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Radioisotope generators used on spacecraft usually operate on the principle of using radiation energy to heat the hot junctions of thermocouples, in which thermal energy and electrical energy are converted.
Modern radioisotope generators have an efficiency of 3 - 5% and a service life of 3 months to 10 years. The technical and economic characteristics of these generators can be significantly improved in the future.
One of these generators, the Soviet experimental radioisotope generator Beta-1, operated successfully for two years, powering the radio transmitter of a meteorological station near Moscow in Khimki. It used cerium-144 as an energy source, placed in anti-radiation containers made of tungsten and lead. Its energy intensity was 440 kWh, the average power was 5 watts, and the output (with accumulation) power when the transmitter was operating was 150 - 200 watts.
The works suggested various options radioisotope generator with a two-stage system for converting nuclear energy into electrical energy, which belongs to the family of photovoltaic nuclear batteries. In such a generator, the energy of nuclear fission fragments is initially converted into radiation through some process of nuclear-stimulated fluorescence (for example, in an aerosol gas-filled converter), and then the photon energy is converted into electricity using a photovoltaic converter. This method of energy conversion has whole line advantages over existing ones. For example, unlike many of the most widely used traditional methods, it does not contain a low-efficiency thermal cycle. Thus, the total efficiency of the system can be about 35%, which is 3 - - 5 times higher than the efficiency of systems using a thermal cycle and solar panels.
The most significant and expensive part of a radioisotope generator development program is its testing. Can be predicted General characteristics of one or another structural element, but to determine the real physical parameters a new node or system as a whole is often only possible through experimentation.
| Scheme of a thermionic radioisotope generator with a heat pipe that automatically stabilizes the heat flow and temperature at the cathode of the converter. |
But this is the solution to the problem of stabilization heat flow and temperature at the cathode of a thermionic radioisotope generator under conditions of a continuous decrease in energy release in the capsule. The excess thermal energy generated in the isotope fuel during the initial period of operation is discharged from a section of the heat pipe protruding beyond the cylindrical thermionic converter.
In addition to constructive improvements and increasing the power of thermoelectric generating plants with nuclear reactors, the Soviet Union is developing designs for radioisotope generators. To generate electric current they use the heat generated during the decay of radioactive isotopes of cobalt, curium, polonium, etc. They have small overall dimensions and operate reliably for a long time without recharging (depending on the half-life of the corresponding radioactive elements) and the amount of energy generated by 1 kg dead weight, superior to electrochemical batteries.
Let us consider the features of the formulation and solution of problem (9.18) for a combined power plant containing a two-stage TEG and a double-circuit PTS with a condensing injector and a single-stage turbine, the working fluid of which is the DFS. The heat supply from the radioisotope generator to the TEG and from it to the PTP is carried out by a liquid metal coolant.
Why are such quantities of the heavy isotope curium needed? It is believed that curium-244 can replace ilutonium-238 in radioisotope generators for space and ocean research. Generators based on 244Csh are less durable than plutonium ones, but their specific energy release is approximately five times greater... Therefore, curium generators are hardly applicable as cardiac stimulants. But in other autonomous energy sources, curium-244 may well replace plutonium. In addition, curium is not as toxic as plutonium. And the maximum power of curium generators (determined by the critical mass) is approximately 10 times greater than that of plutonium generators: 162 and 18 kilowatts, respectively.
On the instructions of the AEC, the potential capabilities of thermoelectric generators based on polonium-210, plutonium-238 and curium-244 with an electrical power of up to 10 kW are being studied in relation to space installations. This power is considered to be the practical limit for radioisotope generators for this purpose. It should be noted that KEA is developing rocket engines with isotope heat sources. The heat released during the decay of polonium-210 is used to heat liquid hydrogen. Such an engine can develop thrust up to 0 11 kg with a specific impulse of 700 - 800 sec.
This type of generator today is the most widely used for powering on-board equipment and heating spacecraft. aircraft. According to , of the nine radioisotope generators in orbit in the United States in 1992, eight were thermoelectric with the Pu238 isotope as fuel. A radioisotope thermoelectric generator (RTG) directly converts thermal energy into electrical energy based on the Seebeck effect.
