I apologize if this sounds really dumb, but nuclear physics is not something I'm versed on...

Reading through the previous posts I get the impression that once you start a fission reaction it won't stop until the uranium is depleted?

Which means that unless cooled down by artificial means, as long as the materials it finds on it's path melt at lower temperatures than what the uranium fuel can reach it will keep melting it's way down?

Have I got this right or I've just read too much sci-fi?