The PACER project, carried out at Los Alamos National Laboratory in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small H-bombs inside an underground cavity.
The proposed system would absorb the energy of the explosion in a molten salt, which would then be used in a heat exchanger to heat water for use in a steam turbine. In the original proposal a huge cavity would be emptied in a salt dome, but further developments used engineered cavities instead. A typical design called for a 12 foot thick steel alloy blast-chamber, 100 feet in diameter and 300 feet tall, to be embedded in a cavity dug into bedrock in Nevada. Hundreds of 50-foot long bolts were to be driven into the surrounding rock to support the cavity. The space between the blast-chamber and the rock cavity walls was to be filled with concrete; then the bolts were to be put under enormous tension to pre-stress the rock, concrete, and blast-chamber. The blast-chamber was then to be partially filled with molten fluoride salts to a depth of 100 feet, a "waterfall" would be initiated by pumping the salt to the top of the chamber and letting it fall to the bottom, and while being surrounded by this falling coolant, a 1 kiloton bomb would be detonated; this would be repeated every 45 minutes. The fluid would also absorb neutrons to avoid damage to the walls of the cavity.
Another example: a 2 kt bomb; this produces an energy of 8 TJ, which would be absorbed by 2,000 tons of flibe, i.e. 4 MJ/kg; the energy the coolant absorbs per kg for heating and evaporation is the same as the energy value of TNT, hence the amount of coolant has to be the same as the TNT-value of the bomb.
As a power source the system is the only one that could be demonstrated to work using existing technology. However it would also require a massive supply of nuclear bombs, making the economics of such a system rather questionable. H-bombs are ignited using a small plutonium-based nuclear bomb, which is very expensive. More plutonium could be produced to lower the cost, but only for another large capital expense.
Equally worrying are the security concerns. A huge number of H-bombs is a clear risk, as is the plutonium needed to build them. Currently these various parts are constructed at different locations and shipped between them in highly guarded convoys, but the number of bombs needed for a PACER system would make this very difficult. The entire system — plutonium production, bomb fabrication and power generation — could be carried out in a single well-guarded site, but only for massive development costs that would likely never be paid off by the electricity they generated.
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