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Radioisotope thermoelectric generators (RTGs) use heat from radioactive decay to produce electricity. They do not use nuclear fission or fusion and are highly radioactive. They are used in spacecraft and other systems where other energy sources are impractical. They use thermocouples to convert heat into electricity, but are inefficient and expensive. Plutonium-238 is the most common isotope used. They can produce electricity for decades without external input or maintenance. The use of RTGs in space has caused environmental concerns.
A radioisotope thermoelectric generator (RTG) is a source of electrical energy that uses the heat produced by radioactive decay. Thermal radioisotope generators are not nuclear reactors and do not use nuclear fission or fusion to produce energy, although they are still highly radioactive. Thermal radioisotope generators are used when other energy sources are too expensive or impractical, such as on spacecraft, automated beacons, and radar systems.
A radioisotope heat generator contains an amount, often several pounds (kilograms) or more, of highly radioactive material. The radioactive material must produce enough heat to provide significant amounts of electricity; at the same time, it must not decay so rapidly that the radioisotope thermal generator is rendered useless after a few years. Plutonium-238 is the most common isotope used for thermal radioisotope generators, although strontium-90 and others have been used in the past. Plutonium-238 is expensive to produce, but it emits low penetrating alpha radiation, which is much easier to shield than the radiation produced by other isotopes.
To convert heat into electricity, radioisotope thermal generators use thermocouples, semiconductor devices that can convert temperature differences directly into electricity. While thermocouples are durable and have no moving parts, they are very inefficient, turning less than 10% of available heat into electrical energy. Thermocouples gradually degrade over time, adding to the losses caused by the slow decay of the radioactive material.
Thermal radioisotope generators are very expensive compared to other energy sources, but are useful in isolated systems with no other energy sources, such as interplanetary spacecraft and unmanned radio beacons. A radioisotope heat generator can produce electricity for decades, without any external input or external maintenance, until the radioactive material decays or the electronics fail. The use of thermal radioisotope generators aboard spacecraft such as Cassini has caused many environmental groups to protest the use of radioactive material in space; if the Cassini rocket launch failed, there was a small chance that plutonium could be released into the atmosphere.
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