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Radioisotope thermoelectric generators (RTGs) convert the heat generated by radioactive material to produce electricity using thermocouples. Thermocouples are devices made up of two different metals, or semiconductors, that produce an electric current when there is a temperature differential between them, known as the Seebeck effect.
RTGs are lightweight, compact, and reliable power systems with no moving parts. The first RTG was built in 1954 by John Birden and Ken Jordan working at Monsanto's Mound Laboratory.
RTGs are primarily used in space missions when solar arrays are not viable power sources (probes that need to work in any environment for long periods of time). These include missions traveling a long distance from the sun where solar energy is not sufficient to power a probe or allowing rovers/landers to operate at night.
Outside of space applications, approximately 1,000 RTGs were used to power Russian lighthouses and navigation beacons. All such RTGs have exhausted their ten-year operating spans. RTGs were also installed in various US commissioned arctic monitoring sites. Given the radiation hazards of RTGs, their Earth-based use is typically restricted to remote, uncrewed facilities.
The Seebeck effect, first discovered by Thomas Seebeck in 1821, produces an electric current when there is a temperature differential between two conductors. The Seebeck effect occurs due to electrons transferring both charge (electric current) and heat. Therefore, due to the laws of thermodynamics, electrons will flow from the high-temperature conductor towards the cooler conductor. When connected through an electrical circuit, a direct current is produced.
The Seebeck effect is weak and typically only produces small voltages (a few microvolts per kelvin of temperature difference at the junction). To produce a significant voltage between two conductors requires a large temperature differential, or connecting numerous Seebeck effect devices together.
The pair of conductors that form the circuit utilizing the Seebeck effect is known as a thermocouple. Thermocouples have many applications including using the temperature-dependent voltage produced to measure temperature, or as thermoelectric generators such as RTGs.
The Peltier effect is the reverse phenomenon of the Seebeck effect, where applying a voltage between two dissimilar conductors can produce a temperature differential. This is commonly used for thermoelectric cooling, in devices known as Peltier coolers or Thermoelectric Coolers (TECs). While the cooling power of these devices is less than that of vapor-compression refrigeration, their small size/weight, high reliability, and lack of moving parts/circulation of liquid means they have many applications such as CPU cooling and cooling semiconductor detectors.
Early RTG research carried out by Bertram Blanke evaluated over 1300 radioactive isotopes for use as the radioactive fuel within RTGs. He found only forty-seven that had the required characteristics.
Suitable radioisotopes for use as fuel in power systems require:
- the production of high energy radiation
- the production of high radiation decay heat
- a long half-life for continual and relatively steady energy production
- a large heat production to mass ratio to be as light as possible for space missions
- low shielding requirements (in particular low neutron output)
- existence in an insoluble form to not be readily absorbed by the body in the event of an accident
- existence in a form with no or minimal chemical toxicity
- high-temperature stability
To build a compact RTG with effective heat generation and low shielding requires radioactive decay products that have short absorption lengths. Radioisotopes generating alpha radiation produce the most heat and are the easiest to shield, making them the most appropriate RTG fuels. However, isotopes yielding beta and gamma radiation can also be viable RTG fuels as long as a suitable material for absorption and heat conversion is utilized.
Plutonium-238 (Pu-238) is the most commonly used fuel in RTGs. With a half-life of 87.7 years (decay constant of 0.0079), the radioisotope alpha decays to uranium, producing a 5.593 MeV alpha particle.
Pellet of Plutonium-238
Plutonium-238 also has some very low branching ratio decay modes including spontaneous fission (1.9 x 10-9), silicon-32 cluster emission (1.4 x 10-16), and magnesium-28 / magnesium-30 double cluster emission (6 x 10-17).
The advantages of Plutonium-238 for use as fuel in radioisotope power systems include high alpha radiation/heat decay output, reduced shielding requirements, a long half-life, and the ability for it to be packaged into small fuel pellets.
