Nuclear Attribution and Hot Cognition

The challenges of identifying the perpetrators of a nuclear attack on the United States and communicating that information to senior leadership were considered in a 2009 workshop sponsored by the Office of the Director of National Intelligence. A declassified report on the workshop was released last week in heavily redacted form. See “Transforming Nuclear Attribution: Culture, Community, and Change (SHARP 2009)” (redacted), Office of the Director of National Intelligence, July 2009.

One of the challenges is that the task may be impossible. “The outcome from the assessment of all the evidence and sources may be that a definitive answer is not achievable.”

In the best of cases, “There will almost certainly be a disconnect between the speed at which the national leadership must respond to the policy/political environment and the slower pace at which forensic evidence, technical analysis, and law enforcement investigations can proceed. This gives rise to an anchoring problem (i.e. a tendency to anchor on the usual suspects in attributing responsibility for an event).”

“Given the magnitude of the likely national response to any substantial WMD event, those involved in the attribution process need to be cautious of leaping to conclusions ahead of the evidence.”

The report considers the problem of “hot cognition,” referring to analysis that is performed under conditions of emotional agitation or distress.

“Hot cognition has an immense potential for distorting our perceptions of the environment and how we interpret information. It leads us to more extreme judgments of information, perhaps far beyond what they warrant. And it may lead us to fill in the gaps of missing or ambiguous information with emotional filler that could seriously distort our assessments.”

The workshop was conducted as part of the ODNI Summer Hard Problem (SHARP) program. In the roughly 50% of the resulting report that was not redacted by ODNI and the Department of Energy, there are a number of passages of interest concerning the psychology of intelligence analysis, and other topics. For example:

*  “Resolving an information need is not just about finding a ‘nugget.’ Information must be actively incorporated into the mind. New information has to be assimilated into a person’s preexisting context or state of information about the world. ‘Meaning construction’ takes place when this new information can connect with what is already understood. Our ability to absorb new information is limited. People selectively attend to new information that connects, and may be oblivious to the rest.”

*  Assembling an “all-star” team of outstanding intelligence analysts to tackle the attribution problem may not be the right approach. “It has been observed in a number of professional level sporting events that all-star teams — that is, teams created by joining the most exceptional players from across the league — rarely produce the best team overall. While their members have exceptional skills and are tremendous atheletes individually, these all-star teams typically do not perform as well as expected, nor do individual all-stars perform as well as they performed on their originating team.”

*  “In the case of a nuclear event, it is likely that individual private citizens will have images stored on cell phones or digital cameras that could help [resolve] the attribution question. [Word deleted – AFTAC?] should make arrangements in advance of any actual emergency that would give the public a way to send information to government servers for analysis.”

*  “For every one casualty actually caused by a [WMD] event, as many as fifty other individuals may descend upon local medical facilities presenting with psychosomatic symptoms.”

*  “Current limits to information sharing exist for good reasons, including the need to protect sources, the need to avoid tainting legal prosecution, and the need to protect rights to privacy. These reasons will remain important in a nuclear emergency, but cannot be allowed to impede the higher priority of protecting thousands or millions of human lives…. We must prepare IT tools and approaches now, that when activated for a nuclear emergency, allow relevant players to share knowledge at the speed of technology, not the speed of bureaucracy.”

*  “Having examined the range of capabilities that the US Government will bring to the issue of nuclear attribution, we conclude that IC, LE, and TNF [intelligence community, law enforcement, and technical nuclear forensics] capabilities, as currently configured, are likely to result in eventual success. By this we mean that we are confident that these efforts would eventually result in identification of those who mounted and sponsored any nuclear-related attack on the US or engaged in related activities. We are far less confident that as currently configured these agencies will be able to deliver meaningful, rapid success.”