It should be said that recently in the United States a lot of attention has been paid to work related to the search for more effective ways conversion of thermal energy RIT on plutonium-238 than thermoelectric. These primarily include work on the creation of thermophotoelectric radioisotope generators and radioisotope generators AMTEC (Alkali metal thermal to electric conversion) using in both cases radioisotope heat sources based on plutonium-238, previously developed for RTGs for space purposes.
In 1965, a Soviet radioisotope hev was demonstrated in Leipzig (GDR): the Beta-2 erator, which also supplied electricity to the instruments of an automatic weather station. Beta-2 was awarded a gold medal at the anniversary Leipzig fair. In the same year, radioisotope generators of a different type with a power of 5 - 50 W were used to power the on-board systems of several artificial Earth satellites of the Cosmos series, the launch of which was provided for by the space research program adopted in the USSR.
But they have no moving parts and do not require maintenance throughout their entire service life, which can be decades.
Encyclopedic YouTube
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Collection of abandoned Sr 90 beta sources from RTGs in Georgia
Subtitles
Application
RTGs are applicable as energy sources for autonomous systems, remote from traditional power sources and requiring several tens to hundreds of watts with a very long operating time, too long for fuel cells or batteries.
In space
RTGs are the main source of power on spacecraft that have a long mission and move far away from the Sun (for example, Voyager 2 or Cassini-Huygens), where the use of solar panels is ineffective or impossible.
Several kilograms of 238 PuO 2 were used on some Apollo missions to power ALSEP instruments. Electricity generator SNAP-27 Systems for Nuclear Auxiliary Power), whose thermal and electrical power was 1480 W and 63.5 W, respectively, contained 3.735 kg of plutonium-238 dioxide.
On the ground
RTGs were used in navigation beacons, radio beacons, weather stations and similar equipment installed in areas where, for technical or economic reasons, it is not possible to use other power sources. In particular, in the USSR they were used as power sources for navigation equipment installed on the coast of the Arctic Ocean along the Northern Sea Route. Currently, due to the risk of leakage of radiation and radioactive materials, the practice of installing maintenance-free RTGs in inaccessible places has been stopped.
In the USA, RTGs were used not only for land-based power sources, but also for offshore buoys and underwater installations. For example, in 1988, the USSR discovered two American RTGs near Soviet communications cables in the Sea of Okhotsk. The exact number of RTGs installed by the United States is unknown; estimates from independent organizations indicated 100-150 installations in 1992.
Fuel
Radioactive materials used in RTGs must meet the following characteristics:
- Sufficiently high volumetric activity to obtain significant energy release in a limited volume of the installation. The minimum volume is limited by the thermal and radiation resistance of the materials; weakly active isotopes impair the energy-mass perfection of the installation. This usually means that the half-life of the isotope must be short enough for high decay rates and the decay must produce a sufficiently large amount of easily utilized energy.
- A sufficiently long period of maintaining power to complete the task. This usually means that the half-life of the isotope must be long enough for a given rate of decline in energy release. Typical half-lives of isotopes used in RTGs are several decades, although isotopes with short half-lives can be used for specialized applications.
- A type of ionizing radiation convenient for energy utilization. Gamma radiation easily escapes from the structure, taking with it decay energy. Neutrons can also escape relatively easily. The high-energy electrons produced during β-decay are well retained, but this produces bremsstrahlung X-rays, which carry away some of the energy. During α-decay, massive α-particles are formed, which effectively release their energy almost at the point of formation.
- A type of ionizing radiation that is safe for the environment and equipment. Significant gamma, x-ray and neutron radiation often require special design measures to protect personnel and nearby equipment.
- The relative cheapness of the isotope and the ease of its production within the framework of existing nuclear technologies.
Plutonium-238 most often used in spacecraft. α-decay with an energy of 5.5 MeV (one gram gives ~0.54 W). Half-life 88 years (power loss 0.78% per year) with the formation of a highly stable isotope 234 U. Plutonium-238 is a nearly pure alpha emitter, making it one of the safest radioactive isotopes with minimal biological containment requirements. However, producing the relatively pure 238 isotope requires the operation of special reactors, which makes it expensive.
