Radioisotope thermoelectric generator

Diagram of an RTG used on the Cassini probe

A radioisotope thermoelectric generator (RTG, RITEG) is an electrical generator 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 generator has no moving parts.

RTGs have been used as power sources in satellites, space probes, and unmanned remote facilities such as a series of lighthouses built by the former Soviet Union inside the Arctic Circle. RTGs are usually the most desirable power source for 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. Safe use of RTGs requires containment of the radioisotopes long after the productive life of the unit.

History

A pellet of ²³⁸PuO₂ to be used in an RTG for either the Cassini or Galileo mission. 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.

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.^[1]

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

A common RTG application is spacecraft power supply. Systems for Nuclear Auxiliary Power (SNAP) units were used for probes that traveled far from the Sun rendering solar panels impractical. As such, they were used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini, New Horizons and the Mars Science Laboratory. RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17 (SNAP 27s). Because the Apollo 13 moon landing was aborted, its RTG rests in the South Pacific ocean, in the vicinity of the Tonga Trench.^[2] RTGs were also used for the Nimbus, Transit and LES satellites. By comparison, only a few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.

In addition to spacecraft, the Soviet Union constructed many unmanned lighthouses and navigation beacons powered by RTGs.^[3] Powered by strontium-90 (⁹⁰Sr), they are very reliable and provide a steady source of power. Critics argue that they could cause environmental and security problems as leakage or theft of the radioactive material could pass unnoticed for years, particularly as the locations of some of these lighthouses are no longer known due to poor record keeping. In one instance, the radioactive compartments were opened by a thief.^[3] In another case, three woodsmen in Georgia came across two ceramic RTG heat sources that had been stripped of their shielding. Two of the three were later hospitalized with severe radiation burns after carrying the sources on their backs. The units were eventually recovered and isolated.^[4]

There are approximately 1,000 such RTGs in Russia. All of them have long exhausted their 10-year engineered life spans. They are likely no longer functional, and may be in need of dismantling. Some of them have become the prey of metal hunters, who strip the RTGs' metal casings, regardless of the risk of radioactive contamination.^[5]

The United States Air Force uses RTGs to power remote sensing stations for Top-ROCC and SEEK IGLOO radar systems predominantly located in Alaska.^[6]

In the past, small "plutonium cells" (very small ²³⁸Pu-powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life".^[7] As of 2004, about 90 were still in use.

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 converts thermal energy directly into electrical energy using the Seebeck effect. It is made of two kinds of metal (or semiconductors) that can both conduct electricity. They are connected to each other in a closed loop. If the two junctions are at different temperatures, an electric current will flow in the loop.

Fuels

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  Inspection of Cassini spacecraft RTGs before launch

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  New Horizons in assembly hall

Criteria for selection of isotopes

The radioactive material used in RTGs must have several characteristics:

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 10 times as much energy as the beta decay of strontium-90 or caesium-137.
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.

The first two criteria limit the number of possible fuels to fewer than 30 atomic isotopes^[8] within the entire table of nuclides.

Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242, americium-241 and thulium isotopes have also been studied.

²³⁸Pu

Plutonium-238 has a half-life of 87.7 years, reasonable power density of 0.54 watts per gram, 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).

Unlike the latter RTG fuels, ²³⁸Pu must be specifically synthesized and is not abundant as a nuclear waste product. At present only Russia has maintained consistent ²³⁸Pu production, while the USA restarted production at circa 1.5 kg a year in 2013 after a c. 25-year hiatus.

At present these are the only countries with declared production of ²³⁸Pu in quantities useful for RTGs. ²³⁸Pu is produced at typically 85% purity and its purity decreases over time.^[9]

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. Because the energy output is lower it reaches lower temperatures than ²³⁸Pu, which results in lower RTG efficiency. ⁹⁰Sr is a high yield waste product of nuclear fission and is available in large quantities at a low price.^[10]

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).^[11]

²⁴¹Am

Americium-241 is a potential candidate isotope with a longer half-life than ²³⁸Pu: ²⁴¹Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the power density of ²⁴¹Am is only 1/4 that of ²³⁸Pu, and ²⁴¹Am produces more penetrating radiation through decay chain products than ²³⁸Pu and needs more shielding. Even so, its shielding requirements in an RTG are the second lowest of all possible isotopes: only ²³⁸Pu requires less. With a current global shortage^[12] of ²³⁸Pu, ²⁴¹Am is being studied as RTG fuel by ESA.^[13] 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 W_e/kg for 5-50 W_e RTGs design, putting ²⁴¹Am RTGs at parity with ²³⁸Pu RTGs within that power range.^[14]

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 1−0.5^1/87.74, or 0.787%, per year.

