Plutonium can form plutonium hydride in the presence of air and water, and that will catch fire all by itself. But normally you won't ever see pure plutonium, as they stabilize it with other metals when making bombs. Still, the point is that plutonium is not indestructible. It's just a metal, a very strange one, but still just a metal.
Ah. Didn't know that.
I guess calling the local fire department to hose it down is out of the question...