Why was PACER abandoned? 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?
 A: 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.
A: 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:


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*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.

*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.

*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.

*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.
A: 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.
A: 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.
