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The PACER project is described in this question: How much of the energy from 1 megaton H Bomb explosion could we capture to do useful work?

Why was it abandoned? It seems that it is the only readily economical and engineeringwise useful path to fusion power, and it seems that its breeder possibilities can easily let it pay for itself for generating fissile elements and helium (which is getting to be rare too nowadays!)

Was it political or technical limitations that killed it? Is there hope for a renewed interest in this in todays energy conscious politics?

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  • $\begingroup$ I think it would have been a lovely trigger for the California seismic fault line. The earth is full of fault lines waiting for the fullness of time to be triggered printable-maps.blogspot.com/2009/04/…. And we only know the recently, within human history, active ones. $\endgroup$
    – anna v
    Commented Feb 26, 2012 at 6:19
  • $\begingroup$ @anna v: Here is a technology which actually produces energy too-cheap-to-meter (running costs at least 1/10 current technology, probably closer to 1/100 or 1/1000), it is carbon neutral, it produces renewable fission and tritium resources and uses unlimited deuterium as the major fuel. it is available today, no R/D required, but is not implementable because of some vague fears? Using this thinking, as cavepeople we would have rejected fire because somebody will get burned. I am a little outraged that this technology is quietly buried. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 2:37
  • $\begingroup$ Have a look at "erthquakes" in hydraulic fracking en.wikipedia.org/wiki/Hydraulic_fracturing . Pacer would be delivering megatons of energy on the crust over and over and over again, and there is conservation of momentum too. We have an old greek proverb :"how many times will the water pot go to the spring?" This is used to mean that a breakable pot after a number of uses will break. I do not think that caution in this case is different than the caution of not lighting cigarets next to the car when filling up. $\endgroup$
    – anna v
    Commented Feb 27, 2012 at 5:32

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It seems, the main reason is politics. The movement towards prohibition of nuclear tests just started. Facility of this kind is an ideal polygon for nuclear tests. Few hundred explosions per year plus mass production would result in few orders of magnitude cheaper and more effective weapons automatically. That time it was not a good idea to boost development of nuclear weapons that much. It was still too complicated for average countries and everyone wanted to postpone the time when these average countries get an access to nuclear weapons.

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  • $\begingroup$ I think so too, but perhaps there are more technical answers forthcoming. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 2:33
  • $\begingroup$ Accept this answer, mostly because you had a lot of interesting comments, and I do believe this is the main reason. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 17:13
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I don't have any specific knowledge about the project, but based on what you've linked to there are quite a few potential issues that could have contributed to it:

  1. Cost. Not only the mega-engineering needed to build the chamber and ensure the stability of the surrounding geology, but also the need to instigate continuous production of nuclear bombs in large numbers. Although it would eventually pay off, it might simply be that the initial investment was unaffordable.
  2. Safety. You'd have to be pretty certain that neither the containment chamber nor the surrounding rock can crack under the force of those explosions, or because of geological movement. There are also safety issues involved in the manufacturing of the bombs and the running of the plant.
  3. Proliferation. You'd be manufacturing massive numbers of bombs that could easily be made into incredibly destructive weapons. You'd have to be pretty certain that none of them could ever find their way into the wrong hands. And if another country decided to copy the project then they'd have loads of bombs too.
  4. Environmental impact. That chamber isn't going to remain in an operational state forever, because it will be absorbing neutrons, which will eventually weaken it. When it reaches the end of its lifespan the only thing you can really do is leave all that accumulated radioactive material inside the chamber for ever, and hope it never leaks out. So you'd have to make sure the surrounding geology was stable over very long time scales and that the chamber was completely resistant to any form of corrosion.

My guess is that when all these factors were added together it simply didn't look like a good investment.