Strontium-90 widely used in ground-based RTGs of Soviet and American production. A chain of two β-decays gives a total energy of 2.8 MEV (one gram gives ~0.46 W). Half-life 29 years with the formation of a stable 90 Zr. Strontium-90 is obtained from spent fuel from nuclear reactors in large quantities. The cheapness and abundance of this isotope determines its widespread use in ground-based equipment. Unlike plutonium, strontium has significant levels of high-permeability ionizing radiation, which places relatively high demands on biological shielding.
There is a concept of subcritical RTGs. A subcritical generator consists of a neutron source and fissile material. Neutrons from the source are captured by atoms of the fissile substance and cause their fission. The main advantage of such a generator is that the decay energy of a reaction with neutron capture can be much higher than the energy of spontaneous fission. For example, for plutonium it is 200 MeV versus 6 MeV of spontaneous fission. Accordingly, the required amount of the substance is much lower. The number of decays and radiation activity in terms of heat release are also lower. This reduces the weight and size of the generator.
Ground RTGs in Russia
During Soviet times, 1007 RTGs were manufactured for ground use. Almost all of them were made on the basis of the isotope strontium-90 (RIT-90). The fuel element is a durable, sealed, welded capsule containing the isotope. Several variants of the RIT-90 were produced with different amounts of isotope. The RTG was equipped with one or more RIT capsules, radiation shielding (often based on depleted uranium), a thermoelectric generator, a cooling radiator, a sealed housing, and electrical circuits. Types of RTGs produced in the Soviet Union:
| Type | Initial activity, kCi | Thermal power, W | Electrical power, W | Efficiency, % | Weight, kg | Start year of release |
|---|---|---|---|---|---|---|
| Ether-MA | 104 | 720 | 30 | 4,167 | 1250 | 1976 |
| IED-1 | 465 | 2200 | 80 | 3,64 | 2500 | 1976 |
| IED-2 | 100 | 580 | 14 | 2,41 | 600 | 1977 |
| Beta-M (English) Russian | 36 | 230 | 10 | 4,35 | 560 | 1978 |
| Gong | 47 | 315 | 18 | 5,714 | 600 | 1983 |
| Horn | 185 | 1100 | 60 | 5,455 | 1050 | 1983 |
| IEU-2M | 116 | 690 | 20 | 2,899 | 600 | 1985 |
| Senostav | 288 | 1870 | - | - | 1250 | 1989 |
| IEU-1M | 340 | 2200 | 120 | 5,455 | 2100 | 1990 |
The service life of installations can be 10-30 years, most of them have expired. An RTG poses a potential danger because it is located in a deserted area and can be stolen and then used as a dirty bomb. Cases have been recorded of RTGs being dismantled by hunters for non-ferrous metals, while the thieves themselves received a lethal dose of radiation.
Currently, the process of their dismantling and disposal is underway under the supervision of the International Atomic Energy Agency and with funding from the United States, Norway and other countries. By the beginning of 2011, 539 RTGs had been dismantled. As of 2012, 72 RTGs are in operation, 3 are lost, 222 are in storage, 31 are in the process of disposal. Four installations were operated in Antarctica.
New RTGs for navigation needs are no longer produced; instead, wind power plants and photoelectric converters, and in some cases diesel generators, are installed. These devices are called APS (alternative power supplies). Consist of a solar panel (or wind generator), a set of maintenance-free batteries, an LED beacon (circular or folding), a programmable electronic unit that sets the algorithm for the beacon’s operation.
Requirements for RTG design
In the USSR, the requirements for RTGs were established by GOST 18696-90 “Radionuclide thermoelectric generators. Types and common technical requirements" and GOST 20250-83 “Thermoelectric radionuclide generators. Acceptance rules and test methods."
