
A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of
nuclear battery that uses an array of
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
s to convert the
heat released by the decay of a suitable
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
material into
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
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 cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
s, batteries, or generators to provide economically, and in places where
solar cell
A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. s are not practical. RTGs have been used as power sources in
satellite
A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
s,
space probe
Uncrewed spacecraft or robotic spacecraft are spacecraft without people on board. Uncrewed spacecraft may have varying levels of autonomy from human input, such as remote control, or remote guidance. They may also be autonomous, in which th ...
s, and uncrewed remote facilities such as a series of
lighthouse
A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lens (optics), lenses and to serve as a beacon for navigational aid for maritime pilots at sea or on inland waterways.
Ligh ...
s built by the
Soviet Union
The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
inside the
Arctic Circle
The Arctic Circle is one of the two polar circles, and the northernmost of the five major circle of latitude, circles of latitude as shown on maps of Earth at about 66° 34' N. Its southern counterpart is the Antarctic Circle.
The Arctic Circl ...
. However, the
Western Bloc
The Western Bloc, also known as the Capitalist Bloc, the Freedom Bloc, the Free Bloc, and the American Bloc, was an unofficial coalition of countries that were officially allied with the United States during the Cold War (1947–1991). While ...
did not use RTGs in this way due to worries about their risk to humans in a radiological accident.
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

The RTG was invented in 1954 by
Mound Laboratories scientists Kenneth (Ken) C. Jordan (1921–2008) and John Birden (1918–2011). They were inducted into the
National Inventors Hall of Fame
The National Inventors Hall of Fame (NIHF) is an American not-for-profit organization, founded in 1973, which recognizes individual engineers and inventors who hold a US patent of significant technology. Besides the Hall of Fame, it also operate ...
in 2013. 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
Polonium-210 (210Po, Po-210, historically radium F) is an isotope of polonium. It undergoes alpha decay to stable 206Pb with a half-life of 138.376 days (about months), the longest half-life of all naturally occurring polonium isotopes (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 United States Atomic Energy Commission (AEC) was an agency of the United States government established after World War II by the U.S. Congress to foster and control the peacetime development of atomic science and technology. President Harry ...
. The project was led by Dr. Bertram C. Blanke.
The first RTG launched into space by the United States was
SNAP 3B in 1961 powered by 96 grams of
plutonium-238
Plutonium-238 ( or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage ...
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.
Several generations of RTG design have been used for probes that traveled far from the Sun, rendering
solar panels
A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...
impractical. As such, they have been used for ''
Pioneer 10
''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...
'' and ''
11''; ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar medium, interstellar space beyond the Sun's heliosphere. It was launched 16 days afte ...
'' and ''
2''; ''
Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
''; ''
Ulysses''; ''
Cassini''; ''
New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
''; and are planned for the
''Dragonfly'' mission to Titan. RTGs were also used instead of solar panels to power the two
Viking
Vikings were seafaring people originally from Scandinavia (present-day Denmark, Norway, and Sweden),
who from the late 8th to the late 11th centuries raided, pirated, traded, and settled throughout parts of Europe.Roesdahl, pp. 9� ...
landers, and for the scientific experiments left on the Moon by the crews of
Apollo 12
Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center in Florida. Commander Charles ...
through
17 (SNAP 27s). Because the
Apollo 13
Apollo 13 (April 1117, 1970) was the seventh crewed mission in the Apollo program, Apollo space program and would have been the third Moon landing. The craft was launched from Kennedy Space Center on April 11, 1970, but the landing was abort ...
Moon landing was aborted, its RTG rests in the
South Pacific Ocean
South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both west and east.
Etymology
The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...
, in the vicinity of the
Tonga Trench
The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurri ...
. The ''
Curiosity
Curiosity (from Latin , from "careful, diligent, curious", akin to "care") is a quality related to inquisitive thinking, such as exploration, investigation, and learning, evident in humans and other animals. Curiosity helps Developmental psyc ...
'' and ''
Perseverance'' Mars rover designs selected RTGs to allow greater flexibility in landing sites and longer lifespan than the solar-powered option, as used in
prior generations of rovers. RTGs were also used for the
Nimbus,
Transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1980 film), a 1980 Israeli film
* ''Transit'' (1986 film), a Canadian short film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countrie ...
and
LES satellites. By comparison, only a few space vehicles have been launched using full-fledged
nuclear reactor
A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
s: the Soviet
RORSAT
Upravlyaemy Sputnik Aktivnyy ( for Controlled Active Satellite), or US-A, also known in the Western world as Radar Ocean Reconnaissance Satellite or RORSAT (GRAU index 17F16K), was a series of 33 Soviet reconnaissance satellites. Launched between ...
series and the American
SNAP-10A.
In addition to spacecraft, the
Soviet Union
The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
built 1,007 RTGs
[ to power uncrewed lighthouses and navigation beacons on the Soviet Arctic coast by the late 1980s.] Many different types of RTGs (including Beta-M
The Beta-M is a radioisotope thermoelectric generator (RTG) that was used in Soviet-era lighthouses and beacons.
