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National Museum of Nuclear Science & History

Nth Country Experiment

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Friday, March 1, 2019

Could any country with the right knowledge and technology build a nuclear bomb? From May 1964 to April 1967, the Lawrence Radiation Laboratory (the predecessor to the Lawrence Livermore National Laboratories and the Lawrence Berkeley National Laboratory) set out to answer this question. The Laboratory hired three physicists who only recently received their Ph.Ds in physics to design a nuclear bomb. D.A. Dobson, D.N. Pipkorn, and R.W. Selden had little to no experience with nuclear physics. The Lab wanted to know “if a few capable physicists, unfamiliar with nuclear weapons and with access only to the unclassified technology, could produce a credible weapon design”[1] with “a militarily significant yield.”[2]

If the scientists succeeded in designing a workable implosion device, the implications were that any country with the right technology and knowledge base could also build one. This finding would have had grave implications for American national security and nuclear proliferation during the Cold War.

For the experiment, the postdocs chose to design an implosion bomb that used plutonium-239, like the “Fat Man” bomb the US dropped on Nagasaki, for several reasons. One, plutonium had an economic advantage over uranium-235 “because [uranium-235] requires the development of reactor technology.”[3] Two, its relatively lower critical mass and low density phase with a greater compressibility compared to uranium-235 allowed for a shorter chain reaction time compared to uranium-235.

Another reason why the postdocs wanted to design an implosion device was because they believed that a gun-type weapon that used uranium-235 was easier for them to build and did not need to be tested. In fact, Manhattan Project scientists did not test the “Little Boy” atomic bomb, which used U-235, because they were so confident that it would work. However, an implosion device would be more difficult to design and “seemed to be a more sophisticated, challenging, and hence appealing problem.”[4]

One challenge the scientists had to overcome was plutonium-239 capturing a neutron and turning into plutonium-240. When plutonium-240 fissions spontaneously, neutrons are released. However, neutrons are important in sustaining a chain reaction (or the self-sustained splitting of atoms as caused by neutrons) in a nuclear weapon. The chain reaction itself is important because the splitting of atoms releases energy. That energy is what gives a nuclear weapon its yield. If enough neutrons are released before a nuclear weapon is detonated, that weapon will fizzle, or fails to to meet expected TNT yield. As a result, a bomb that has a significant quantity of plutonium-240 will fizzle.[5]

Another problem was the timing of the lens detonation. An implosion device looks like a soccer ball because of the 32 lenses of explosives that surround the plutonium core. In order to produce enough energy to create a sustained chain reaction, all the lenses needed to explode at the same time. As physicist Cameron Reed explains, “[g]iven the speed of the explosive, they all had to trigger within a microsecond. There were 32 segments surrounding that core altogether. If the implosion is, say, a little off-centered, the core might shoot out one side, and you get a much less efficient nuclear explosion.”[6]

Information about nuclear reactions, such as fission and fusion, was not secret by the 1960s. In fact, Dwight D. Eisenhower’s Atoms for Peace program and the USSR’s equivalent program unintentionally spread the knowledge and technology to build a nuclear weapon. These programs were specifically geared towards spreading the peaceful uses of the atom–namely through nuclear energy. However, the technology and the knowledge to build and run facilities to produce nuclear energy are the same ones that can produce a nuclear weapon. In this regard, finding this type of information was not hard for the postdocs. As Selden recounted, “You just go to the library and you start looking under all the subjects, you look under plutonium and uranium and high explosives and you look under nuclear physics and you just keep going and you find articles and stumble across things and books and publications.”[7] Dobson added, “That kind of information greatly reduces the amount of preliminary experimentation you’d have to do.”[8]

After three years, Dobson and Selden (Pipkorn quit after a few months and was replaced by Selden)[9] completed their design. While their design was too big to be delivered by missile, airplanes or trucks could deliver it.[10] Furthermore, according to Dobson, they “weren’t trying to get fancy and optimize”[11] the possible yield of the weapon. Essentially, they were not focusing on designing a weapon that would cause most or all of the plutonium-239 to fission, which would maximize the yield of the weapon. For example, the Little Boy bomb used about 64 kg of uranium-235, or about 140 Lbs.[12] Theoretically, if all the uranium atoms in the bomb split, that bomb would have a yield of 1, 280 kilotons of TNT.[13] However, the Little Boy bomb had a yield of about 15 kilotons. This  difference of yield can be in part explained by the bomb’s design not allowing the maximum amount of uranium-235 to fission. For Dobson and Selden, optimizing the theoretical yield of the plutonium-239 was not a priority; rather, they were only focused on designing a weapon with a “militarily significant” yield.

