|Fig. 1: The crater left after the Sedan Nuclear Test. Source: Wikimedia Commons).|
Operation Plowshare was an initiative of U.S. Atomic Energy Commission aimed developing peaceful uses for nuclear explosives, involving 35 explosions across 27 tests that ran from 1961 to 1973. An initial classified conference on the potential civilian uses on nuclear explosions was held at Lawrence Livermore National Laboratory in February 1957.  The Plowhare program was initially classified entirely due the similarity in nuclear explosions for peaceful and non-peaceful purposes. Following the first U.S. nuclear test to be contained underground, the Rainier test in September of 1957, interest in the technology increased, and on June 6th, 1958 the Atomic Energy Commission formally announced the Plowshare program to the public. 
Plowshare's initial proponents such as Dr. Edward Teller had high hopes for nuclear explosives. Teller hoped that progress in thermonuclear explosives would be such as to make them cost-competitive with conventional explosives at the size of a 1000 or even 100 tons of TNT equivalent.  Consequently, throughout the program, proponents argued for a wide variety of potential benefits of nuclear explosions. As part of their Understanding the Atom Series, the Atomic Energy Commission put out a book titled Plowshare in which they claimed the "uses are as varied as chemical high explosives", and listed potential applications as varied as making steam and directly converting mineral deposits into chemicals. However, two applications get the bulk of attention. One was "geographical engineering" - large scale earth moving of the sort helpful in digging canals, clearing passes through mountains, and creating harbors. The second was increasing production of oil and natural gas by using explosions to access sand and shale reserves. 
The majority of the 35 nuclear explosions carried out by Operation Plowshare were to develop nuclear devices for the earth excavation applications. Initial efforts to demonstrate the power of nuclear explosives to move Earth centered on Project Chariot, an proposal to use 5 nuclear explosions to excavate a harbor in Northern Alaska, along with an effort to use nuclear explosives to dig canals.  Project Chariot was originally tentatively scheduled for 1960, but a moratorium on nuclear testing went into effect in 1958 and remained in place till the fall of 1961. The delay proved fatal to Project Chariot. Local criticism and concerns about the testing grew over time, as did concerns about the practicality of the remote proposed site. However, having spent $4 million dollars and not wanting to be seen validating public fears by giving in to them, the Atomic Energy Commission was careful to simultaneously announce the cancellation of Project Chariot and the advent of Project Sedan, a test who provide much of the same information relating to excavation as Project Chariot at a lower cost. Project Sedan would involve a single nuclear explosion that would be a cratering shot: neither above ground nor contained fully underground. Instead, it would be placed a depth calculated to just break the surface, with majority of debris falling into the crater created and not leaving the site. 
The Sedan test was carried out on July 6th, 1962. While most of the radioactive debris fell into the crater (pictured above right) as planned, the cloud of dust gas rose to 12,000 feet in the air and spread radioactivity across Nevada and Utah. In the Salt Lake City area, there were detections of anomalous levels of radioactivity in milk caused by cows grazing on grass exposed to fallout from the test. While the AEC managed to calm public fears, and human exposure to radiation remained with acceptable limits for both the workers at the site and citizens in the surrounding areas, Sedan demonstrated the need to minimize radioactive fallout if widespread civilian use of nuclear explosives for earth excavation was to be a reality. 
As the Sedan Test demonstrated, a key engineering challenge was to modify the devices in order to minimize the level of radioactivity produced. In addition to safety and alleviating public fears, the Limited Nuclear Test Ban treaty signed in 1963 required that no "radioactive debris to be present outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted."  The scientists and engineers of Operation Plowshare made significant progress towards reducing radioactive fallout associated with nuclear explosions. Since fission explosions necessarily produced radioactive products, one step was reduce the size of the fission explosion necessary to trigger the fusion explosion stage of the thermonuclear Plowshare devices. This was necessary but not sufficient step, since the fusion explosion could release large numbers of energetic neutrons that could activate materials surrounding the explosion. In order to address this concern, two steps in addition to minimizing the fission yield were proposed. First, components of devices using materials that produced biologically objectionable products could be replaced with other materials that produced relatively less harmful products, such as substituting plastics for tin. Second, devices could be surrounded with neutron absorbing shielding in order to prevent activation of earth or any other substances surrounding the test. Simulations showed that such shielding would be highly effective because high energy neutrons that has passed through the shielding into the soil would be reflected back into and absorbed by the shielding.  Plowshare's progress towards cleaner nuclear device designs was not just theoretical; some of the later Plowshare tests such as Cabriolet test in 1968 produced no offsite fallout. 
The ultimate cause of the demise of Operation Plowshare was economic rather than technical, compounded by shifts in political and public attitudes.  One of the projects that justified the use of nuclear explosives to move earth on a massive scale was the Interoceanic Canal Study, which ran from 1965-1970 in order to determine whether nuclear explosives could use to excavate a sea- level canal between the Atlantic and Pacific Oceans in Central America. By 1968, it was the only major earth moving application being seriously considered. As part of this effort, the Operation Buggy nuclear test demonstrated a sequence of 5 nuclear explosions in a row could dig a trench with a minimum of fallout outside the site. In spite of this success on a technical level, not only were public safety concerns were raised when considering larger-scale tests, cost estimates were higher than conventional digging techniques, leading to recommendation in 1970 that nuclear explosives not be considered for the canal project.  With earth excavation no longer a promising application, the last Plowshare test focused on nuclear explosives to stimulate natural gas production. Plowshare carried out three tests for the stimulation of natural gas production: Gasbuggy in 1967, Rulison in 1969, and Rio Blanco, the last Plowshare nuclear test in 1973. All of the tests had significant support from partners in the oil and gas industry. However, unlike with earlier tests, the AEC now faced and had to win litigation in court against wary local citizens to carry out the Rulison and Rio Blanco tests. Further, although the tests did significantly boost natural gas production, the natural gas was slightly radioactive. Although the radiation levels were not necessarily harmful, adding about 1% to the background radiation of the average home, none of the gas was sold.  Even if the gas has been sold, the economics were unpromising. It was estimated that even after 25 years of gas production from the fields, only 15-40% of the cost of carrying out the tests could be recovered. Operation Plowshare was discontinued at end of fiscal year 1975. 
© Shashwat Udit. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 R. M. Lessler, "Reduction of Radioactivity Produced by Nuclear Explosives," in Symposium on Engineering with Nuclear Explosives Proceedings, Volume 2, U. S. Atomic Energy Commission, CONF-700101(Vol.2), May 1970, pp. 1563-1568.