Incidents with RTGs in the CIS
| date | Place | |
|---|---|---|
| 1983, March | Cape Nutevgi, Chukotka | Severe damage to the RTG on the way to the installation site. The fact of the accident was hidden by the staff and discovered by the Gosatomnadzor commission in 1997. As of 2005, this RTG was abandoned and remained at Cape Nutevgi. As of 2012, all RTGs have been removed from the Chukotka Autonomous Okrug. |
| 1987 | Cape Nizkiy, Sakhalin region. | During transportation, the helicopter dropped an IEU-1 type RTG, which belonged to the USSR Ministry of Defense, into the Sea of Okhotsk. As of 2013, search work continues, with interruptions. |
| 1997 | Tajikistan, Dushanbe | Three expired RTGs were stored, disassembled by unknown persons, in a coal warehouse in the center of Dushanbe, and an increased gamma background was recorded nearby. |
| 1997, August | Cape Maria, Sakhalin region. | During transportation, the helicopter dropped an IEU-1 type RTG into the Sea of Okhotsk, which remained at the bottom at a depth of 25-30 m. After 10 years, it was picked up and sent for disposal. |
| 1998, July | Korsakov port, Sakhalin region. | An RTG belonging to the Russian Ministry of Defense was found disassembled at a scrap metal collection point. |
| 1999 | Leningrad region. | The RTG was looted by non-ferrous metal hunters. A radioactive element (background near - 1000 R/h) was found at a bus stop in Kingisepp. |
| 2000 | Cape Baranikha, Chukotka | The natural background near the device was exceeded several times due to a malfunction of the RTG. |
| 2001, May | Kandalaksha Bay, Murmansk region. | 3 radioisotope sources were stolen from lighthouses on the island, which were discovered and sent to Moscow. |
| 2002, February | Western Georgia | In the area of the village of Liya, Tsalenjikha district local residents two RTGs were found, which they used as heat sources and then dismantled. As a result, several people received high doses of radiation. |
| 2003 | O. Nuneangan, Chukotka | It was established that the external radiation of the device exceeded the permissible limits by 5 times due to flaws in its design. |
| 2003 | O. Wrangel, Chukotka | Due to erosion of the coast, the RTG installed here fell into the sea, where it was washed away by soil. In 2011, a storm washed up on the coast. The radiation protection of the device is not damaged. In 2012, it was removed from the territory of the Chukotka Autonomous Okrug. |
| 2003 | Cape Shalaurov Izba, Chukotka | The background radiation near the installation was 30 times higher due to a flaw in the RTG design. |
| 2003, March | Pihlisaar, Leningrad region. | The RTG was looted by non-ferrous metal hunters. The radioactive element was released onto the ice surface. The hot capsule with strontium, having melted the ice, sank to the bottom; the background nearby was 1000 R/h. The capsule was soon found 200 m from the lighthouse. |
| 2003, August | Shmidtovsky district, Chukotka | The inspection did not find RTG type "Beta-M" No. 57 at the installation site near the Kyvekvyn River; According to the official version, it was assumed that the RTG was washed into the sand as a result of a strong storm or that it was stolen. |
| 2003, September | Golets Island, White Sea | Northern Fleet personnel discovered the theft of metal from an RTG biological shield on Golets Island. The door to the lighthouse room was also broken into, where one of the most powerful RTGs with six RIT-90 elements, which were not stolen, was stored. |
| 2003, November | Kola Bay, Olenya Bay and South Goryachinsky Island | Two RTGs belonging to the Northern Fleet were looted by non-ferrous metal hunters, and their RIT-90 elements were found nearby. |
| 2004 | Priozersk, Kazakhstan | An emergency situation occurred as a result of the unauthorized dismantling of six RTGs. |
| 2004, March | p. Valentin, Primorsky region | An RTG belonging to the Pacific Fleet was found dismantled, apparently by non-ferrous metal hunters. The radioactive element RIT-90 was discovered nearby. |
| July, 2004 | Norilsk | Three RTGs were discovered on the territory of the military unit, the dose rate at a distance of 1 m from which was 155 times higher than the natural background. |
| July, 2004 | Cape Navarin, Chukotka | Mechanical damage to the RTG body of unknown origin, as a result of which depressurization occurred and part of the radioactive fuel fell out. The emergency RTG was removed for disposal in 2007, the affected areas of the adjacent territory were decontaminated. |
| September, 2004 | Land Bunge, Yakutia | Emergency release of two transported RTGs from a helicopter. As a result of the impact with the ground, the integrity of the radiation protection of the hulls was compromised; the gamma radiation dose rate near the impact site was 4 mSv/h. |
| 2012 | O. Lishny, Taimyr | At the installation site of the RTG of the Gong project, its fragments were discovered. It is assumed that the device was washed out to sea. |
see also
Notes
- Konstantin Lantratov. Pluto has become closer (Russian) // Kommersant newspaper: article. - Kommersant, 2006. - Issue. 3341. - No. 10.
- Alexander Sergeev. Probe to Pluto: an impeccable start to a great journey (Russian). - Elements.Ru, 2006.
