What are possible power sources for satellites?
Plutonium, the active ingredient in most radioactive thermal generators (RTGs) is a toxic heavy metal like lead. If it is powdered and inhaled, it is a cancer-causing agent. But this isn't likely to happen. RTGs work by a much different technology than conventional nuclear power plants in the U.S. or Europe. The plutonium (a few kilograms of it) is sealed inside a hard, radiation-proof shell. The shell is designed to survive all conceivable accidents, so even in the unlikely event a launch goes wrong, none of the radioactive particles will escape.
Because RTGs use radioactive decay, each launch of any vehicle with an RTG requires Presidential approval. Given that they are the only reasonable way to power satellites beyond the orbit of Mars (as solar panels stop becoming effective), RTGs are necessary. Thus, NASA engineers strive to make them the safest, most indestructible parts of a spacecraft.
RTG's can not explode like a nuclear weapon. Nuclear weapons are made of high-grade uranium and have to be arranged very carefully to go into fission. RTGs, at best, can produce a warm fizzle.
What are radioactive thermal generators?
What are fuels are used in radioisotope thermoelectric generators?