Even though they solely relied on unclassified sources, the Atomic Energy Commission (AEC) Regulations classified their design and the results of the Nth Country Experiment as top secret. This is one reason  why the Nth Country document is heavily redacted.[14] The redactions leave many questions about the results of the experiment. Whether or not they were successful in designing a credible weapon with a significant military yield remains undetermined.

However, the physicists discovered two major technical problems that a nuclear-weapon-seeking country would face. First, the country would have to acquire plutonium-239, most likely with a plutonium production reactor since it is not a naturally occurring element (except in very small quantities). Additionally, the country would have to construct uranium and plutonium processing plants and be able to extract plutonium from spent fuel, such as in the PUREX (the Plutonium-Uranium Extraction) process. They concluded that building one plant would require about $60 million while the yearly operating costs would be about $10 million.[15]


Critiques of the Nth Country Experiment

Although the results of the experiment are unknown, Selden and Dobson were confident in their success in completing their objectives. After the experiment, they presented their work to people with the proper security clearance. Based on the reactions of “stunned” civilian officials and the unsurprised scientists, they concluded that they succeeded in fulfilling the requirements of the Nth Country Experiment. They believe that they designed an implosion device that had significant military yield, using only open sources.[16]

However, some experts doubt this confidence. For example, F.S. Eby and L.S. Germain wrote “Critique of the Nth Country Design,” which is included in the original Nth Country Report. The nature of these critiques are ambiguous because of the heavy redaction in this document. However, there seemed to have been some disagreements on the calculation and yield. Eby and Germain state that the “detailed LRL [Lawrence Radiation Laboratory] design calculations, using codes unavailable to the Nth Country physicists, disagree with both of these numbers.”[17] Furthermore, they point to a discrepancy in the postdocs’ calculations, but what that discrepancy is remains unknown.[18]

Eby and Germain even took issue with the postdocs’ premise to use plutonium-239 instead of uranium-235. While they agreed “Pu-239 has a long range economic advantage over U- 235…it is doubtful that the weapon scientists would be called upon to make decisions concerning the overall economy of the nation. Thus, it may be that they have directed themselves to plutonium implosion systems for reasons which are not completely valid in the context of the study.”[19] To summarize, Eby and Germain question one of the assumptions the postdocs made in choosing to design an implosion weapon: a weapons scientist from an Nth Country most likely would not factor the economic advantage of plutonium-239 over uranium-235 when designing and building an atomic bomb.

Ultimately, Eby and Germain had little confidence in the success of the Nth Country Experiment. They even went as far to say that the postdocs “correctly observe that they have very little firm information about the criticality of their system. [Redacted] In light of this extreme sensitivity, it would seem that confidence in the expected yield is unwarranted.”[20] Dobbs and Selden previously stated that they were confident that the implosion design would have a militarily significant yield. Due to the redactions, what the exact yield they calculated remains unknown. Eby and Germain counter that the open source resources that were available to the postdocs did not provide the necessary information to determine whether or not their design could sustain a chain reaction.

Unfortunately, the full context of their critiques is lost because of heavy redactions. However, as Alex Wellerstein, a historian of science who specializes in the history of nuclear weapons and nuclear secrecy, points out, Eby and Germain’s comments are “along a similar theme: there were a lot of unknown unknowns for the postdocs, and their confidence that their weapon would work as planned is probably unwarranted. At no place in the Summary Report does it imply that anyone but the postdocs thought the design would work as expected.”[21]


Nuclear Terrorism?

The Nth Country Experiment has implications for today’s global nuclear security. Today, there are concerns that not just countries, but also terrorist groups, can build a nuclear weapon. If three postdocs could design an implosion bomb, what would stop a terrorist group from doing the same? Experts are split on this issue.

Selden and Dobson believe that their experiment proves this is a possibility. Selden argues, “It’s certainly possible for a terrorist group if they’re really technically savvy and have a lot of resources.” Furthermore, the internet could give terrorists enough information  that “[put them] in the right ballpark.”[22] However, the current trend seems to be that terrorists prefer uranium-235 over plutonium-239 since uranium-235’s lower radioactivity makes it harder to detect.[23] As well, the simpler gun-type design could be employed in place of the implosion design.[24] Ted Taylor, a former physicist at the Los Alamos National Laboratory during the Cold War, explained that terrorists may not be interested in a “militarily significant yield,” like the Nth Country Experiment’s postdocs were. Instead, they might be satisfied with a bomb with a sub-kiloton yield that is strategically placed.[25]