- Timoshenko, Alexey Space age —man turned out not needed (Russian) (inaccessible link - story) . gzt.ru (September 16, 2010). Retrieved October 22, 2010. Archived April 19, 2010.
- Energy of pure science: Current from the collider (Russian) // physics arXiv blog Popular mechanics: article. - 08/12/10.
- NASA conducted the first test drive of the new Mars rover (Russian). Lenta.ru (July 26, 2010). Retrieved November 8, 2010. Archived February 3, 2012.
- Ajay K. Misra. Overview of NASA Program on Development of Radioisotope Power Systems with High Specific Power // NASA/JPL: review. - San Diego, California, June 2006.
- World Information Service on Energy.
- Alaska fire threatens air force nukes. Drits M. E. et al.
- Properties of elements. - Directory. - M.: Metallurgy, 1985. - 672 p. - 6500 copies. Venkateswara Sarma Mallela, V Ilankumaran, N.Srinivasa Rao.
- Trends in Cardiac Pacemaker Batteries // Indian Pacing Electrophysiol J: article. - October 1, 2004. - Iss. 4 . - No. 4 .
Plutonium Powered Pacemaker (1974) (English) . Oak Ridge Associated Universities (March 23, 2009). Retrieved January 15, 2011. It so happened that in the “Peaceful Space Atom” series we are moving from the fantastic to the widespread. Last time we talked about power reactors, the obvious next step is to talk about radioisotope reactors. thermoelectric generators
. Recently there was an excellent post on Habré about the RTG of the Cassini probe, and we will look at this topic from a broader point of view.
Physics of the process
Unlike a nuclear reactor, which uses the phenomenon of a nuclear chain reaction, radioisotope generators use the natural decay of radioactive isotopes. Recall that atoms are made up of protons, electrons and neutrons. Depending on the number of neutrons in the nucleus of a particular atom, it can be stable or exhibit a tendency to spontaneous decay. For example, the cobalt atom 59 Co with 27 protons and 32 neutrons in the nucleus is stable. This cobalt has been used by mankind since the times of Ancient Egypt. But if we add one neutron to 59 Co (for example, by putting “regular” cobalt in a nuclear reactor), we get 60 Co, a radioactive isotope with a half-life of 5.2 years. The term “half-life” means that after 5.2 years, one atom will decay with a 50% probability, and about half of a hundred atoms will remain. All “ordinary” elements have their own isotopes with different half-lives:
3D isotope map, thanks to LJ user crustgroup for the picture.
By selecting a suitable isotope, it is possible to obtain an RTG with the required service life and other parameters:
| Isotope | Method of obtaining | Specific power, W/g | Volumetric power, W/cm³ | Half life | Integrated isotope decay energy, kWh/g | Working form of isotope |
|---|---|---|---|---|---|---|
| 60 Co (cobalt-60) | Irradiation in the reactor | 2,9 | ~26 | 5,271 years | 193,2 | Metal, alloy |
| 238 Pu (plutonium-238) | atomic reactor | 0,568 | 6,9 | 86 years old | 608,7 | Plutonium carbide |
| 90 Sr (strontium-90) | fission fragments | 0,93 | 0,7 | 28 years | 162,721 | SrO, SrTiO 3 |
| 144 Ce (cerium-144) | fission fragments | 2,6 | 12,5 | 285 days | 57,439 | CeO2 |
| 242 Cm (curium-242) | atomic reactor | 121 | 1169 | 162 days | 677,8 | Cm2O3 |
| 147 Pm (promethium-147) | fission fragments | 0,37 | 1,1 | 2.64 years | 12,34 | Pm 2 O 3 |
| 137 Cs (cesium-137) | fission fragments | 0,27 | 1,27 | 33 years | 230,24 | CsCl |
| 210 Po (polonium-210) | bismuth irradiation | 142 | 1320 | 138 days | 677,59 | alloys with lead, yttrium, gold |
| 244 Cm (curium-244) | atomic reactor | 2,8 | 33,25 | 18.1 years | 640,6 | Cm2O3 |
| 232 U (uranium-232) | irradiation of thorium | 8,097 | ~88,67 | 68.9 years | 4887,103 | uranium dioxide, carbide, nitride |
| 106 Ru (ruthenium-106) | fission fragments | 29,8 | 369,818 | ~371.63 days | 9,854 | metal, alloy |
The fact that isotopes decay independently means that the RTG cannot be controlled. Once loaded with fuel, it will heat up and produce electricity for years, gradually degrading. Decreasing the amount of fissile isotope means there will be less nuclear decay, less heat and less electricity. Plus, the drop in electrical power will be aggravated by the degradation of the electric generator.
