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The world is full of nuclear warheads being stockpiled. Controlled fusion power seems a long way away. Could we put these warheads to better use by exploding them in a controlled way and capturing the energy they produce?

By useful work I mean the power is then available for the national grid to boil kettles or have a shower!

Extra points for looking at the practicalities of building a facility to do this (although I guess that would get the question closed :-(... )

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By "how much of the energy", I take it you mean to include thermal efficiency of a thermal cycle. And although we have many nuclear weapons, they would only suffice for a small fraction of our power production for a limited time unless we made more. –  Alan Rominger Feb 24 '12 at 14:08
    
But... we did get quite good a making them, so if we could build a facility to harness the energy would this not be a quicker (if not far more ugly) way of implementing "fusion power"? –  Matt Luckham Feb 24 '12 at 14:10
    
@MattLuckham I read somewhere recently (i think on physics.SE) that it would require a humungous underground facility and still would be dangerous. Aside from the fact that a continuous process is more easily managed than a discrete one. –  Manishearth Feb 24 '12 at 14:14
    
@Manishearth: That was me, here: physics.stackexchange.com/questions/21365/… . I found this information in an online article critical of hot-fusion in general, and I believe the proposal for energy production using H-bombs is from 1970s China. I can't find the relevant article again, sorry, it was years ago. –  Ron Maimon Feb 24 '12 at 18:38
    
@RonMaimon Los Alamos had a project in the '70s called "PACER". There is a brief overview at p. 8 of this magazine and a way more detailed description in LA-5764-MS (warning: 22 MB file). –  mmc Feb 24 '12 at 20:16

3 Answers 3

up vote 6 down vote accepted

During the 1970s, the Los Alamos National Laboratory carried out the PACER project, to explore the use of thermonuclear explosions as a way of generating electrical power and breeding nuclear materials. The general layout of the initially proposed fusion power plant can be seen in the following illustration:

enter image description here

The system parameters were under exploration, but one of the ideas was to explode about 800 50 kT thermonuclear devices per year. As the conversion efficiency was expected to be about 30%, the generated electrical power would have been

$\displaystyle 0.3 \cdot 800 \cdot 50\,{\rm kT \cdot yr^{-1}}\frac{4.2 \cdot 10^{12}\,{\rm J \cdot kT^{-1}}}{3.15 \cdot 10^7\,{\rm s \cdot yr^{-1}}} \approx 1.6\,{\rm GW}$,

about 80% of the nominal power, because that was the assumed capacity factor.

Heat loss wasn't much of a problem because of scaling properties. As the thermal conductivity of rock salt is about $10\,{\rm W \cdot m^{-1} \cdot K^{-1}}$, assuming a crudely simplified geometry consisting of a flat plate of about $1\,{\rm km^2}$ with $ 100\,{\rm m}$ of thickness and the whole $500\,{\rm K}$ thermal gradient applied, the resulting thermal flux is about

$\displaystyle 10\,{\rm W \cdot m^{-1} \cdot K^{-1}} \cdot \frac{10^6\,{\rm m^2}}{100\,{\rm m^{-1}}}500\,{\rm K} = 50\,{\rm MW}$,

less than 1% of the thermal power.

The technical limiting factors were the relatively low temperature achievable inside a rock salt cavity and the large cavity sizes required to avoid contact of the walls with the unmixed fireball.

Obviously, there were also safety and public perception problems.

See page 8 of this magazine for an overview and LA-5764-MS for the details (warning: 22 MB PDF file).

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Awesome answer! Thank you mmc! Full marks from me! –  Matt Luckham Feb 25 '12 at 23:21
    
+1: Is it possible to scale up pacer to megaton-range devices? Aren't the bombs in the 10 kiloton range only 80% fusion, and so reliant on the non-renewable production of plutonium? A megaton range device can be 99.9% fusion. Also, do you know how much of the heat is wasted in the PACER? If you are 20% fission and waste 80% of the heat, you might as well use a normal nuclear power plant, so I hope that you can actually extract most of the energy from fusion. I think this is a terrific idea that might get a different reception today, now that cold-war fears have abated. –  Ron Maimon Feb 26 '12 at 1:46
    
Reading the linked papers, I get that it is possible to use the breeder aspects of the PACER using a Uranium or Thorium casing for each bomb which acts to convert U and Th to fissile material, which can then be bred in a regular fission reactor to produce Plutonium, which can then drive a PACER system. The whole PACER cycle might allow a complete utilization of all fission resources, plus a certain fraction of the fusion resources. It's a pity this project is abandoned. Who can people write to suggest a second look? Which congressional committee is responsible for this stuff? –  Ron Maimon Feb 26 '12 at 2:25
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This is a foolhardy project, imo. Contemplate this map of known fault lines in the earth printable-maps.blogspot.com/2009/04/… . Earthquakes happen because stress is built up and they are triggered by a straw that brakes the camel's back. Megatons underground would be a very useful trigger. And note the "known" faults. There can always be inactive faults for centuries that may suddenly activate. Can any sane government undertake such research, particularly in California? –  anna v Feb 26 '12 at 5:41
    
@RonMaimon Essentially all the thermal energy of the bombs was extracted, but the conversion efficiency to electricity was relatively low (~30%) (though it's not much better in current nuclear power plants). This was limited by the need of a secondary circuit to avoid running the turbines with radioactive (and salty) steam. –  mmc Feb 26 '12 at 14:08

I will give you an analogy.

