce any decay heat.90Sr has a highfission product yield in the fission of both235
U and239
Pu and is thus available in large quantities at a relatively low price if extracted fromspent nuclear fuel.[45] Starting from the oxide or the native metal, one pathway to obtaining SrTiO3 is to let it transform tostrontium hydroxide in aqueous solution, which absorbscarbon dioxide from air to become less solublestrontium carbonate. Reaction of strontium carbonate withtitanium dioxide at high temperature produces the desired strontium titanate pluscarbon dioxide. If desired, the strontium titanate product can then be formed into a ceramic-like aggregate viasintering.
Some prototype RTGs, first built in 1958 by the US Atomic Energy Commission, have usedpolonium-210. This isotope provides phenomenal power density (pure210Po emits140 W/g) because of its highdecay rate, but has limited use because of its very short half-life of 138 days. A half-gram sample of210Po reaches temperatures of over 500 °C (900 °F).[46] As210Po is a pure alpha-emitter and does not emit significant gamma or X-ray radiation, the shielding requirements are as low as those for238Pu. 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 of209
Bi, the only naturally occurring isotope ofbismuth. It is this accidental production that is cited as an argument against the use oflead-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 howtritium is economically recovered from the heavy water moderator inCANDUs.
Americium-241 is a candidate isotope with much greater availability than238Pu. Though241Am has a half-life of 432 years, which is about five times longer than that of238Pu and could hypothetically power a device for centuries, missions with more than 10 years were not the subject of the research until 2019.[47] The power density of241Am is only one-fourth that of238Pu, and241Am produces more penetrating radiation through decay chain products than238Pu and needs more shielding. Its shielding requirements in a RTG are the third lowest: only238Pu and210Po require less. With a current global shortage[48] of238Pu,241Am is being studied as RTG fuel byESA[47][49] and in 2019, UK'sNational Nuclear Laboratory announced the generation of usable electricity.[50] An advantage over238Pu is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of241Am 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.[47]Americium-241 is currently used in small quantities in household smoke detectors and thus its handling and properties are well-established. However, it decays toneptunium-237, the most chemically mobile among the actinides.
Curium-250 is the isotope with the lowest atomic number that primarily decays by spontaneous fission, a process that releases many times more energy than alpha decay. Compared to plutonium-238, curium-250 has 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 thancalifornium-252 but not negligible) some applications require a further shielding againstneutron radiation. As lead, which is an excellent shielding material against gamma rays and beta ray inducedBremsstrahlung, is not a good neutron shield (insteadreflecting most of them), a different shielding material would have to be added in applications where neutrons are a concern.
Most RTGs use238Pu, which decays with a half-life of 87.7 years. RTGs using this material therefore diminish in power output by a factor of 1 – (1/2)1/87.7, or 0.787%, per year.
One example is theMHW-RTG used by theVoyager 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 convertthermal energy intoelectrical 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 forVoyager 1 and to 319 W forVoyager 2.[51] By 2022, these numbers had dropped to around 220 W.[52]
NASA has developed amulti-mission radioisotope thermoelectric generator (MMRTG) in which the thermocouples would be made ofskutterudite, acobaltarsenide (CoAs3), which can function with a smaller temperature difference than the currenttellurium-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 nextNew Frontiers mission.[53]
Radioactive materials in RTGs are dangerous and can be used for malicious purposes. They are not useful for a genuinenuclear weapon, but still can serve in a "dirty bomb". TheSoviet Union constructed many uncrewed lighthouses and navigation beacons powered by RTGs usingstrontium-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.[12]In another case, three woodsmen inTsalenjikha Region, Georgia found two ceramic RTGorphan 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, and the third died. The units were eventually recovered and isolated.[54] There are about 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.[55] Transforming the radioactive material into an inert form reduces the danger of theft by people unaware of the radiation hazard (such as happened in theGoiânia accident in an abandoned Cs-137 source where the caesium was present in easily water-solublecaesium 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.
RTGs pose a risk ofradioactive 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.[56][57]
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 129radioisotope 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.[58] 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.[59] The launch was successful andCassini–Huygens reachedSaturn.
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 ofiridium metal and encased in high-strengthgraphite 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 highlyinsoluble.
Theplutonium-238 used in these RTGs has ahalf-life of 87.74 years, in contrast to the 24,110 year half-life ofplutonium-239 used innuclear weapons andreactors. Due to the shorter half-life, plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. 17.3curies (640 GBq)/g compared to 0.063 curies (2.3 GBq)/g[60]). 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,[61] 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 theskeleton, the surface of which is likely to absorb the isotope, and theliver, where the isotope will collect and become concentrated.
A case of RTG-related irradiation is theLia radiological accident inGeorgia, December 2001.Strontium-90 RTG cores were dumped behind, unlabelled and improperly dismantled, near the Soviet-builtEnguri Dam. Three villagers from the nearby village ofLia were unknowingly exposed to it and injured; one of them died in May 2004 from the injuries sustained. TheInternational Atomic Energy Agency led recovery operations and organized medical care. Two remaining RTG cores are yet to be found as of 2022.
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There have been several known accidents involving RTG-powered spacecraft:
ASNAP-19C RTG was lost near the top ofNanda Devi mountain in India in 1965, when it was stored in arock formation near the top of the mountain in the face of a snowstorm. It was intended to power a CIA remote automated intelligence station collecting telemetry from the Chinese rocket testing facility atLop Nur. The seven capsules were probably carried down the mountain onto a glacier by a subsequent avalanche and have never been recovered. It is most likely that they melted through the glacier and were pulverized,[66] whereupon the238Pu–Zr alloy fuel oxidized soil particles that are moving in a plume under the glacier.[67][page needed] As the glaciers from these peaks feed some of India's largest rivers, including theGanges, there are concerns about massive radioactive contamination originating from these RTGs. Accounts from the operatives who attempted the installation recounted that thesherpas who aided their mission jockeyed to carry the capsules as they produced heat; referring to the capsules asGuru Rinpoche, the warmth offered some respite from the freezing winds, but some of the operatives thought the capsules were inadequately shielded and irradiated the men around them.[68]
ManyBeta-M RTGs produced by the Soviet Union to powerlighthouses andbeacons have becomeorphaned sources of radiation. Its design allowed for the use of normal industrial bolts instead of intrinsically safe or safety interlocked bolts (most likely to reduce cost), and did not require the use of intrinsically safe opening mechanisms or made any use of tamper resistant shielding systems. Several of these units have also been illegally dismantled for scrap metal, or been exposed to storm conditions, freezing and water penetration, common issues in those abandoned in the harsh Russian arctic.[1] Some have even fallen into the ocean, or have defective shielding due to poor design or physical damage. TheUS Department of Defense cooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used byterrorists to construct adirty bomb.[12] 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 sensitiveintestinal 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.[citation needed]
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