Design
The Beta-M contains a core made up of strontium-90, which has a half-life of 28.79 years. The service life of these genera ...
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
Looting is the act of stealing, or the taking of goods by force, typically in the midst of a military, political, or other social crisis, such as war, natural disasters (where law and civil enforcement are temporarily ineffective), or rioting. ...
or by the natural forces of ice/storm/sea.[ In 1996, a project was begun by ]Russian
Russian(s) may refer to:
*Russians (), an ethnic group of the East Slavic peoples, primarily living in Russia and neighboring countries
*A citizen of Russia
*Russian language, the most widely spoken of the Slavic languages
*''The Russians'', a b ...
and international supporters to decommission the RTGs in the lighthouses, and by 2021, all RTGs had been removed.
As of 1992, the United States Air Force
The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
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
Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
, use RTGs. The units use strontium-90
Strontium-90 () is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.79 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine a ...
, and a larger number of such units have been deployed both on the ground and on the ocean floor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
than have been used on spacecraft, with public regulatory
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
documents suggesting that the US had deployed at least 100–150 during the 1970s and 1980s.["Alaska fire threatens air force nukes"]
WISE. 16 October 1992. Accessed 15 March 2021.
In the past, small "plutonium cells" (very small Pu-powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life".["Nuclear-Powered Cardiac Pacemakers"](_blank)
Los Alamos National Laboratory
Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...
. , about ninety were still in use. By the end of 2007, the number was reported to be down to just nine. The Mound Laboratory Cardiac Pacemaker program began on 1 June 1966, in conjunction with NUMEC. 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
Nuclear technology is technology that involves the nuclear reactions of atomic nucleus, atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in s ...
: the main component is a sturdy container of a radioactive material (the fuel). Thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
s are placed in the walls of the container, with the outer end of each thermocouple connected to a heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is thermal management (electronics), ...
. 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
The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including:
* Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
directly into electrical energy
Electrical energy is the energy transferred as electric charges move between points with different electric potential, that is, as they move across a voltage, potential difference. As electric potential is lost or gained, work is done changing the ...
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 temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
s, an electric current will flow in the loop. Typically a large number of thermocouples are connected in series to generate a higher voltage.
RTGs and 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
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events.
Chain reactions are one way that sys ...
. 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 nuclear accident
A nuclear and radiation accident is defined by the International Atomic Energy Agency (IAEA) as "an event that has led to significant consequences to people, the environment or the facility." Examples include radiation poisoning, lethal effect ...
. Chain reactions do not occur in RTGs. Heat is produced through spontaneous radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
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. 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
Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...
. This way a long-lived neutron source
A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear p ...
is produced. Because the system has a criticality close to but less than 1, i.e. K < 1, a subcritical 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 the Pu needed per mission. The idea was proposed to NASA in 2012 for the yearly NASA NSPIRE competition, which translated to Idaho National Laboratory at the Center for Space Nuclear Research (CSNR) in 2013 for studies of feasibility. However the essentials are unmodified.
RTG have been proposed for use on realistic interstellar precursor missions and interstellar probe
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar medium, interstellar space, which is typically defined as the region beyond the Heliopause (astronomy), heliopause. It also r ...
s.[ An example of this is the Innovative Interstellar Explorer (2003–current) proposal from NASA.]
An RTG using Am was proposed for this type of mission in 2002. This could support mission extensions up to 1000 years on the interstellar probe, because Am decays more slowly than Pu.[ Other isotopes for RTG were also examined in the study, looking at traits such as watt/gram, half-life, and decay products.][ An interstellar probe proposal from 1999 suggested using three advanced radioisotope power sources (ARPS).] The RTG electricity can be used for powering scientific instruments and communication to Earth on the probes.[ One mission proposed using the electricity to power ]ion engines
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. An ion thruster creates a cloud of positive ions from a neutral gas by ionizing it to extract some electrons from its atoms. The i ...
, calling this method radioisotope electric propulsion (REP).[
A power enhancement for radioisotope heat sources based on a self-induced electrostatic field has been proposed. 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
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. 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.
** 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
*** 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[ Reactor heat fed a thermoelectric conversion system for electrical production.][
**** not really an RTG, the ASRG uses a ]Stirling
Stirling (; ; ) is a City status in the United Kingdom, city in Central Belt, central Scotland, northeast of Glasgow and north-west of Edinburgh. The market town#Scotland, market town, surrounded by rich farmland, grew up connecting the roya ...
power device that runs on radioisotope (see Stirling radioisotope generator)
Terrestrial
Fuels
Inspection of '' '' RTGs before launch">Cassini-Huygens'' RTGs before launch
file:New Horizons 1.jpg, ''New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' in assembly hall
The radioactive material used in RTGs must have several characteristics:[
# Its ]half-life Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''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
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but ...
with shorter half-lives could be used for specialized applications.
# For spaceflight use, the fuel must produce a large amount of power per mass
Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
and volume
Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...
(density
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
). Density and weight are not as important for terrestrial use, unless there are size restrictions. The decay energy
The decay energy is the energy change of a nucleus having undergone a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energ ...
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.
# Radiation must be of a type easily absorbed and transformed into thermal radiation, preferably alpha radiation
Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an atom ...
