Project PACER, carried out at Los Alamos National Laboratory in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small hydrogen bombs —or, as stated in a later proposal, fission bombs—inside an underground cavity. As an energy source, the system is the only fusion power system that could be demonstrated to work using existing technology. It would also require a continuous supply of nuclear bombs and contemporary economics studies demonstrated that these could not be produced at a competitive price compared to conventional energy sources. The earliest references to the use of nuclear explosions for power generation date to a meeting called by Edward Teller in 1957. Among the many topics covered, the group considered power generation by exploding 1 Mt bombs in a diameter steam-filled cavity dug in granite. This led to the realization that the fissile material from the fission sections of the bombs, the "primaries", would accumulate in the chamber. Even at this early stage, physicist John Nuckolls became interested in designs of very small bombs, and ones with no fission primary at all. This work would later lead to his development of the inertial fusion energy concept. The initial PACER proposals were studied under the larger Project Plowshares efforts in the United States, which examined the use of nuclear explosions in place of chemical ones for construction. Examples included the possibility of using a large nuclear devices to create an artificial harbour for mooring ships in the north, or as a sort of nuclear fracking to improve natural gas yields. Another proposal would create an alternative to the Panama Canal in a single sequence of detonations, crossing a Central American nation. One of these tests, 1961's Project Gnome, also considered the generation of steam for possible extraction as a power source. LANL proposed PACER as an adjunct to these studies. Early examples considered 1000 ft diameter water-filled caverns created in salt domes at as much as deep. A series of 50-kiloton bombs would be dropped into the cavern and exploded to heat the water and create steam. The steam would then power a secondary cooling loop for power extraction using a steam turbine. Dropping about two bombs a day would cause the system to reach thermal equilibrium, allowing the continual extraction of about 2 GW of electrical power. There was also some consideration given to adding thorium or other material to the bombs to breed fuel for conventional fission reactors. In a 1975 review of the various Plowshares efforts, the Gulf University Research Consortium considered the economics of the PACER concept. They demonstrated that the cost of the nuclear explosives would be the equivalent of fuelling a conventional light-water reactor with uranium fuel at a price of $328 per pound. Prices for yellowcake at that point were $27 a pound, and are around $45 in 2012. GURC concluded that the likelihood of PACER being developed was very low, even if the formidable technical issues could be solved. The report also noted the problems with any program that generated large numbers of nuclear bombs, saying it was "bound to be controversial" and that it would "arouse considerable negative responses". In 1975 further funding for PACER research was cancelled. Despite the cancellation of this early work, basic studies of the concept have continued. A more developed version considered the use of engineered vessels in place of the large open cavities. A typical design called for a 4 m thick steel alloy blast-chamber, 30 m in diameter and 100 m tall, to be embedded in a cavity dug into bedrock in Nevada. Hundreds of 15 m 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 30 m, a "waterfall" would be initiated by pumping the salt to the top of the chamber and letting it fall to the bottom. While surrounded by this falling coolant, a 1-kiloton fission 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.
