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Basics of an RTG

An RTG is a Radioisotope Thermoelectric Generator.  Essentially these radioactive isotopes decay to more stable atoms.  As the isotopes decay they give off heat and that heat is converted to electricity.  The conversion to electricity is done using thermocouples.

An RTG is used on space craft instead of solar panels when the mission is very long or to an outer planet where the light energy provided by the sun is too weak (low density).  The Voyager and Cassini missions used RTGs.

Benefits of an RTG

For space the benfits of an RTG are obvious.

1. Long term power (in the range of hundreds of watts)
2. No moving parts - like solar arrays that have open up after launch
3. Degradation of output power happens but much slower than with solar panels

These advantages lead to a stable power source capable of operating for decades in extreme environments.

Fuel Source

From Wikipedia:

Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242and thulium isotopes have also been studied. Of the above, ²³⁸Pu has the lowest shielding requirements and longest half-life. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25 mm of lead shielding to control unwanted radiation. ²³⁸Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is needed in a ²³⁸Pu RTG, as the casing itself is adequate.

²³⁸Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO₂). ²³⁸Pu has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels.

The criteria leading these choices of fuels are

1. Must have a half-life short enough that its decay produces substantial heat
2. Must ahve a half-life long enough that the heat generated is relatively stable for decades
3. For space, the energy output per density must be high
4. The radiation produced should be high energy with low penetration - preferably alpha-radiation - so that shielding can be minimal

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