Feasibility of a Low-Yield Gun-Type Terrorist Fission Bomb


Edward Friedman and Roger Lewis’s essay “A Scenario for Jihadist Nuclear Revenge,” published in the Spring 2014 edition of the Public Interest Report, is a sobering reminder of both the possibility of a terrorist nuclear attack based on stolen highly-enriched uranium and the depressing level of public ignorance of such threats. Articles exploring the issue of terrorists or rogue sub-national actors acquiring and using a nuclear weapon or perpetrating some other type of nuclear-themed attack have a long history and have addressed a number of scenarios, including a full-scale program to produce a weapon from scratch, use of stolen reactor-grade plutonium, an attack with a radiological dispersal device, and the vulnerability of research reactors.[5]Equally vigorous are discussions of countermeasures such as detecting warheads and searching for neutron activity due to fissile materials hidden inside cargo containers. An excellent summary analysis of the prospects for a terrorist-built nuclear weapon was prepared almost three decades ago by Carson Mark, Theodore Taylor, Eugene Eyster, William Maraman and Jacob Wechsler, who laid out a daunting list of materials, equipment, expertise and material-processing operations that would be required to fabricate what the authors describe as a “crude” nuclear weapon – a gun or implosion-type device similar to Little Boy or Fat Man. The authors estimated that such a weapon might weigh on the order of a ton or more and have a yield of some 10 kilotons. Perpetrators would face a serious menu of radiological and toxicological hazards involved in processing fissile materials. For example, both uranium (U) and plutonium (Pu) are chemically toxic; also, U can ignite spontaneously in air and Pu tends to accumulate in bones and kidneys. Of course, longer-term health effects might be of little concern to a group of suicidal terrorists.

While the difficulties of such a project might provide reassurance that such an effort has a low probability of being brought to fruition, we might ask if nuclear-armed terrorists along the lines envisioned by Friedman and Lewis would be willing to settle for a relatively low-yield device to achieve their ends. A bomb with a yield of 10 percent of that of Little Boy would still create a devastating blast, leave behind a radiological mess, and generate no small amount of social and economic upheaval. Such a yield would be small change to professional weapons engineers, but the distinction between one kiloton and 15 kilotons might largely be lost on political figures and the public in the aftermath of such an event. Timothy McVeigh’s 1995 Oklahoma City truck bomb used about 2.5 tons of explosive; a one-kiloton detonation would represent some 400 such explosions and make a very powerful statement.

Motivated by Friedman and Lewis’s scenario, I consider the feasibility of an extremely crude gun-type U-235 device configured to be transported in a pickup truck or similar light vehicle. My concern is not with the difficulties perpetrators might face in acquiring fissile material and clandestinely preparing their device, but rather with the results they might achieve if they can do so. The results reported here are based on the basic physics of fission weapons as laid out in a series of pedagogical papers that I have published elsewhere. The essential configuration and expected yield of the device proposed is described in the following section; technical details of the physics computations are gathered in the Appendix.

A Crude Gun-Type Fission Bomb

The bare critical mass of pure U-235 is about 46 kg; this can be significantly lowered by provision of a surrounding tamper. I frame the design of a putative terrorist bomb by assuming that perpetrators have available 40 kg of pure U-235 to be packaged into a device with a length on the order of 2-3 meters and a total estimated weight of 450 kg (1000 pounds), of which 200 kg is budgeted for tamper material. The 40-kg core is subcritical, and the uranium need not be divided up into target and projectile pieces as in the Friedman-Lewis scenario, although the design suggested here could easily be modified to accommodate such an arrangement.

As sketched below, I assume that the uranium is formed into a cylindrical slug of diameter and length Lcore. The core and a plug of tamper material are to be propelled down an artillery tube into a cylindrical tamper case such that the core will be located in the middle of the case once assembly is complete; the assembled core-plus-tamper is assumed to be of diameter and length Ltamp. The choice of tamper material is a crucial consideration; it can seriously affect the predicted yield. In the case of Little Boy, readily-available tungsten-carbide (WC) was employed. Beryllium oxide (BeO) has more desirable neutron-reflective properties, but is expensive and its dust is carcinogenic; more importantly, an effort to acquire hundreds of kilograms of it is likely to bring unwanted attention. I report results for both WC and BeO tampers.

Figure 1: Sketch of a cylindrical tamper case and core/tamper-plug projectile assembly. A 40-kg U-235 core of normal density will have Lcore = 14 cm.