. Beta radiation
A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...
can emit considerable gamma
Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...
/ X-ray radiation through bremsstrahlung
In particle physics, bremsstrahlung (; ; ) is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic ...
secondary radiation production and therefore requires heavy shielding. Isotopes must not produce significant amounts of gamma, neutron radiation
Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides— ...
or penetrating radiation in general through other decay mode
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
s or decay chain
In nuclear science a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements.
Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...
products.
The first two criteria limit the number of possible fuels to fewer than thirty nuclides[NPE chapter 3 Radioisotope Power Generation]
within the entire table of nuclides.
Plutonium-238
Plutonium-238 ( or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage ...
, curium
Curium is a synthetic chemical element; it has symbol Cm and atomic number 96. This transuranic actinide element was named after eminent scientists Marie and Pierre Curie, both known for their research on radioactivity. Curium was first inten ...
-244, strontium-90
Strontium-90 () is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.79 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine a ...
, and most recently americium-241
Americium-241 (Am, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . Am is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It ...
are the most often cited candidate isotopes, but 43 more isotopes out of approximately 1,300 were considered at the beginning in the 1950s.
The table below does not necessarily give power densities for the pure material but for a chemically inert
In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (negative standard Gibbs free en ...
form. For actinide
The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...
s this is of little concern as their oxides are usually inert enough (and can be transformed into ceramics
A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porce ...
further increasing their stability), but for alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s and alkaline earth metal
The alkaline earth metals are six chemical elements in group (periodic table), group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar p ...
s like caesium or strontium respectively, relatively complex (and heavy) chemical compounds have to be used. For example, strontium is commonly used as strontium titanate
Strontium titanate is an oxide of strontium and titanium with the chemical formula strontium, Srtitanium, Tioxygen, O3. At room temperature, it is a centrosymmetric paraelectricity, paraelectric material with a Perovskite (structure), perovskite st ...
in RTGs, which increases molar mass
In chemistry, the molar mass () (sometimes called molecular weight or formula weight, but see related quantities for usage) of a chemical substance ( element or compound) is defined as the ratio between the mass () and the amount of substance ...
by about a factor of 2. Furthermore, depending on the source, isotopic purity may not be obtainable. Plutonium extracted from spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...
has a low share of Pu-238, so plutonium-238 for use in RTGs is usually purpose-made by neutron irradiation of neptunium-237
Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be ...
, further raising costs. Caesium in fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
s is almost equal parts Cs-135 and Cs-137, plus significant amounts of stable Cs-133 and, in "young" spent fuel, short lived Cs-134. If isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
, a costly and time-consuming process, is to be avoided, this has to be factored in, too. While historically RTGs have been rather small, there is in theory nothing preventing RTGs from reaching into the megawatt range of power. However, for such applications actinides are less suitable than lighter radioisotopes as the critical mass
In nuclear engineering, critical mass is the minimum mass of the fissile material needed for a sustained nuclear chain reaction in a particular setup. The critical mass of a fissionable material depends upon its nuclear properties (specific ...
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
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of thes ...
under any circumstances, RTGs of arbitrary size and power could be assembled from them if enough material can be produced. In general, however, potential applications for such large-scale RTGs are more the domain of small modular reactor
The small modular reactor (SMR) is a class of small nuclear fission reactor, designed to be built in a factory, shipped to operational sites for installation, and then used to power buildings or other commercial operations. The term SMR refers t ...
s, microreactors or non-nuclear power sources.
238Pu
Plutonium-238
Plutonium-238 ( or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage ...
has a half-life of 87.7 years, reasonable power density
Power density, defined as the amount of power (the time rate of energy transfer) per unit volume, is a critical parameter used across a spectrum of scientific and engineering disciplines. This metric, typically denoted in watts per cubic meter ...
of 0.57 watts per gram, and exceptionally low gamma and neutron radiation levels. Pu 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. plutonium(IV) oxide
Plutonium(IV) oxide, or plutonia, is a chemical compound with the formula Pu O2. This high melting-point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature and ...
(PuO).
However, plutonium(IV) oxide containing a natural abundance of oxygen emits neutrons at the rate of roughly n/sec/g of Pu. This emission rate is relatively high compared to the neutron emission rate of plutonium-238 metal. The metal containing no light element impurities emits roughly n/sec/g of Pu. These neutrons are produced by the spontaneous fission of Pu.
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 phase O exchange method. Regular production batches of PuO particles precipitated as a hydroxide were used to show that large production batches could be effectively O-exchanged on a routine basis. High-fired PuO microspheres were successfully O-exchanged showing that an exchange will take place regardless of the previous heat treatment history of the PuO.
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.
Unlike the other three isotopes discussed in this section, Pu must be specifically synthesized and is not abundant as a nuclear waste product. At present only Russia has maintained high-volume production, while in the US, no more than were produced in total between 2013 and 2018.
The US agencies involved desire to begin the production of the material at a rate of 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 per year by 2025.
has been used by the Soviet Union in terrestrial RTGs. Sr decays by
, which quickly β-decays again. It has a lower decay energy than Pu, 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.