Radioisotope thermoelectric generator
A radioisotope thermoelectric generator(RTG) is an electrical generator
which obtains its power from radioactive decay. A radioactive material decays and releases heat which is then converted into elecricity through the seebeck effect using thermocouples. satellites, space probes and unmanned remote facilities all have used them as th epower sources.

Design
The nuclear technology guides its design.The
main component is a sturdy container of a radioactive material on whose walls the thermocouple is kept and the he outer end of
each thermocouple connected to a heat sink. Radioactive decay of the fuel
produces heat which flows through the thermocouples to the heat sink,
generating electricity in the process.

Fuels
The criteria for the selection of the fuel is:
- The half-life must be long enough that it will produce energy at a
relatively continuous rate for a reasonable amount of time
- amount of energy
per mass and volume
- high energy radiation must be produced which is easily absorbed and transferred into thermal radiation.


SELECTION OF ISOTOPES
Plutonium-238, Curium- 244 and Strontium-90 are the most commonly used isotopes.

Efficiency
efficiencies above 10% have never been achieved as the thermocouples are very inefficient. A thermionic converter depending on the principle of thermionic emission can achieve efficiencies between 10-
20%, but require higher temperatures.

Life span
The radioactive material decays and reduces in its radiation and also the the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade. The produce a decrease in the power production.

For full details refer this pdf;
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http://en.wikipediawiki/Radioisotope_the..._generator