Peter Zimmerman, a former Chief Scientist of the Senate Foreign Relations Committee, and Jeffrey Lewis, the founding publisher of, even go as far to imagine how a terrorist group might build a nuclear bomb in their article “The Bomb in the Backyard.” While this thought experiment does not refer to the Nth Country Experiment itself, it does bolster Selden and Dobson’s belief that the Nth Country Experiment proves the possibility of nuclear terrorism. In the article, Zimmerman and Lewis break down the costs of building a gun-type weapon that uses uranium-235. These costs include hiring physicists, personnel, equipment, uranium, and a 150-acre ranch in Texas or Wyoming to prevent outsiders from hearing them “firing the gun during their tests.”[26] In 2006, when the article was published, the total cost came out to be about $5,433,000.[27] The most difficult part of building a nuclear weapon would be “casting the uranium metal, which melts at high temperatures, into appropriate shapes.”[28]

However, Wellerstein casts doubt on this possibility. In response to Ted Taylor’s argument about terrorist being satisfied with a sub-kiloton yield, he argues that if the terrorists’ goal was to build a sub-kiloton weapon, “then you probably [wouldn’t] be worried about a complicated implosion design from scratch anyway (much less many of the other design decisions that the postdocs made)” and adds that there is a technical gap between designing a nuclear weapon on paper and actually building one. Wellerstein concludes that “the Nth Country Experiment is probably useless on that point as well.”[29]

John Mueller, a Senior Fellow at the Cato Institute, states that researching, developing, and building a nuclear weapon can be cost prohibitive, and terrorist groups may not have the interest to invest money in this endeavor. For example, Al-Qaeda allegedly allocated only $2,000 to $4,000 for WMD research, and most of it was reserved for chemical weapons. Additionally, Al-Qaeda leader Ayman al-Zawahiri was quoted as saying that the project was “wasted time and effort.”[30] The question of a terrorist group building and using a nuclear bomb in an attack remains inconclusive.

While the legacy of the Nth Country Experiment remains disputed, the “Nth Country Problem” has had a long history in US strategy and thinking. Even a decade before the Nth Country Experiment, many in the intelligence and defense communities thought about whether any country, big or small, could develop nuclear weapons. Now, this question now encompasses non-state actors.[31] Perhaps  the experiment’s legacy has less to do with definitively answering this question about nuclear proliferation, and more with  keeping nuclear proliferation at the forefront of US thinking.


[1] “Summary Report of the Nth Country Experiment” (Report, Livermore, California, 1967, 7.

[2] Ibid., 33.

[3] Ibid., 12.

[4] Dan Stober, “No Experience Necessary,” Atomic Bulletin of Scientists 59, no. 2 (2003): 59.

[5] “Nth Country Experiment,” 12.

[6] “‘Fat Man’ Atomic Bomb,” The Atomic Heritage Foundation, accessed February 25, 2019,

[7] Stober, “No Experience Necessary,” 59.

[8] Ibid.

[9] Ibid.

[10] Ibid., 60.

[11] Ibid., 60-1

[12] “Little Boy and Fat Man,” The Atomic Heritage Foundation, published July 23, 2014,

[13] 1 kg of uranium-235 will have an approximate yield of 20,000 tons of TNT. Please see The Los Alamos Primer for more information on the math behind the conversion.

[14] “Nth Country Experiment,” 33-4.

[15] Ibid., 64.

[16] Stober, “No Experience Necessary,” 61.

[17] F.S. Eby and L.S. Germain, “Critiques of the Nth Country Weapons Design,” (Report, Livermore, California, 1967,

[18] Ibid., 23.

[19] Ibid., 24.

[20] Ibid., 22.

[21] Alex Wellerstein, “Re-examining the The Nth Country Experiment (1967),” The Nuclear Secrecy Blog, published January 4, 2012.

[22] Stober, “No Experience Necessary,” 62.

[23] Matthew Bunn, Martin B. Malin, Nickolas Roth, William H. Tobey, “Preventing Nuclear Terrorism,” Project on Managing the Atom (Report, Cambridge, 2016), ii.

[24] Ibid., 63.

[25] Wellerstein, “Re-examining the The Nth Country Experiment (1967).”

[26] Peter D. Zimmerman and Jeffrey G. Lewis, “The Bomb in the Backyard,” Foreign Policy, no. 57 (2006): 37.

[27] Ibid., 36.

[28] Ibid.

[29] Wellerstein, “Re-examining the The Nth Country Experiment (1967).”

[30] John Mueller, “Think Again: Nuclear Weapons,” Foreign Policy, published December 18, 2009,

[31] William Burr, “1960s Nth Country Experiment,” The National Security Archives, published July 1, 2003,