With a decay constant of 0.0079, a kg of plutonium-238 (around 2.5 x 1024 atoms) equates to roughly 2 x 1022 alpha decays a year or 6.3 x 1014 decays every second. At 5.593 MeV per alpha particle, 1 kg of plutonium-238 produces roughly 3.5 x 1015 MeV per second or 550 joules per second (W). Therefore an estimate of the output from an RTG containing 1 kg of plutonium-238 is 0.5 kW of thermal power. Assuming a typical high-end efficiency of the thermocouple surrounding the fuel to be 7%, this returns an output of 39 We / kg of electrical power.
With a long half-life, it takes 87.7 years for the heat output from plutonium-238 fuel to drop by half, with only about a 0.8% drop per year from the beginning of mission (BOM) output. Plutonium-238 offers a high radiation output to generate enough heat to be useful as RTG fuel but with a long enough half-life for long multi-decade missions.
To improve safety, the fuel is used in the form of plutonium oxide (PuO2). In this ceramic form, it primarily breaks into large pieces rather than fine particles.
Plutonium-238 produces a decay chain resulting in the stable element Lead-206. The entire decay chain is shown below.
Plutonium-238 can be produced in multiple ways:
- Hitting uranium-238 with deuterons (an isotope of the hydrogen nucleus containing one proton and one neutron) to produce two neutrons and neptunium-238, which beta decays into plutonium-238.
- Irradiating americium in a nuclear reactor or by irradiating neptunium-237 (a minor actinide derived from reprocessing spent nuclear fuel). Both require subsequent chemical reactions that involve dissolving the components in nitric acid to extract the plutonium-238.
- Reactor-grade plutonium from spent nuclear fuel consists of multiple plutonium isotopes including 1% - 2% plutonium-238. However, due to the difficult processes involved in separating the isotope, spent nuclear fuel is not an effective source of plutonium-238 for RTGs.
After a break of thirty years, Congress began providing funds to NASA and the Department Of Energy (DOE) Office of Nuclear Energy in 2011 to resume US domestic production of plutonium for civil space applications using a series of specialized facilities. US production of plutonium oxide for space exploration was previously performed at the DOE's Savannah River Plant in South Carolina but ceased in the 1980s with purchases of heat sources during the break in US production made from Russia.
US production of plutonium-238 is performed at Oak Ridge National Laboratory (ORNL), beginning in 2015. The lab has consistently been increasing its production with the aim of producing 1.5 kg per year by 2026.
ORNL receives neptunium-237 feedstock from Idaho National Laboratory (INL), where it is mixed with aluminum and pressed into pellets. The pellets are put into tubes and irradiated in ORNL’s High Flux Isotope Reactor (HFIR), which causes the neptunium to transmute into plutonium-238. The pellets are moved to shielded hot cells in ORNL’s Radiochemical Engineering Development Center, plutonium-238 is separated from the neptunium through a series of chemical processes and converted to an oxide powder before shipping to Los Alamos for fabrication into ceramic pellets. Leftover neptunium is recycled to produce more plutonium-238.
The production of plutonium-238 is very expensive. Excluding the upfront investments needed to re-establish production in the US, estimates suggest it costs $4 million to make 1 pound (0.45 kg) of plutonium-238.
Strontium-90 is a beta emitting radioisotope with a 28.1 year half-life that has found use as an RTG fuel. It was the fuel used by most of the Soviet ocean bottom and Arctic devices. It produces roughly 15% less thermal output (0.46 kW / kg) compared to plutonium-238, and its outputs deteriorate roughly three times as fast. Plus its lower surface temperature reduces the efficiency of the thermoelectric conversion, making a Strontium-90 RTG between 50% to 100% heavier than a plutonium-238 RTG of the same output.
The advantage of strontium-90 is that it is a high-yield fission product, produced by about 5% of all fission reactions and it is feasible to mine strontium-90 from used nuclear fuel. There is also a precedence for widely licensing small quantities of sealed strontium-90 in the US.