One example is the 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.^[15]

Efficiency

RTGs use thermocouples to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%. Thermoelectric materials in space missions to date have included silicon–germanium alloys, lead telluride and tellurides of antimony, germanium and silver (TAGS). Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.

Thermoelectric effect
Principles Thermoelectric effect Seebeck effect Peltier effect Thomson effect Seebeck coefficient Ettingshausen effect Nernst effect
Applications Thermoelectric materials Thermocouple Thermopile Thermoelectric cooling Thermoelectric generator Radioisotope thermoelectric generator Automotive thermoelectric generator

A thermionic converter—an energy conversion device which relies on the principle of thermionic emission—can achieve efficiencies between 10–20%, but requires higher temperatures than those at which standard RTGs run. Some prototype ²¹⁰Po RTGs have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these unfeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes.

Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light emitted by a hot surface rather than visible light into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and can be overlaid on top of thermocouples, potentially doubling efficiency. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%,^[16] but have not been tested with actual radioisotopes. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells and silicon thermocouples degrade faster than metal thermocouples, especially in the presence of ionizing radiation.

Dynamic generators can provide power at more than 4 times the conversion efficiency of RTGs. NASA and DOE have been developing a next-generation radioisotope-fueled power source called the Stirling Radioisotope Generator (SRG) that uses free-piston Stirling engines coupled to linear alternators to convert heat to electricity. SRG prototypes demonstrated an average efficiency of 23%. Greater efficiency can be achieved by increasing the temperature ratio between the hot and cold ends of the generator. The use of non-contacting moving parts, non-degrading flexural bearings, and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibration can be eliminated as a concern by implementation of dynamic balancing or use of dual-opposed piston movement. Potential applications of a Stirling radioisotope power system include exploration and science missions to deep-space, Mars, and the Moon.

The increased efficiency of the SRG may be demonstrated by a theoretical comparison of thermodynamic properties, as follows. These calculations are simplified and do not account for the decay of thermal power input due to the long half-life of the radioisotopes used in these generators. The assumptions for this analysis include that both systems are operating at steady state under the conditions observed in experimental procedures (see table below for values used). Both generators can be simplified to heat engines to be able to compare their current efficiencies to their corresponding Carnot efficiencies. The system is assumed to be the components, apart from the heat source and heat sink.,^[17][18][19]

The thermal efficiency, denoted η_th, is given by:

Where primes ( ' ) denote the time derivative.

From a general form of the First Law of Thermodynamics, in rate form:

Assuming the system is operating at steady state and ,

η_th, then, can be calculated to be 110 W / 2000 W = 5.5% (or 140 W / 500 W = 28% for the SRG). Additionally, the Second Law efficiency, denoted η_II, is given by:

Where η_th,rev is the Carnot efficiency, given by:

In which T_{heat sink} is the external temperature (which has been measured to be 510 K for the MMRTG (Multi-Mission RTG) and 363 K for the SRG) and T_{heat source} is the temperature of the MMRTG, assumed 823 K (1123 K for the SRG). This yields a Second Law efficiency of 14.46% for the MMRTG (or 41.37% for the SRG).

Safety

Diagram of a stack of general purpose heat source modules as used in RTGs

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.^[20][21]

However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the Cassini–Huygens probe launched in 1997 estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its 3 RTGs (or from its 129 radioisotope heater units) during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million.^[22] If an accident which had the potential to cause contamination occurred during the launch phases (such as the spacecraft failing to reach orbit), the probability of contamination actually being caused by the RTGs was estimated at about 1 in 10.^[23] In any event, the launch was successful and Cassini–Huygens reached Saturn.