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  • $\begingroup$ Factor 4 doesn't seem right: the chamber is carved out of salt because the weakened parts will dissolve into the water, and any cracks self-repair. It is placed deep underground so that any leeching water will not contaminate groundwater. The radioactive materials are part of the economic output, they are to be chemically removed from the water, and form the breeder program. You can also use a completely different liquid, not water, if you are worried about contamination. I think the whole thing can be environmentally relatively ok, although it makes a lot of radioactive stuff. $\endgroup$
    – Ron Maimon
    Commented Feb 26, 2012 at 18:52
  • $\begingroup$ Factor 1 was analyzed in estimate in the document, and it seems to be competitive with other power sources. The chamber itself is a side-effect of certain mining operations, and the explosion effect on the chamber was already known. The issues with mass-producing hydrogen bombs was already known in 1974, and it can only be cheaper today. I agree with point 3, but it seems a shame to base rejection on this. Point 2 is also dubious, because the cracks in salt can be repaired, and the salt formations are much larger than the cavity, so that the leaks should be contained for a long time. $\endgroup$
    – Ron Maimon
    Commented Feb 26, 2012 at 18:55
  • $\begingroup$ @RonMaimon I wouldn't be so quick to dismiss the environmental issues with a subterranean cavity considering the economics of nuclear geologic repositories. Isolating the radioactive material isn't the kind of thing that you show in principle how it works and then proceed. It would be subject to endless scrutiny and failure mechanisms do exist. $\endgroup$
    – Alan Rominger
    Commented Feb 26, 2012 at 20:20
  • $\begingroup$ @Zassounotsukushi: The point is that the cavity is constant use--- you are extracting the water for removing the breeder materials, and reusing the radioactive components. You would get buildup of radioactive materials, sure, but they are all in one spot, and they are chemically separated at the plant, and can be bred and rebred by putting on the bomb casings until they are either safe or until they are fuel. If you have short half-life isotopes, you let them decay, for intermediate or long, you put them on the bomb-case for one more run-through. It's a continuous recycling program. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 2:18
  • $\begingroup$ I was partially going by PACER's wikipedia page, which describes a much more highly engineered solution in later versions of the proposal, including a building a metal-lined chamber, reinforcing the surrounding rock and using molten salts rather than water as a coolant. It doesn't say why it was changed but I assumed it was because there were engineering/safety related reasons why a simple hollowed rock dome wouldn't be suitable. If this is the case then the added complexity might be what killed the project. It would be good to know. $\endgroup$
    – N. Virgo
    Commented Feb 27, 2012 at 13:56
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Sorry for answering my own question, but I thought of a tentative answer--- there is an uncontrollable problem, which is the unknown chemistry you will generate in the water tank. As the thing operates, you have a constant neutron and fissile material flux which will produce a mix of plutonium, uranium, fission products, pusher products (isotopes near lead), various breeder elements, and various neutron absorption products on the salt, on the water, on the plutonium, on the lead, which will eventually produce every element under the sun in some proportion.

The chemistry of all these elements in solution is completely unknown. For all we know, they will form some plutonium compound that will produce a chemical plutonium polymer muck at the bottom of the chamber. Worse yet, this sludge could flow from one part to another, producing a critical fissioning mass which could sit there, making a meltdown which could wreck the containment.

The thing will also produce hydrogen gas. It could find a way to make polymers from hydrogen and trans-uranics, and these sludges would be highly radioactive, and they could clog the pipes with impossible to clean gunk of unknown chemistry, or it could just make a standard chemical explosion with hydrogen. The unknown compounds could be chemically explosive in much worse ways, even underwater, or otherwise chemically annoying.

I don't know any way to test this other than a trial run. It might not be a problem. But if residues collect in miniscule amounts, the radioactive chemical explosions might not begin until a few years of running. The moment you have to close a plant, the disposal problem becomes a nightmare of radioactivity. Although, I suppose you could just leave it where it is.

Whether such a thing should kill the project is a matter of judgement. One could try to figure out all the chemistry (this would be an enormous RD project), or just experiment with one power plant for 10 years in the middle of Antarctica. I still think the promise is greater than the danger.