Curium-244 has many attractive properties for use in RTGs. It decays to plutonium-240 producing 5.9MeV alpha particles at a half-life of 18.1 years. In the oxide form (244Cm2O3), it has a high power output of 2.58 kW / kg and a power density of 27 W/cm3 higher than most competing materials. The isotope is present in wastes from the reprocessing of irradiated fuel in nuclear reactors with proposed methods of recovery at lower costs than plutonium-238 production.
The radioisotope emits comparatively little gamma radiation, but does emit high-energy neutrons due to spontaneous fission of a very small fraction of nuclei.
Polonium-210 has been investigated as a heat source for RTG space applications and was used by the Soviet Union in the Lunokhod lunar rovers. It has a short half-life of 140 days emitting 5.3 MeV alpha particles. Its half-life means it is only practical for short missions that require light power sources (shorter half-life equates to higher activity).
Americium-241 is being studied as a potential RTG fuel. It has a longer half-life of 432 years, giving it the potential to power missions for centuries. The radioisotope emits 5.4MeV alpha particles. Americium-241 fueled RTGs are not expected to match the power output of plutonium-238, but medium-sized RTGs (5-50W) are predicted to provide an electrical power output of roughly 2 We / kg.
The use of americium-241 as a source of heat for radioisotope power systems has been under development by the European Space Agency since 2009. Compared to producing plutonium-238 in Europe, americium-241 fuel can be produced economically in both an oxide or ceramic form. A chemical separation method developed by the UK’s National Nuclear Laboratory produces high isotopic purity, extracting it from stored separated plutonium produced during the reprocessing of civil fuel.
Radioisotope thermoelectric generators have been used in NASA missions for almost six decades. Early RTGs carried small amounts of radioactive material that were designed to burn up at high altitudes during an accidental reentry. The first RTG launched into space was the Systems for Nuclear Auxiliary Power (SNAP) 3 design, launched in 1961 aboard Transit Navy Navigational Satellite Transit 4A.
Notable NASA missions that utilized RTGs include the following:
- Six Apollo missions (12-17), powering the Apollo Lunar Surface Experiment Packages (ALSEPs). The SNAP-27 RTG used in the ALSEPs was designed to provide 63 watts of power at 16 VDC (volts of direct current) one year after placement on the lunar surface. RTGs were chosen due to their lightweight, reliability, and ability to provide power throughout the long lunar day-night cycles. The sealed fuel capsule was kept separate in the lunar module with the device assembled on the lunar surface after landing. All five SNAP-27 RTG placed on the Moon (Apollo 13 was aborted in transit to the Moon) exceeded their mission lifetimes and were still operating when NASA shut down the stations on September 30th, 1977.
- The Pioneer 10 and 11 missions, launched in 1972 and 1973 respectively, were each powered by four SNAP-19 RTGs delivering approximately 165 watts of electricity at launch. Pioneer 10 was the first spacecraft to survive passing through the asteroid belt and the high radiation environment of Jupiter. Pioneer 11 was the second probe to investigate Jupiter and the first to explore Saturn and its rings.
- The Viking missions to Mars (1 and 2) launched in 1975 were each made up of an orbiter (solar powered) and an RTG powered lander. The landers contained two SNAP-19 RTGs delivering approximately 85 watts of electrical power.
- The Voyager probes 1 and 2 launched in 1977 to explore Jupiter, Saturn, Uranus, and Neptune required significant power upgrades compared to previous missions. This led to the development of the multihundred-watt (MHW) RTG, utilizing a new heat source with twenty-four pressed plutonium oxide fuel spheres.
- The Mars Science Laboratory (MSL) Curiosity rover and Mars 2020 Perseverance Mars rover are both powered by the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) design.
