An RTG uses no moving parts to create electricity. Waste heat is radiated into space from an array of metal fins. Plutonium, like all radioactive materials and many non-radioactive materials, can be a health hazard under certain circumstances and in sufficient quantity. RTGs are designed, therefore, with the goal of surviving credible launch accident environments without releasing plutonium. The safety design features of RTGs are tested by the US Department of Energy to verify the survival capabilities of the devices.
Presidential approval is required for the launch of RTGs.
Prior to the launch of a spacecraft carrying an RTG a rigorous safety analysis and review is performed by the Department of Energy, and the results of that analysis are evaluated by an independent panel of experts. Since they remain thermally hot, RTGs must be located on the spacecraft in such a way to minimize their impact on infra-red detecting science instruments. Galileo's RTGs are mounted behind shades to hide the near-infrared mapping spectrometer from their radiant heat. Shades are used on Cassini for similar reasons. RTGs performance degrades in flight about one to two percent per year, slightly faster degradation than for photovoltaics.