There is a simplified version of the RTG, in which the decay of the isotope is used only for heating, without generating electricity. This module is called a heating unit or RHG (Radioisotope Heat Generator).
Converting heat into electricity
As in the case of a nuclear reactor, the output we get is heat, which must be somehow converted into electricity. For this you can use:- Thermoelectric converter. By connecting two conductors from different materials(for example, chromel and alumel) and heating one of them, you can get a source of electricity.
- Thermionic converter. In this case, a vacuum tube is used. Its cathode heats up, and the electrons receive enough energy to “jump” to the anode, creating an electric current.
- Thermophotovoltaic converter. In this case, a photocell operating in the infrared range is connected to the heat source. The heat source emits photons, which are captured by a photocell and converted into electricity.
- Alkali metal thermoelectric converter. Here, an electrolyte made from molten sodium and sulfur salts is used to convert heat into electricity.
- Stirling's engine - heat engine to convert the temperature difference into mechanical work. Electricity is obtained from mechanical work using some kind of generator.
Story
The first experimental radioisotope energy source was introduced in 1913. But only from the second half of the 20th century, with the spread of nuclear reactors in which isotopes could be produced on an industrial scale, RTGs began to be actively used.USA
In the USA, RTGs were dealt with by the organization SNAP, already familiar to you from the previous post.SNAP-1.
It was an experimental RTG using 144 Ce and a Rankine cycle generator (steam engine) with mercury as a coolant. The generator successfully operated for 2,500 hours on Earth, but did not fly into space.
SNAP-3.
The first RTG to fly into space on the Transit 4A and 4B navigation satellites. Energy power 2 W, weight 2 kg, used plutonium-238. 
Sentry
RTG for meteorological satellite. Energy power 4.5 W, isotope - strontium-90.
SNAP-7.
A family of ground-based RTGs for beacons, light buoys, weather stations, sonic buoys and the like. Very large models, weight from 850 to 2720 kg. Energy power - tens of watts. For example, SNAP-7D - 30 W with a weight of 2 tons.
SNAP-9
Serial RTG for Transit navigation satellites. Weight 12 kg, electrical power 25 W.
SNAP-11
Experimental RTG for Surveyor lunar landing stations. It was proposed to use the isotope curium-242. Electrical power - 25 W. Not used.
SNAP-19
Serial RTG, used in many missions - Nimbus meteorological satellites, Pioneer probes -10 and -11, Viking Martian landing stations. Isotope - plutonium-238, energy power ~40 W. 
SNAP-21 and -23
RTGs for underwater use using strontium-90.
SNAP-27
RTGs for powering scientific equipment of the Apollo program. 3.8 kg. plutonium-238 gave an energy power of 70 W. Lunar scientific equipment was turned off back in 1977 (people and equipment on Earth required money, but there was not enough of it). RTGs in 1977 produced from 36 to 60 W of electrical power. 
MHW-RTG
The name stands for “multi-hundred-watt RTG.” 4.5 kg. plutonium-238 produced 2400 W of thermal power and 160 W of electrical power. These RTGs were installed on the Lincoln Experimental Satellites (LES-8,9) and have been providing heat and electricity to Voyagers for 37 years. As of 2014, RTGs provide about 53% of their initial power. 
GPHS-RTG
The most powerful of the space RTGs. 7.8 kg of plutonium-238 provided 4400 W of thermal power and 300 W of electrical power. Used on the Ulysses solar probe, Galileo, Cassini-Huygens probes and flying to Pluto on New Horizons. 
MMRTG
RTG for Curiosity. 4 kg of plutonium-238, 2000 W thermal power, 100 W electrical power. 
Warm lamp cube of plutonium.
US RTGs with time reference.
