plutonium­-238, generates so much heat when it decays, that it was used as a long-lasting thermoelectric generator in early heart pacemakers. Nowadays it's been replaced by better batteries, but it's ...

Beginning in the late 1950s, the U.S. met this challenge by developing nuclear batteries known as radioisotope thermoelectric generators (RTGs) and producing plutonium-238 (Pu-238) as their fuel, ...