Molotov bombs are made by filling a plastic bottle with gasoline, attaching a wick ingeniously, lighting it and throwing it on a target, usually a car or a policeman controlling a demonstration.

Now there are a number of BTUs of energy in this bottle of gasoline. I can ask paraphrasing you

Could we put these Molotov s to better use by exploding them in a controlled way and capturing the energy they produce?

You know the answer. It is the car engine. Controlled fusion research is trying to create the corresponding "car engine" to utilize the energy in the H bomb.

You could not control gasoline explosion in a better way than in a car engine. People would laugh if one proposed to explode a barrel of gasoline and devise a system to use the energy, no?

The same is true for fusion, it has to be incremental.

Ponder on dynamite, or nitroglycerin or a number of other explosives, which are not used for energy production because the explosions cannot be incrementally useful and safe.

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I like your analogy. However, there is a lot of energy locked up in a h bomb. Just say we could tap into 10%. I have calculated that is 155,000 mega watts of power. You know it was not that long ago we were going to use H bombs to power a spacecraft so I don't see why the idea is so crazy! en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) –  Matt Luckham Feb 24 '12 at 16:39
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From your link, it is atomic bombs, not Hydrogen ones. Atomic bombs can be made by bringing two pieces of critical mass together, and that can be easier to control : reactors are like slow bombs. There exist even nuclear submarines. Hydrogen has to reach very high temperatures before it can fuse, that is why the H bomb is triggered by an A bomb.en.wikipedia.org/wiki/Hydrogen_bomb –  anna v Feb 24 '12 at 19:39
    
Good point! I picked a fusion bomb as the vast amount of stocked pile nuclear war heads are fusion bombs. Your right though all of these will have a fission trigger so our "facility" will get pretty contaminated! I found this link, but it is still only theoretical:- en.wikipedia.org/wiki/Pure_fusion_weapon looks like my idea may not have the legs I thought! –  Matt Luckham Feb 24 '12 at 19:50
    
This answer is not so good--- the gasoline engine is not the optimal way to extract the energy in a gasoline fuel, it is just the most convenient for a self-propelling vehicle, because it is small. The best way is to burn the gasoline in a high temperature environment and run an efficient Carnot cycle to make electricity. This would work for a nuclear explosion just as well, but containing the heat requires a large amount of solid material surrounded by insulation. The solid material will be churned by the explosion, and made radioactive, but it might work if you dispose into undersea faults. –  Ron Maimon Feb 24 '12 at 22:17
    
@anna v: Project Orion was definitely based on thermonuclear, hydrogen bombs, but small-yeild ones, of around 10-100 Kilotons or so. There was no question of "control", that's just something you made up. You don't control the explosion, you just use it to blast things away from the spaceship at a good fraction of the speed of light, to have good thrust. Orion was only abandoned because of the test-ban treaty, and the idea might still be revived. It is the only way you can have feasable commercial spacetravel with known technology. –  Ron Maimon Feb 24 '12 at 22:18

if 1 ton of TNT releases about 4.184 gigajoules, which is 1162.2 kWh, therefore 1 megaton is equal to 4.184 x 1000000 = 4184000 gigajoules which equals 1,162,222,222 kWh. the average 2000 ft2 home in boston MA uses 820 kWh/month so 9840 kwh/year. 1162222222/9840 = 118,112 homes for a year. Maybe not the best method to use the energy, but hey why not?

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Love this answer! Let's build it! –  Matt Luckham Feb 24 '12 at 17:21
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Simply computing the energy generated does not address the questions. As it stands there is not even a theoretical way to hardness the power of a fusion explosion on that scale. –  dmckee Feb 24 '12 at 20:25
    
@dmckee There was a proposal in the '70s to generate electricity from ~100 kT explosions in underground cavities (the yields were limited by cavity stability considerations). –  mmc Feb 24 '12 at 21:24
    
@mmc: I was thinking of using wet-sand in the cavity, not water, and having a much bigger cavity for multi-megaton explosions. The issue, I thought, would be heat-containment--- so that you don't waste most of the heat. But the stuff in the article is good enough for fusion power today, if the security factors could be overcome. –  Ron Maimon Feb 25 '12 at 1:46

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