Summary table:
| Name | Media (quantity on device) | Maximum power | Isotope | Fuel weight, kg | Total weight, kg | |
|---|---|---|---|---|---|---|
| Electric, W | Thermal, W | |||||
| MMRTG | MSL/Curiosity rover | ~110 | ~2000 | 238 Pu | ~4 | <45 |
| GPHS-RTG | Cassini (3), New Horizons (1), Galileo (2), Ulysses (1) | 300 | 4400 | 238 Pu | 7.8 | 55.9-57.8 |
| MHW-RTG | LES-8/9, Voyager 1 (3), Voyager 2 (3) | 160 | 2400 | 238 Pu | ~4.5 | 37.7 |
| SNAP-3B | Transit-4A (1) | 2.7 | 52.5 | 238 Pu | ? | 2.1 |
| SNAP-9A | Transit 5BN1/2 (1) | 25 | 525 | 238 Pu | ~1 | 12.3 |
| SNAP-19 | Nimbus-3 (2), Pioneer 10 (4), Pioneer 11 (4) | 40.3 | 525 | 238 Pu | ~1 | 13.6 |
| modification of SNAP-19 | Viking 1 (2), Viking 2 (2) | 42.7 | 525 | 238 Pu | ~1 | 15.2 |
| SNAP-27 | Apollo 12-17 ALSEP (1) | 73 | 1,480 | 238 Pu | 3.8 | 20 |
USSR/Russia
There were few space RTGs in the USSR and Russia. The first experimental generator was the Limon-1 RTG based on polonium-210, created in 1962:
.The first space RTGs were Orion-1 with an electrical power of 20 W on polonium-210 and launched on the communications satellites of the Strela-1 series - Kosmos-84 and Kosmos-90. Heating units were installed on Lunokhods -1 and -2, and an RTG was installed on the Mars-96 mission: 
At the same time, RTGs were very actively used in lighthouses, navigation buoys and other ground-based equipment - the BETA, RTG-IEU series and many others. 
Design
Almost all RTGs use thermoelectric converters and therefore have the same design:
Prospects
All flying RTGs are distinguished by very low efficiency - as a rule, electrical power is less than 10% of thermal power. Therefore, at the beginning of the 21st century, NASA launched the ASRG project - RTG with a Stirling engine. An increase in efficiency to 30% and 140 W of electrical power with 500 W of thermal power was expected. Unfortunately, the project was stopped in 2013 due to cost overruns. But, theoretically, the use of more efficient heat-to-electricity converters can seriously increase the efficiency of RTGs.Advantages and disadvantages
Advantages:- Very simple design.
- It can work for years and decades, gradually degrading.
- Can be used simultaneously for heating and power supply.
- Does not require management or supervision.
- Requires rare and expensive isotopes as fuel.
- Producing the fuel is difficult, expensive and slow.
- Low efficiency.
- Power is limited to hundreds of watts. An RTG with a kilowatt electrical power is already poorly justified; a megawatt RTG is practically meaningless: it will be too expensive and heavy.
The combination of such advantages and disadvantages means that RTGs and heating units occupy their niche in space energy and will continue to do so. They make it possible to simply and efficiently heat and power interplanetary spacecraft with electricity, but one should not expect any energy breakthrough from them.
Sources
In addition to Wikipedia, the following were used:- Paper "Space Nuclear Energy: Opening the Final Horizon".
- Topic “Domestic RTGs” on “Cosmonautics News”.
Tags:
- RTG
- MKA
Compared to nuclear reactors that use a chain reaction, RTGs are much more compact and simpler in design. The output power of RTGs is very low (up to several hundred watts) with low efficiency. But they have no moving parts and do not require maintenance throughout their entire service life, which can be decades.
Application
RTG of the New Horizons spacecraft
RTGs are generally the most suitable energy source for autonomous systems requiring tens to hundreds of watts with very long operating times, too long for fuel cells or batteries.
In space
Diagram of the RTG used on the Cassini-Huygens spacecraft
RTGs are the main source of power for missions that have a long mission and are very far away (for example, Voyager 2 or Cassini-Huygens), where the use of solar panels is ineffective or impossible.
Plutonium-238 in 2006, during the launch of the New Horizons probe, found its use as a power source for spacecraft equipment. The radioisotope generator contained 11 kg of high-purity 238 Pu dioxide, producing an average of 220 W of electricity throughout the journey (240 W at the beginning of the journey and, according to calculations, 200 W at the end).