The plutonium-238 used in these RTGs has a half-life of 87.74 years, in contrast to the 24,110 year half-life of plutonium-239 used in nuclear weapons and reactors. A consequence of the shorter half-life is that plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. 17.3 curies (640 GBq)/g compared to 0.063 curies (2.3 GBq)/g^[24]). 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,^[25] plutonium-238 is around 275 times more toxic by weight than plutonium-239.

The alpha radiation emitted by either isotope will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk is the skeleton, the surface of which is likely to absorb the isotope, and the liver, where the isotope will collect and become concentrated.

There have been several known accidents involving RTG-powered spacecraft:

1. The first one was 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.^[26] The 17,000 Ci (630 TBq) plutonium metal fuel in its SNAP-9a RTG was injected 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.
2. The second was the Nimbus B-1 weather satellite whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Launched from the Vandenberg Air Force Base, its SNAP-19 RTG containing relatively inert plutonium dioxide was recovered intact from the seabed in the Santa Barbara Channel five months later and no environmental contamination was detected.^[27]
3. In 1969 the launch of the first Lunokhod lunar rover mission failed, spreading polonium 210 over a large area of Russia ^[28]
4. 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 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.
5. Mars 96 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.^[29]

A SNAP-27 RTG deployed by the astronauts of Apollo 14 identical to the one lost in the reentry of Apollo 13

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.

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.^[3]

28 U.S. space missions have safely flown radioisotope energy sources since 1961.^[30]

Nuclear fission

RTGs and nuclear power reactors use very different nuclear reactions. Nuclear power reactors use controlled nuclear fission in a chain reaction. The rate of the reaction can be controlled with neutron absorbers, so power can be varied with demand or shut off entirely for maintenance. However, care is needed to avoid uncontrolled operation at dangerously high power levels.

Chain reactions do not occur in RTGs, so heat is produced at an unchangeable, though steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. An accidental power excursion is impossible. However, if a launch or re-entry accident occurs and the fuel is dispersed, the combined power output of the radionuclides now set free does not drop. In an RTG, heat generation cannot be varied with demand or shut off when not needed. Therefore, auxiliary power supplies (such as rechargeable batteries) may be needed to meet peak demand, and adequate cooling must be provided at all times including the pre-launch and early flight phases of a space mission.

Subcritical multiplicator RTG

Because of the shortage of plutonium-238, a new kind of RTG assisted by subcritical reactions has been proposed.^[31] 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. K_eff < 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 almost 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.^[32]. However the essentials are unmodified.

RTG for interstellar probes

RTG have been proposed for use on realistic interstellar precursor missions and interstellar probes.^[33] An example of this is the Innovative Interstellar Explorer (2003–current) proposal from NASA.^[34] An RTG using ²⁴¹Am was proposed for this type of mission in 2002.^[33] This could support mission extensions up to 1000 years on the interstellar probe, because the power output would be more stable in the long term than plutonium.^[33] Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products.^[33] An interstellar probe proposal from 1999 suggested using three advanced radioisotope power sources (ARPS).^[35]

The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.^[33] One mission proposed using the electricity to power ion engines, calling this method radioisotope electric propulsion (REP).^[33]

Electrostatic-boosted radioisotope heat sources

A power enhancement for radioisotope heat sources based on a self-induced electrostatic field has been proposed.^[36] According to the authors, enhancements of up to 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

* The ASRG is not really an RTG: it uses a Stirling power device that runs on radioisotope (see Stirling radioisotope generator).

** The BES-5 Buk (БЭС-5) reactor was a fast breeder reactor which used thermocouples based on semiconductors to convert heat directly into electricity.^[41][42]

*** 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.^[40]Reactor heat fed a thermoelectric conversion system for electrical production.^[40]

Terrestrial

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.^[44] 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,^[45] while the Cassini spacecraft has 32.7 kg.^[46]

See also

References

Notes

External links

Wikimedia Commons has media related to Radioisotope thermoelectric generators.

• NASA Radioisotope Power Systems website – RTG page
• NASA JPL briefing, Expanding Frontiers with Radioisotope Power Systems – gives RTG information and a link to a longer presentation
• SpaceViews: The Cassini RTG Debate
• Stirling Radioisotope Generator
• DOE contributions – good links
• Idaho National Laboratory – Producer of RTGs
• Idaho National Laboratory MMRTG page with photo-based "virtual tour"

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