Adopted parameters and calculated results are gathered in Table 1. Technical details are described in the Appendix; the last line of the table gives estimated yields in kilotons. To estimate these yields I used a FORTRAN version of an algorithm which I developed to simulate the detonation of a spherical core-plus-tamper assembly (see the numerical simulation paper cited in footnote 10). A spherical assembly will no doubt give somewhat different results in detail from the cylindrical geometry envisioned here, but as the program returns an estimated yield for a simulation of Little Boy in good accord with the estimated actual yield of that device, we can have some confidence that the results given here should be sensible.

For both configurations in Table 1, the sum of the core, tamper, and artillery-tube masses is about 315 kg (700 lb). With allowance for a breech to close off the rear end of the tube, neutron initiators, detonator electronics, propelling chemical explosives and an enclosing case (which need not be robust if the weapon is not to be lifted), it appears entirely feasible to assemble the entire device with a total weight on the order of 1,000 pounds. Beryllium oxide is clearly preferable as the tamper material, but even with a tungsten-carbide tamper the yield is about 10 percent of that of Little Boy. In open terrain a 2-kiloton ground-burst creates a 5-psi overpressure out to a radius of about one-third of a mile; such an overpressure is quite sufficient to destroy wood-frame houses.

In summary, the sort of vehicle-deliverable makeshift gun-type fission weapon envisioned by Friedman and Lewis appears to be a very plausible prospect; yields on the order of a few kilotons are not out of reach. In view of the fact that all of the calculations in this paper are based on open information, there are sure to be nuances in the physics and particularly the engineering involved that would make realization of such a device more complex than is implied here. But this exercise nevertheless serves as a cautionary tale to emphasize the need for all nuclear powers to rigorously secure and guard their stockpiles of fissile material.

Technical Appendix

Refer to Table 1 and the figure above. A 40-kg U-235 core of normal density (18.71 gr cm-3) will have Lcore = 13.96 cm. The first three lines of Table 1 give adopted atomic weights, densities, and elastic-scattering cross sections for each tamper material. The next two lines give the tamper size and plug mass, and the sixth line the total length of the core-plus-plug bullet.

To estimate the yield of the proposed device I assumed for sake of simplicity that the core is spherical (radius ~ 8 cm) and surrounded by a snugly-fitting 200-kg tamper. Each fission was assumed to liberate 180 MeV of energy and secondary neutrons of average kinetic energy 2 MeV. The number of initiator neutrons was assumed to be 100, radiation pressure was assumed to dominate over gas pressure in the exploding core, and the average number of neutrons per fission was taken to be n =  2.637.

Lines 7 and 8 in Table 1 refer to two important considerations in bomb design: the speed with which the core seats into the tamper and the propellant pressure required to achieve this speed. The core material will inevitably contain some U-238, which, because of its high spontaneous fission rate (~ 7 per kg per second), means that there will be some probability for premature initiation of the chain reaction while the core and tamper are being assembled. (There is no danger of pre-detonation before seating as 40 kg is less than the “bare” critical mass of U-235. The danger during seating arises from the fact that the tamper lowers the critical mass.) The key to minimizing this probability lies in maximizing the assembly speed. If our 40-kg core contains 10 percent by mass U-238, the pre-detonation probability can be kept to under 10 percent if the time during which the core is in a supercritical state during assembly is held to no more than four milliseconds (see the pre-detonation paper cited in footnote 10). The seventh line of Table 1 shows corresponding assembly speeds based on this time constraint and the core-plug lengths in the preceding line. These speed demands are very gentle in comparison to the assembly speed employed in Little Boy, which was about 300 m s-1.

