Would it be possible in a Fukushima-style meltdown to halt the process (edit: I mean halting the meltdown process, not the actual nuclear processes) by dumping large amounts of iron into the vessel? The iron would melt and mix with the uranium, which would 1) Dilute the fuel to slow the reaction 2) Absorb and transport away heat 3) Encase the fuel in a massive block of metal 3) Iron doesn't contain carbon, as steel would. Carbon moderates the neutrons.
Of course, this would render the reactor useless, but if it is already melting down, it is already useless. It would slow the reaction and lower the temperature so you stop building up hydrogen gas in the containment vessel. Other metals might be theoretically better, but iron is cheap, strong, and readily available.