European Space Agency

Outside an atmosphere like Earth's, everything is already exposed to intense ionizing radiation from the sun/stars. A bit more from an RTG, even a big one, is a drop in the ocean. If we found signs of extraterrestrial life, then we'd want to be extra cautious about not sterilizing by accident, but that's not currently a major concern. And of course, any sort of nuclear rocket propulsion would need to be handed with utmost care, but it's also not a major issue once it's outside the atmosphere.