Contain pellets of plutonium 238 in the form of an oxide pressed into ceramic pellets
The natural radioactive decay of the plutonium produces heat (no fission or fusion) some of which is converted into electricity by an array of thermocouples made of silicon-germanium junctions.
Presidential approval is required for the launch of RTGs, review of the spacecraft is done by the Department of Energy and the results are reviewed by an independent group.