Other NASA missions powered by RTGs include Nimbus III, Galileo, Ulysses, Mars Pathfinder, Cassini, and New Horizons.
RTGs have also been used in Soviet and Russian spacecraft including many Kosmos probes between 1965 (Kosmos 84) and 1988 (Kosmos 1932).
Reports suggest China’s Chang’e 3 lunar lander uses a Pu-238 fueled RTG.
RTGs launched by the US
RTGs pose significant safety challenges due to the radioactive fuel used, including the following:
- The dose from proximity to the device, primarily caused by neutral particles (neutrons, gamma rays) escaping the shielding
- The release and dispersal of radioactive material
- The theft of radioactive material
While nearly all charged particles are stopped locally, a non-trivial level of neutral particles (neutrons, gamma rays) escape the fuel and pose health risks for personnel during terrestrial shipping, integration, and testing. The radiation field from RTG fuels can also cause damage to instrumentation, materials, and electronics.
The GPHS-RTG that flew on the Ulysses, Galileo, Cassini, and New Horizons missions produced a neutron dose rate of between 20 and 50 mrem (0.2 mSv - 0.5 mSv) and a gamma dose rate between 5 mrem/h to 10 mrem/h (0.05-0.1 mSv/h) at 1 m from the center of the RTG. The average American receives a dose of roughly 620 mrem (6.2 mSv) each year.
In the event of a mission accident, there is the potential for the release and dispersal of the fuel into the environment and subsequent exposure to humans. To minimize this risk, radioisotope power systems have multiple safety features. This includes producing fuel in ceramic forms such that it breaks into large pieces rather than being vaporized into fine particles, which can be a health hazard when inhaled. The ceramic form also prevents the material from being absorbed into the body if ingested.
Model of the MMRTG
The MMRTG flown on MSL and Mars 2020 has safety features including the ceramic form of the plutonium dioxide fuel, iridium metal cladding, graphite sleeves that protect the fuel clads, and carbon-fiber material that forms the aeroshell model of an MMRTG, including its internal General Purpose Heat Source (GPHS) modules.
Plutonium-238 is a dangerous carcinogenic substance that is hard to locate once it enters the body and has been absorbed. The main health hazards come from the alpha (α) radiation that delivers localized damage to cells increasing the risk of cancer. Traces of Plutonium-238 get lodged in soft tissues, like in the bone marrow, the liver, on bone surfaces, and other non-calcified bony structures. The primary threat to human health is from inhaling this radioactive substance. This can damage the cells and tissues of the lungs and the bronchial tubes. The substance can also enter the body through cuts and abrasions and be absorbed into the bloodstream.
On April 21, 1964, during the launch of the Transit 5BN-3 navigational satellite, a computer malfunction prematurely shut down an upper stage booster, causing it to fail to reach orbit. The satellite and its SNAP-9A RTG power system (25W) reentered the Earth's atmosphere, burning up completely at an altitude of roughly 50km. In the event of accidental reentry, early RTG designs were intended to burn up in the upper atmosphere to disperse their fuel over a large area.
The RTG burned up above Madagascar and traces of plutonium were found in the area months later. The accident released approximately 7.4 x 1014 Bq (20,000 Ci) of Pu-238 and led to a change in RTG design to survive reentry intact, maintaining the containment and confinement of the fuel.
Intact Nimbus B-1 heat sources during the underwater recovery operation.
On May 18, 1968, the Nimbus B-1 meteorological satellite launch at Vandenberg Air Force Base, California was terminated shortly after launch (roughly one minute) by the range safety office to protect the public and property in the area from an erratically ascending launch vehicle. The launch vehicle and satellite were completely destroyed, but the two SNAP-19B2 RTGs aboard survived intact with no release of radioactive fuel.
The plutonium heat sources were recovered from the Santa Barbara Channel near the California coast after five months in 300 feet of seawater. The fuel capsules were reused for a later mission.
