OK, so I spruced up the sublimely boring image accompanying this interesting question over at Popular Science with a picture of the thermite reaction. Sorry, but I couldn’t find a picture of burning plutonium. C’mon Wikipedia! What’s taking so long?
The real answer, it turns out, is something like “at first it would be very exciting, and then it would be very boring.” Here’s a characteristically droll quote from my old quantum mechanics instructor, John Stanton:
The oxygen gas would react with lithium or sodium and ignite, raising the temperature in the container to the point that all hell would break loose. Powdered graphite carbon would ignite, too. There are roughly 25 radioactive elements, and they would make your flaming stew a little dangerous. Flaming plutonium is a very bad thing.
Of the rather boring low-energy end products, Stanton says:
“Thermodynamics wins again. Things will always achieve equilibrium, and in this case that’s a mix of common, stable compounds.”