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Radionuclide battery

Lexicon> Letter R> Radionuclide Battery

Acronym: RTG = radioisotope thermoelectric generator

Definition: a device that generates electrical energy from heat that emits a radioactive substance

English: radioisotope thermoelectric generator

Categories: electrical energy, nuclear energy

Author: Dr. Rüdiger Paschotta

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Original creation: November 18, 2014; last change: 01/09/2021

URL: https://www.energie-lexikon.info/radionuklidbatterie.html

A Radionuclide battery is a compact device that generates electrical energy with a thermoelectric generator, which in turn uses heat generated by a radioactive substance. As a rule, such devices only generate a relatively small amount of electrical power, e.g. B. a few dozen or a few hundred watts, but this is very reliable for many years. The applications (see below) are mainly in space. It is a use of nuclear energy on a very small scale.

Other names for radionuclide batteries are Nuclear battery and Plutonium battery (as far as they contain plutonium).

Texture of a radionuclide battery

In the center of a radionuclide battery there is a certain amount (usually a few grams to a few kilograms) of an artificially produced highly radioactive material in which heat is generated as a result of the radioactive decay. This material is surrounded by a suitable cover that shields most of the radioactive radiation and should also prevent the radioactive material from escaping. The radiation of the material should of course be well shieldable (not too penetrating), which is why gamma emitters are hardly an option, but alpha emitters and possibly beta emitters are best.

A suitable radioactive material has a half-life that is long enough for the planned application (i.e. several times longer than the period of use), but on the other hand should not be so long, as otherwise a correspondingly larger amount of material would be required for the required heat output. Often used materials are transuranic elements such as plutonium 238 (in the form of ceramic plutonium dioxide, PuO2), Curium 244, Americium 241 and Polonium 210 - all extremely dangerous and extremely expensive. Typical fission products from nuclear reactors (e.g. cesium 137 or strontium 90) would be much cheaper, but less suitable because of their radiation that is more difficult to shield.

The thermoelectric generator is located outside the above-mentioned casing and is connected on its outer side to the housing of the radionuclide battery with good heat conductivity. The surface of the housing must be able to give off enough heat at all times (e.g. via a radiator) so that the device does not overheat. Since the efficiency of thermoelectric generators is relatively low (usually a few percent), the device emits many times more heat than electrical energy.

Additional equipment may be necessary, for example, to let off the helium gas, which arises as a decay product of alpha radiation, or to protect against external influences, for example when a satellite crashes into the earth's atmosphere.

Applications of radionuclide batteries

Photovoltaics as an alternative to radionuclide batteries cannot always be used.

Radionuclide batteries are used almost exclusively in space travel, namely to power satellites and space probes. They enable a reliable supply of such devices with electrical energy even in areas far from the sun, where the intensity of the solar radiation is too low for the effective operation of solar cells. Probably the best-known example of this were the two Voyager probes, which had to be supplied with electricity for several decades on their long journey. Even satellites orbiting the earth that have particularly low orbits (especially spy satellites) often use radionuclide batteries, as large “solar sails” with solar cells on such orbits would have too much air resistance. In other cases, e.g. B. in missions to Jupiter or Saturn, there is also the problem of the life span of solar cells under the influence of the strong radiation in the radiation belts of such planets.

Of course, it is a major disadvantage that radionuclide batteries contain a more or less large amount of a highly dangerous radioactive material, depending on their performance. This leads to corresponding dangers, especially when starting with a rocket; As is well known, it is relatively common for a rocket to crash on launch. This can destroy a radionuclide battery, releasing dangerous material. Such accidents have indeed happened and have led to a globally measurable radioactive pollution of the atmosphere. International protests against the use of such devices have meant that they are now less used or better protected.

Even pacemakers with plutonium batteries were used!

In the 1970s, several hundred pacemakers were powered by small radionuclide batteries based on plutonium. At that time it was difficult to manufacture batteries with a long enough lifespan. Radiation during operation was not a significant problem, but these devices had to be disposed of correctly after use, which could not be guaranteed in all cases, especially in the USSR. After all, the half-life of the plutonium 238 used is only around 87 years, which is far less than that of the known plutonium 239 (around 24,000 years).

Larger radionuclide batteries that v. a. were and in some cases still are used in the USSR are very dangerous.

Otherwise, a number of relatively powerful radionuclide batteries have been used in the USSR for applications such as lighthouses in remote areas where no power grid exists. Because of the large amount of highly radiating material that these devices contain, great dangers arise as long as strict safety requirements are not fully met.

Modified technical approaches

There are also Radionuclide heating elementsthat do not contain a thermoelectric generator, i.e. only give off heat. They are used, for example, to protect important components of space probes from being too cold.

The concept of Stirling generator A radionuclide heating element heats a Stirling engine, which in turn drives a generator. This functional principle enables a significantly higher degree of efficiency (often over 20%) and is therefore suitable for higher outputs. However, moving parts are used here, which leads to a less robust and reliable function than a radionuclide battery. So far, this approach does not seem to have been used - probably because it is difficult to achieve a sufficiently high level of reliability for applications in space travel, while the use of a large amount of highly radioactive material is hardly an option for terrestrial applications.

At a thermophotovoltaic generator a type of solar cell is used that is fed with thermal radiation from a very hot radionuclide. This makes it possible to achieve a similarly high level of efficiency as with Stirling generators without having to accept moving parts. However, such a solar cell degrades very quickly under the influence of the strong radiation.

For even greater performance, a nuclear reactor would be used in which a nuclear chain reaction is exploited. The advantages of this approach are that a much higher power and power density can be achieved and that the power can be regulated according to demand. However, because of the need to achieve the so-called critical mass of the radioactive material, nuclear reactors are relatively large and heavy. This is the main reason why radionuclide batteries are far more suitable for small electrical outputs (e.g. a few dozen watts).

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See also: radioactivity, thermoelectric generator, nuclear energy
as well as other articles in the categories of electrical energy, nuclear energy