To achieve the assembly speed I assume that (as in Little Boy), the core-plus-plug is propelled along a tube by detonation of a conventional explosive adjacent to the rear end of the tamper plug in the tail of the weapon. To estimate the maximum pressure required, I assumed that the propulsion is provided by the adiabatic expansion (in which no heat is gained or lost) of the detonated explosive. Adiabatic expansion of gas to propel a projectile confined to a tube has been extensively studied; an expression appearing in Rohrbach et. al. can be used to estimate the initial pressure required given the cross-sectional area of the tube, the mass of the projectile, the length of the tube, a value for the adiabatic exponent   _gand the assembly speed to be achieved. This pressure also depends on the initial volume of the detonated explosive; for this I adopted a value of 0.004 m3, about the volume of the core-plug assemblies. The eighth line of Table 1 shows the estimated necessary initial pressures (neglecting any friction between the projectile and the tube) for a travel length of 1.5 meters for g = 1.4; this value of  is characteristic of a diatomic gas. These pressures are very modest, and would set no undue demands on the tube material. Stainless steel, for example, has an ultimate strength of ~ 500 MPa (~75,000 psi); such a tube of inner diameter 7 cm, thickness 1 cm, and length 2 meters would have a mass of about 75 kg. This would bring the sum of the core, tamper, and tube masses to ~ 315 kg (700 lb).

A final technical consideration is the so-called fizzle yield that this makeshift weapon might achieve, that is, its yield if the chain reaction should begin at the moment when the core achieves first criticality. As described by von Hippel and Lyman in Mark (footnote 3), the fizzle yield as a fraction of the nominal design yield can be estimated from the expression Yfizzle/Ynominal ~ (2t F/a tO)3/2, where t  is the average time that a neutron will travel before causing a fission, F is the natural logarithm of the number of fissions that have occurred when the nuclear chain reaction proper can be considered to have begun, a is a parameter in the exponential growth rate of the reaction set by the masses and sizes of the core and tamper, and tO is the time required to complete the core assembly. As described by Mark, t ~ 10-8 sec and F ~ 45. For the design posited here, a~ 0.32 for the WC tamper and ~ 0.47 for the BeO tamper; see Reed (2009) in footnote 10 or Sect. 2.3 of the last reference in footnote 10 regarding the computation of a. Taking tO = 0.004 sec gives Yfizzle/Ynominal ~ 1.9 x 10-5 for the WC tamper and 1.0 x 10-5 for the BeO tamper. With nominal yields of 1.4 and 4.9 kt, the estimated fizzle yields are only ~ 27 and 50 kilograms equivalent. While the perpetrators of such a device might be willing risk such a low yield in view of the low pre-detonation probability involved, they would be well-advised to increase the assembly speed as much as possible.

Table 1: Adopted and calculated parameters for a simple gun-type fission weapon, assuming a 40-kg core of U-235.

*Fission-spectrum averaged elastic-scattering cross-sections adopted from Korea Atomic Energy Research Institute Table of Nuclides,

Edward A. Friedman & Roger K. Lewis, “A Scenario for Jihadist Nuclear Revenge,” Federation of American Scientists Public Interest Report 67 (2) (Spring 2014).

Robert Harney, Gerald Brown, Matthew Carlyle, Eric Skroch & Kevin Wood, “Anatomy of a Project to Produce a First Nuclear Weapon,” Science and Global Security 14 (2006): 2-3, 163-182.

J. Carson Mark, “Explosive Properties of Reactor-Grade Plutonium,” Science and Global Security 4 (1993): 1, 111-128.

J. Magill, D. Hamilton, K. Lützenkirchen, M. Tufan, G. Tamborini, W. Wagner, V. Berthou & A. von Zweidorf, “Consequences of a Radiological Dispersal Event with Nuclear and Radioactive Sources,” Science and Global Security 15 (2007): 2, 107-132.

Steve Fetter, Valery A. Frolov, Marvin Miller, Robert Mozley, Oleg F. Prilutsky, Stanislav N. Rodinov & Roald Z. Sagdeev, “Detecting nuclear warheads,” Science and Global Security 1 (1990): 3-4, 225-253.

J. I. Katz, “Detection of Neutron Sources in Cargo Containers,” Science and Global Security 14 (2006): 2-3, 145-149.

J. Carson Mark, Theodore Taylor, Eugene Eyster, William Maraman & Jacob Wechsler, “Can Terrorists Build Nuclear Weapons?” Paper Prepared for the International Task Force on the Prevention of Nuclear Terrorism. Nuclear Control Institute, Washington, DC (1986). Available at

Cristoph Wirz & Emmanuel Egger, “Use of nuclear and radiological weapons by terrorists?” International Review of the Red Cross 87 (2005): 859, 497-510.