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  • $\begingroup$ This is hardly the main reason. Salt is good also because Na is well known to have only short living isotopes (it is used in breeder reactors as a heat exchanger due to this fact), Cl is also light. Only admixture of heavy elements might result in isotopes which are able to produce fissioning products with small critical mass. To get a plutonium you should start at least from plumbum. Though, I'd also prefer to experiment with such plant at least on the Moon. $\endgroup$
    – Misha
    Commented Feb 27, 2012 at 4:15
  • $\begingroup$ @Misha: But you necessarily have Pu and heavy elements from the bombs, and all the fission products--- the light stuff isn't going to be radioactive, but it might chemically bind Pu into polymers which then can precipitate out and start a Pu reaction. You are using 2 bombs a day, each with several Kg of Pu and many tons of heavy pusher, which is lead, or thorium, or uranium, or breeder stuff, or something else that's necessarily heavy. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 4:23
  • $\begingroup$ I've heard, almost all plutonium in H-bombs detonator burns out. First, fission in detonator is quite effective, second high neutron flow in the middle of explosion burns the rest. I would count on few [tens] grams from each explosion. Pusher is a problem. However, there was a significant progress in this direction. And who knows what would come out in mass-production where people could experiment a lot with light materials like berillium. $\endgroup$
    – Misha
    Commented Feb 27, 2012 at 5:14
  • $\begingroup$ @Misha: interesting! But with two bombs a day, even a few grams will build up over a few years to a critical mass with the appropriate chemistry. As far as pusher, it has to be heavy, because it has to stay in place during the ablation cycle, so it has to withstand its own ablation pressure by inertia long enough to spark the seondary. So you are really limited in material choice. Further, the accumulation of possibly chemically explosive fission products will be a headache in the pipes. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 14:57
  • $\begingroup$ I am not sure that pusher has to be heavy. Under these conditions anything behaves more as a gas. It is heavy due to the fact that nuclear head is supposed to be installed to the rocket or something. Probably, 10 times more iron would go, but not suitable for nowadays use due to the size and weight. Even if it has, there is some choice: one of bismuth, mercury or another most likely is able to produce isotopes with "green" products only. $\endgroup$
    – Misha
    Commented Feb 27, 2012 at 17:13
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The day gasonline hits $10 a gallon, the PACER project will be back on track. It's only a matter of time.

A one megaton H-bomb is equivalent to one megaton of TNT which is more or less equivalent to one megaton of gasoline. At ten bucks a gallon that's worth $2000/ton, or 2 billion dollars. I think if we can extract useful energy with an efficiency of 10%, that will be a totally adequate return.

And am I mistaken or did they make bombs as big as 50 megatons??? That's a lot of gasoline.

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    $\begingroup$ There is no limit to H-bomb power, but here the goal is to keep the cavity structurally intact, so 1 megaton is an extreme upper limit, realistically 200KT. A megaton is a metric billion kilograms, and a US gallon is 2.7 Kg, so you have 380,000,000 , and you should compare with raw unrefined fuel cost of coal, which gives you about 300,000,000 (300 million) dollars. The cost of a 1 megaton warhead is about 300,000 dollars, so even with 100KT warheads it's at least 100 times cheaper today (no research). There are capital costs in setting up the plant, etc, but the running cost is much lower. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 2:15
  • $\begingroup$ You miscalculated in your answer, using your parameters, and your misinterpretation of megaton as 2000 US gallon tons, the cost equivalent of a warhead in gasoline is 20 billion dollars, not 2 billion. But the true cost is closer to 100,000,000 USD as in the previous comment, still dwarfing the cost of a warhead at 300,000 USD. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 2:35
  • $\begingroup$ You're neglecting the fact that I live in Canada, and our gallon is different. $\endgroup$
    – Marty Green
    Commented Feb 27, 2012 at 7:17
  • $\begingroup$ So is our dollar, for that matter. $\endgroup$
    – Marty Green
    Commented Feb 27, 2012 at 7:17
  • $\begingroup$ Come on, the difference is 30% at most. $\endgroup$
    – Ron Maimon
    Commented Feb 27, 2012 at 17:10

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