Radioisotope thermoelectric generator

RTGs are applicable as energy sources for autonomous systems that are remote from traditional power sources and need several tens-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 for spacecraft that perform a long task and are far away from the Sun (for example, Voyager 2 or Cassini-Huygens ), where the use of solar panels is inefficient or impossible.

Plutonium-238 in 2006, when the New Horizons probe was launched, Pluto found its application as a power source for spacecraft equipment ^[1] . The radioisotope generator contained 11 kg of high-purity ²³⁸ Pu dioxide, producing an average of 220 watts of electricity along the entire route ( 240 watts at the beginning of the path and, according to calculations, 200 watts towards the end) ^{[2] [3]} .

The Galileo and Cassini probes were also equipped with energy sources, for which plutonium served as fuel ^[4] . Mars rover “ Curiosity ” receives energy due to plutonium-238 ^[5] . The rover uses the latest generation of RTGs called the Multi-Mission Radioisotope Thermoelectric Generator . This device produces 125 watts of electric power , and after 14 years - 100 watts ^[6] .

Several kilograms of ²³⁸ PuO _{2 were} used on some Apollo flights to power ALSEP instruments. The SNAP-27 power generator ( Systems for Nuclear Auxiliary Power ), whose thermal and electric power was 1480 W and 63.5 W, respectively, contained 3.735 kg of plutonium-238 dioxide.

On Earth

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 ground power sources, but also for marine buoys and underwater installations. For example, in 1988, the USSR discovered two American RTGs next to Soviet communications cables in the Sea of ​​Okhotsk. The exact number of RTGs installed by the USA is unknown; estimates of independent organizations indicated 100-150 installations for 1992 ^[7] .

Plutonium-236 and plutonium-238 have been used for the manufacture of atomic electric batteries, whose service life reaches 5 years or more. They are used in current generators that stimulate the work of the heart ( pacemaker ) ^{[8] [9]} . As of 2003, there were 50–100 people in the United States with a plutonium pacemaker ^[10] . Prior to the ban on the production of plutonium plutonium-238 in the United States, it was expected that its use could extend to suits of divers and astronauts ^[11] .

Radioactive materials used in RTGs must comply with the following characteristics:

• A sufficiently high volumetric activity to obtain significant energy release in a limited installation volume. The minimum volume is limited by the thermal and radiation resistance of materials, weakly active isotopes worsen the energy-mass perfection of the installation. Usually this means that the half-life of the isotope should be small enough for a high decay rate and the decay should give a lot of easily recyclable energy.
• A sufficiently long time to maintain power to complete the task. Usually this means that the half-life of the isotope should be large enough for a given rate of incidence of energy release. Typical half-lives of isotopes used in RTGs are several decades, although isotopes with a short half-life can be used for specialized applications.
• A type of ionizing radiation that is convenient for energy recovery. Gamma radiation easily flies out of the structure, taking with it the decay energy. Neutrons can also fly off relatively easily. The high-energy electrons formed during β decay are well retained, however, in this case, bremsstrahlung X-rays are generated, which carries away part of the energy. During α decay , massive α particles are formed that efficiently give off their energy almost at the point of formation.
• Safe type of ionizing radiation 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 its simplicity in the framework of existing nuclear technologies.

Plutonium-238 , curium -244, and strontium-90 are the most commonly used isotopes. Other isotopes such as polonium-210 , promethium -147, cesium-137 , cerium -144, ruthenium -106, cobalt-60 , curium-242 and thulium isotopes have also been studied. For example, polonium-210 has a half-life of only 138 days with a huge initial heat release of 140 watts per gram. Americium- 241 with a half-life of 433 years and a heat release of 0.1 W / gram ^[12] .

Plutonium-238 is most often used in spacecraft. 5.5 MeV alpha decay (one gram gives ~ 0.54 W ). The half-life is 88 years (power loss 0.78% per year) with the formation of a highly stable isotope ²³⁴ U. Plutonium-238 is an almost pure alpha emitter, making it one of the safest radioactive isotopes with minimal biological protection requirements. However, obtaining a relatively pure 238th isotope requires the operation of special reactors, which makes it expensive ^{[13] [14]} .

