Radioisotope thermoelectric generator

A radioisotope thermoelectric generator (RTG) uses the fact that radioactive materials (such as plutonium) generate heat as they decay into non-radioactive materials. The heat used is converted into electricity by an array of thermocouples which then power a lighthouse, or an interplanetary space probe. Voyager 1 and Voyager 2 both use RTGs.

A thermocouple is a device that converts thermal energy directly into electrical energy. It is made of two kinds of metal that can both conduct electricity. They are connected to each other in a closed loop. If the two junctions are at different temperatures, an electric current will flow in the loop.

This is a much different technique from that used by nuclear power stations. That process is called fission, and gets very high efficiency rates by splitting atoms of unstable radioactive materials (such as uranium) into smaller parts. Fission generates very large amounts of heat, but is much more complex and not as reliable as simply using the heat produced by radioactive decay. Fission causes a huge release of energy and uses fuel rapidly. An RTG gives a steadier and much smaller amount of energy.

For Russian RITEG lighthouses, see Bellona's report (http://www.bellona.no/en/international/russia/nuke-weapons/nonproliferation/28067).