Project PACER

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This article is about a 1970s nuclear power experiment. For other meanings, see Pacer (disambiguation).

Project PACER, carried out at Los Alamos National Laboratory (LANL) in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small hydrogen bombs (fusion 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. However it would also require a large, continuous supply of nuclear bombs.
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 1,000 foot 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.^[1]
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 single nuclear device to create an artificial harbour for mooring ships in the north, or as a sort of nuclear fracking to improve natural gas yields. 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.^[2]
Early examples considered 1,000 foot diameter water-filled caverns created in salt domes at as much as 5,000 feet deep and then filled with water. 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. Dropping about two bombs a day would cause the system to reach thermal equilibrium, allowing the continual extraction of about 2 GWp.^[3] There was also some consideration given to adding thorium or other material to the bombs to breed fuel for conventional fission reactors.^[4]
In a 1975 review of the various Plowshares efforts, the Gulf University Research Consortium (GURC) 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,^[5] and are around $45 in 2012.^[6] GURC concluded that the likelihood of PACER being developed was very low, even if the formidable technical issues could be solved.^[5] 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".^[7][8] In 1975 further funding for PACER research was canceled.^[9]
In spite of 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 (100 ft) in diameter and 100 m (300 ft) tall,^[10] to be embedded in a cavity dug into bedrock in Nevada. Hundreds of 15 m (45 ft) 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 (100 ft), 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 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.^[11][12]

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: 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 0.3⋅800⋅50kT⋅yr−14.2⋅1012J⋅kT−13.15⋅107s⋅yr−1≈1.6GW, 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 10W⋅m−1⋅K−1, assuming a crudely simplified geometry consisting of a flat plate of about 1km2 with 100m of thickness and the whole 500K thermal gradient applied, the resulting thermal flux is about 10W⋅m−1⋅K−1⋅106m2100m−1500K=50MW, 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). +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 1   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.