scrap iron for stopping a meltdown 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.
 A: One the fuel rods are hardly likely actually melt- they have a melting temperature of 2000C+- temperatures like that are only achievable in furnaces or jet engines. The concern is that it might melt-disintegrate the reactor base. It didn't happen in Fukushima. 
The Hydrogen came from the Zirconium reacting with water that jackets the fuel rods (unclear how much is left). Sure the fuel rods produce tritium but that is very tiny quantities (large for nuclear regulators). 
They could have just enclosed the reactor in concrete and used it for effective passive cooling but it would have left the reactor radioactive for thousand+ years and left the site unusable indefinitely.      
A: Bad idea. There are many parts of the structure which would become part of the molten iron. Ideally you'd want a material that melts at a temperature much lower than iron, so that structural elements remain unaffected. Quite a few salts have such melting points.
A lower temperature of the resulting molten material would also prevent a number of radioactive elements from being vaporized. It's not just the Uranium that's radioactive; in fact that's pretty harmless.
A: If you want a better reactor design, look at this type.
