Can a nuclear reactor meltdown be contained with molten lead? If lead can absorb or block radiation, would it be possible to pump molten lead into a reactor core which is melting, so that it would eventually cool and contain the radiation?
Is there something that can be dumped into the core that will both stop the reaction (extremely rapidly) AND will not combine with radioactive material and evaporate into the atmosphere, thus causing a radioactive cloud?
 A: This would guarantee a meltdown. 
They're trying to get heat out of the core because---thought the fission chain reaction has been suppressed---various unstable fission daughters continue to decay. Adding hot lead would add heat to the system and not stop this behavior. Total disaster.
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If the core does slag out, it will probably end in a hot (thermodynamically and radiologically) heap at the bottom of the primary containment vessel. Presumably you leave it there to cool a bit, they pour on a lot of concrete and post "Keep Away" signs.
No need to use anything as expensive lead, as you just want to pile on enough mass to suppressed the primary flux. Dirt would do, but concrete will make it hard for stupid people to go digging in the pile.
Adding some boron to the mix would be helpful because otherwise neutrons get everywhere. Sneaky little bastards that they are.
A: Hmm, lets see. The melting point of lead is fairly low, 327.46 °C and it is a good absorber of radioactivity.
I think the problem with the reactors  is not the heat per se, but the exposure of the fuel rods to the air without cooling because of escaping steam not being replenished by cooling water. The remaining steam etc may blow out the thick container and radioactive material will disperse some distance while the fuel rods will melt.
I expect that the design is such that even in melt the fuel rods and melted control rods will be sub critical.
The problem with the suggestion is, when they have troubles pouring cool seawater in, how ever will they be able to pour hot liquid lead? It is not that the reactor is a pot whose cover can come out!
A better question/suggestion would be, since lead has such a low melting point, and the operating temperature of a boiling water reactor is 250C, why don't they set up a sort of cladding to be passively  used in such emergencies: the inner layer of something that can melt when temperatures get higher than 500C, and then lead melts and floods out the system before the fuel rods get exposed, covering them. The answer is similar to why they were not pouring seawater on all reactors from the first minute: economics, trying to save the reactors for production later.
A: Gamma rays is not a concern at all, just stay 1000m from the reactor, and no gamma rays will reach you from there. So no reason to block gamma rays. (Other types of radiation cannot go even 100m in air).
The problem is release of radioactive gazes & particles, which fly around and end up somewhere in human body. That's why the only thing we could do now is to try to completely isolate reactor from environment using some concrete.
A: One of the problems is that lead does not absorb neutrons, it reflects them. That has been used to reduce the critical size, i.e favour the nucluear reaction by sending the neutron back into the fissile material. And indeed, the first deadly criticality accident was caused by a similar neutron deflector. This is not something someone wants in a nuclear reactor.
Water is much better and is currently used at Fukushima power plant because :


*

*it is much easier to manipulate and to get in large quantity than anything else;

*it is a good coolant, with high vaporization latent heat;

*its a good solvant and you an dissolve neutron poisons in it, like boric acid, which absorbs the neutrons;

*it is not toxic.

A: Lead, seawater, or anything with a low boiling point at low pressures would be a waste of time in an actual nuclear core melt down situation. Possibly one could pump sand into the reactor. Hopefully the sand, probably melted by the high temperatures around the pool of melted reactor rods, might dilute the pool enough to eventually stop the nuclear reaction. Certainly looks like the nuclear community has not fully thought out the worst case nuclear scenario; otherwise we would not be having this discussion.  What a mess huh?
A: The question amounts to whether this thing can be buried away.  Yes, and that will have to happen, and the sooner the better.  The complex of 4 reactors is what is called in the military a cluster f**k --- a total loss and disaster.  Gorbachev in the wave of the Chernobyl meltdown ordered the military to fly helicopters, and servicemen who died of radiation sickness, to the reactor and bury the thing in neutron moderating boron salts, sand and concrete.  The current situation is a slow motion Chernobyl situation, and the “Chernobyl option” seems to be the best shot at stemming this unfolding disaster.
Molten lead is not a particularly good idea of course.  The radionuclides would dissolve in the liquid lead, and the hot liquid lead would be a source for fires.  Pouring molten lead would make the situation worse.
A: The heat of vaporization is not a temperature, but a capacity for holding heat (whose units are btu/lb, or some equivalent to that) which is at the boiling (vaporization) temperature of a substance. Yes, water does have one of the highest heats of vaporization of any substance around, the Fukushima reactors have not melted yet, and so it does make sense to put lots of water on them right now to cool them down to avoid a run-away meltdown. My suggestion deals with a hypothetical, in which say not enough water was put on a reactor, not enough heat could be taken away, and the core actually melted into a pool of molten liquid radioactive nasty. (pray that don't actually happen). My thought was to take something like liquid silica (sand) that might dilute such a liquid mass such as to stop the heat generating and nuclear reactions from taking place. Water could not do that. Once the molten nasty is diluted no longer producing so much heat and radioactivity, one would then entomb it in more sand and cement. 
What I am also wondering about is if anyone in the nuclear community has actually thought out the worst case scenario so far, and what actual plans are out there to deal with a total runaway meltdown?
A: The GE Mark V containment system used at Fukushima has a design basis that calls for the concrete containment vessel to withstand a complete meltdown. This is typical of reactors in the US. There are many penetrations of the vessel and some of them may be leaking, but that does not necessarily mean the vessel was breached.
Pebble bed reactors can be shut down completely and not melt down. The next reactors in the US will also be able to lose all coolant flow and not melt down.
A: A problem that seems to have been overlooked is the atomic mass of lead.
Lead, although heavier than all non radioactive elements, is lighter than all
radioactive elements.  The result of injecting molten lead into a molten core
would be a layer of molten lead floating on top of the molten fuel.
Further, depending on the structural integrity of the pressure vessel and the 
amount of lead to be added, the additional weight could rupture the vessel.
A: Meltdown containment: Sounds like an option would be boron mixed with wet wet cement pumped through long flexible tubing with a water flush to keep it moving and a pump replaceable and far away from the meltdown far away from being affected. Forget recovery (containment). Pile it high and thick over the top. Dig deep underneath and do the same. Bury it.
A: Fingers can be dipped in molten lead at 500 degrees centrigrade and above without getting hurt even trivially. Further Molten lead contains Uranium blasts. After pouring molten lead use common lenses[bvery large ones] to concentrate on the reactors with will send neutrinos and which in turn would destroy all uranium and convert it into a harmless alloy of sorts and will keep molten due to sun's heat. 
A: No. a meltdown of uranium would sink in lead due to uranium's higher density. that is the whole problem. there is really nothing that can stop it once it gets going. the only thing at that point is the dispersal of the fuel to lower the thermal neutron density that is causing the heat. this will happen when enough of the rock below mixes with the uranium. if there is sufficient lead base you might stop it. it will be molten once this issue starts.