In 1969, an explosion during the launch of a Lunokhod lunar rover by the USSR in Baikonur spread radioactive material over a large area of Russia. The RTG aboard was powered by polonium-210 a short half-life (140 days) isotope emitting alpha particles.
In April 1970, Apollo 13 was aborted in transit to the Moon due to an oxygen tank explosion in the service module. To power the ALSEP, a SNAP-27 RTG was aboard the lunar lander with the fuel canister and thermoelectric generator housing stored separately. The lunar lander served as the crew's lifeboat until returning to the command module for atmospheric reentry.
The lunar module was jettisoned shortly before reaching Earth and burned up in the atmosphere over the southwest Pacific. Upon reentry, the graphite reentry cask which contained the RTG fuel sank to the bottom of the Pacific ocean (five to six miles deep) in the vicinity of the Tonga Trench. No detected release of radioactive material was found by atmospheric and oceanic monitoring. Apollo 13's RTG fuel canister containing 3.9 kg of plutonium was never recovered.
On November 16, 1996, shortly after launch, the Russian spacecraft Mars 96 reentered the Earth's atmosphere over the southern Pacific ocean. The booster rocket malfunctioned during the fourth stage, causing a failed launch. The mission carried 200 g of plutonium.
Many Russian terrestrial RTG installations may be lost or abandoned. These devices contain strontium-90 heat sources.
During Operation Hat, a CIA operation, a SNAP RTG was lost on Nanda Devi in the Himalayas in autumn 1965.
The US DOE develops the Radioisotope Power Systems (RPSs) used by NASA. Using specialized facilities at national labs including Oak Ridge to provide the heat source materials and hardware, Los Alamos for purifying and encapsulating the plutonium-238, and Idaho National laboratory for assembling, testing, and assuring the final delivery of the RPS.
The following companies are involved in radioisotope power:
- Teledyne
- Infinite Power
- Kinetic Energy Australia—granted a provisional worldwide patent for Infinite Power's "Power Cell"
- Tractebel
- Advanced Cooling Technologies, Inc
- Aerojet Rocketdyne Holdings
- Lockheed Martin
- Nanohmics
- Battelle Energy Alliance—operating contractor for Idaho National laboratory
- Atlas Energy Systems
Startup companies working in radioisotope power include the following:
- Zeno Power Systems
- NDB
- Arkenlight
RHUs are small devices that use heat from radioisotopes to keep spacecraft components and systems warm in the cold space environment to complete their mission. This heat is transferred to spacecraft structures, systems, and instruments directly, without moving parts or intervening electronic components.
Unlike RTGs, RHUs do not convert heat into electricity, using the heat directly to maintain the performance of a probe.
RHUs have flown on numerous missions including the Mars Exploration Rovers (MERs), Cassini, Mars Pathfinder, Galileo, Voyager 1 and 2, Pioneer 10 and 11, and the Apollo 11 mission to provide heat for the solar-powered Early Apollo Science Experiment Package (EASEP) batteries.
Fission power systems use nuclear reactors to produce electricity powering space missions.
In the US, the SP-100 Space Reactor Power System (SRPS) is being developed by GE, under contract to the US Department of Energy, to provide electrical power in the range of 10s to 100s of kW.
NASA's fission surface power project is developing systems to reliably generate electricity while exploring the surface of other worlds such as the Moon and Mars. The aim is a small, lightweight system that could provide up to 10 kilowatts of electrical power continuously for at least ten years. The project expands on the efforts of the agency’s Kilopower project, which ended in 2018.
The fission surface power project is managed by NASA’s Glenn Research Center in Cleveland. The technology development and demonstration are funded by the Space Technology Mission Directorate’s Technology Demonstration Missions program, which is located at Marshall Space Flight Center in Huntsville, Alabama. NASA is partnered with DOE and its national laboratories on the fission surface power project. The space agency will define the mission and system requirements.
Illustration of a conceptual fission surface power system on the Moon
