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, ...

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 Nowadays it's been replaced by better batteries, but it's ...