The Pluto probe,like many spacecraft before it, is powered by Radioisotope Thermoelectric Genereators (RTGs) which are cylinders about three feet long and two feet in diameter with a hole down the middle. You connect a bunch of thermocouples between the outside of the cylinder, which gets very cold, and the inside. Then, if you heat the inside, you get electricity. In theory you could heat it by burning coal or wood but the logistics of a long distance space mission require the use of plutonium. Now plutonium is radioactive and decays spontaneously so it's always hot. Just stick it in there and, voile, electricity.

Since the plutonium insn't involvded in any chemical or nuclear reaction, it can be totally sealed up. In actuality, it's put in small pellets which are individually sealed and then groups of pellets are further sealed. The result is that if the launch vehicle blows up, it is virtually certain that the plutonium will be entirely contained within the RTG. Even if it blows apart, the pellets are harmless unless you do something stupid like eat one.

NASA did a thourough study of what would have happened if the space shuttle Challenger had been carrying one or more RTGs and the answer is nothing.

Bev and Jerry