Strontium-90 was 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 ). The half-life of 29 years with the formation of a stable ⁹⁰ Zr . Strontium-90 is obtained from spent fuel from nuclear reactors in large quantities. The low cost and abundance of this isotope determines its widespread use in ground equipment. Unlike plutonium-238, strontium-90 creates a significant level of ionizing radiation of high permeability, which makes relatively high demands on biological protection ^[14] .

There is a concept of subcritical RTGs ^{[15] [16]} . A subcritical generator consists of a neutron source and fissile material. Source neutrons are captured by nuclei of fissile material and cause their fission. The main advantage of such a generator is that the energy released during the fission reaction is much higher than the alpha decay energy. For example, for plutonium-238 it is about 200 MeV versus 5.6 MeV released by this nuclide during alpha decay. Accordingly, the amount of substance required is much lower. The number of decays and radiation activity in terms of heat release is also lower. This reduces the weight and size of the generator.

During the USSR, 1007 RTGs were manufactured for ground operation. Almost all of them were made on the basis of a radioactive fuel element with a strontium-90 isotope (RIT-90). The fuel element is a solid, sealed welded capsule with an isotope inside it. Several versions of RIT-90 with different amounts of isotope were produced ^[17] . The RTG was equipped with one or more RIT capsules, radiation protection (often based on depleted uranium ), a thermoelectric generator, a cooling radiator, a sealed case, and electrical circuits. Types of RTGs produced in the Soviet Union: ^{[17] [18]}

The service life of plants can be 10-30 years , most of them have ended. The RTG is a potential danger, as it is located in a deserted area and can be stolen, and then used as a dirty bomb . There were recorded cases of RTG decommissioning by non-ferrous metal hunters ^[19] , while the abductors themselves received a lethal dose of radiation ^[20] .

Currently, the process of their dismantling and disposal is under the supervision of the International Atomic Energy Agency and funded by the United States, Norway and other countries ^[17] . By the beginning of 2011, 539 RTGs were dismantled ^[21] . As of 2012, 72 RTGs are in operation, 2 have been lost, 222 in storage, 32 in the process of disposal ^{[22] [23]} . Four installations were operated in Antarctica ^[24] .

New RTGs for navigational needs are no longer being produced; instead, wind power plants and photovoltaic converters are installed ^[20] , in some cases diesel generators. These devices are called AIPs ( alternative power supplies). They consist of a solar panel (or wind generator), a set of maintenance-free batteries, an LED beacon (circular or wing), a programmable electronic unit that defines the algorithm of the beacon.

RTG Design Requirements

In the USSR, the requirements for RTGs were established by GOST 18696-90 “Radionuclide Thermoelectric Generators. Types and general specifications. ” and GOST 20250-83 “Radionuclide thermoelectric generators. Acceptance rules and test methods. "

• The power of the equivalent dose of ionizing radiation on the outer surface of the RTG should not exceed 2.0 mSv / h, and at a distance of 1 m from it - 0.1 mSv / h.
• The RTG design should ensure that no radionuclides exit from it and preserve the protective characteristics of radiation protection when the RTG falls on a solid base from a height of 9 m, and also after exposure to a temperature of 800 ° C for 30 minutes.
• The temperature of all accessible surfaces of the RTG should not exceed 80 ° C ^[25] .

RTG incidents in the CIS

Data sources - NPO Bellona ^[26] and IAEA ^[17]

• Radioisotope Energy Sources
• Thermoelectric generator

• Detailed description of models with a pivot table
• Power plants for space
• Organization of handling III at the enterprises of sea and river transport
• Inventory and disposal of III in the territory of the CIS countries
• RTGs
• Order No. 1084 “On the approval and enforcement of guidelines on the procedure for monitoring radiation safety during the decommissioning, transportation and long-term storage of radioisotope thermoelectric generators” (neopr.) . Rostekhnadzor (December 14, 2006). Date of treatment July 8, 2013.