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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?

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Could lead stop the neutrons? –  JeffG Mar 15 '11 at 11:52
    
Neutrons are shielded by wax brick walls traditionally today one would use polyethlene, maybe. –  Georg Mar 15 '11 at 13:12

10 Answers 10

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.

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Yes, it would guarantee a melt down, but the question is whether the melt could be contained under some tons of lead and thus not be dangerous. Lead is also a good heat conductor and radiation absorber. –  anna v Mar 15 '11 at 12:35
    
I do not understand the "this would guarantee a meltdown" : lead -cooled reactors ( en.wikipedia.org/wiki/Lead-cooled_fast_reactor )have been built. But clearly water is better for various reasons now. –  Frédéric Grosshans Mar 18 '11 at 11:30
    
lead only disallows ionising radiation. It doesn't absorb neutrons. So, fission would continue, hence heat generation will continue, which has to be removed to prevent meltdown. btw, current fourth generation reactors (in research) are planning to use molten mixture of lead and bismuth as coolant. –  Vineet Menon Mar 13 '12 at 4:51

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.

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I'll certainly buy that they cannot add lead to an already build reactor, unless they can tap the existing seawater makeup lines, and the components on that pipeline are unlikely to take the heat. But I was thinking something along the same lines as you suggested... Lead has a low melting point and good radiation stopping power. Might it be possible to introduce lead slurry or use water to convey lead pebbles into the core? It seems that the lead would actually draw heat FROM the core, as it melts. –  JeffG Mar 15 '11 at 11:52
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@JeffG if you read the answer by @Georg you will see that once the temperatures have gone up the way they did at this time, lead would be futile, because it would not longer be liquid, the same as waterO one would have to consider lead steam which is not a nice exhaust for the environment. A nuclear engineer would have to answer if it could be used as a precautionary system, before temperatures run away. But as in all industry projects, there is a cost benefit analysis, and the fact that lead would destroy completely the reactor would be against using it. Costs are in billion dollars. –  anna v Mar 15 '11 at 12:33
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On the plus side - pumping tons of lead vapor into the environment means people wouldn't have to worry about the health effects of any radionuclide. –  Martin Beckett Mar 15 '11 at 16:54

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.

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+1 anyway for the gamma rays vs radioactive particles even if I do not completely agree with "the only thing we could do now". The reason they try to cool down the core is to prevent an explosion of the confinement structure, and the building of concrete structure around the reactors seems even harder that cooling them down... –  Frédéric Grosshans Mar 18 '11 at 14:30

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.
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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?

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The relevant temperature is not the boiling point, but the vaporization heat capacity, which is high for water. When 1kg of water boils it takes 2 MJ of heat from the reactor away. That why the japanes authorities try to get as much water at they can to the Fukushima reactor. It's not useless. It's just that they have not managed to put enough water for now. –  Frédéric Grosshans Mar 18 '11 at 14:25

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.

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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.

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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?

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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.

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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.

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Molecular mass $\neq$ density or miscibility. –  Manishearth Mar 12 '12 at 5:44
    
Also, tritium certainly is lighter than lead. (Elements aren't radioactive; isotopes are). –  MSalters Mar 13 '12 at 11:55

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