B. Cameron Reed, “Arthur Compton’s 1941 Report on explosive fission of U-235: A look at the physics.” American Journal of Physics 75 (2007): 12, 1065-1072; “A brief primer on tamped fission-bomb cores.” American Journal of Physics 77 (2009): 8, 730-733; “Predetonation probability of a fission-bomb core.” American Journal of Physics 78 (2010): 8, 804-808; “Student-level numerical simulation of conditions inside an exploding fission-bomb core.” Natural Science 2 (2010): 3, 139-144; “Fission fizzles: Estimating the yield of a predetonated nuclear weapon.” American Journal of Physics, 79 (2011): 7, 769-773; The Physics of the Manhattan Project (Heidelberg, Springer-Verlag, 2010).

Z. J. Rohrbach, T. R. Buresh & M. J. Madsen, “Modeling the exit velocity of a compressed air cannon,” American Journal of Physics 80 (2012): 1, 24-26.

Cameron Reed is the Charles A. Dana Professor of Physics at Alma College, where he teaches courses ranging from first-year algebra-based mechanics to senior-level quantum mechanics. He received his Ph.D. in Physics from the University of Waterloo (Canada). His research has included both optical photometry of intrinsically bright stars in our Milky Way galaxy, and the history of the Manhattan Project. His book The History and Science of the Manhattan Project was recently published by Springer.

A Scenario for Jihadist Nuclear Revenge

The Greatest Threat

The weapon was ready, a simple fission device similar to the bomb that destroyed Hiroshima. It had been finally assembled in a rented storage space on the outskirts of Las Vegas. Gulbuddin Hekmatyar had spent years quietly contemplating while meticulously planning this diabolical, logistically challenging mission. Among other things, the plot necessitated recruiting and directing a number of operatives, some technically skilled, located in several countries. All were individuals devoted to his cause and committed to the Jihadist goal of detonating a nuclear bomb in an American city. He chose Las Vegas because the city epitomized western decadence.

The bomb’s essential component – 140 pounds of highly enriched uranium (HEU) – had been stolen or secretly purchased, bit-by-bit, mostly from Pakistan, but also from India, North Korea, Russia, Ukraine and Kazakhstan. It took years to collect, hide and safeguard all the necessary HEU in northwest Pakistan. From there, small pieces of the fissile material and some structural bomb components, hidden and well shielded in multiple shipments, were transported to U.S. ports or border crossings, and eventually to Las Vegas and the rented storage space. With all the components covertly acquired or fabricated by dedicated Jihadists in Pakistan and the United States, the relatively simple bomb finally could be assembled. It was comprised of a long, large-bore, artillery-type barrel; a heavy-duty breech; and enough chemical explosive to propel an appropriately shaped, 70-pound HEU piece through the barrel at very high velocity into another appropriately shaped, 70-pound piece of HEU affixed to the end of the barrel. Slamming together these two sub-critical masses would create the critical mass needed for an explosive chain reaction.

It was time. The plan of attack was straightforward and foolproof. Weighing under a ton and less than a dozen feet long, the weapon in its lead-lined crate fit easily inside a small, rented truck. A lone, suicidal operative drove and parked the truck near the Strip, then activated the electronic device triggering the detonator that set off the chemical explosion. The HEU bullet accelerated through the barrel and merged almost instantaneously with the target HEU. Within microseconds, the critical mass exploded, releasing kilotons of energy, a blinding, rapidly expanding ball of light, heat and deadly radiation. A shock wave propagated through the atmosphere, flattening almost every building within a half-mile of the detonation point. Tens of thousands were dead or injured. Las Vegas was in ruins. The threat had become reality.

When President Obama declared in 2009 that “nuclear terrorism is the most immediate and extreme threat to global security,” it was scarcely noticed. Yet when questionable sources announced that the Mayan Calendar predicted the end of the world in 2012, media and public attention was astonishing. The apocalyptic prediction arising from myth took hold, while a warning of potential catastrophe based in reality, put forth by Barack Obama in Prague, passed us by. Supernatural doomsday scenarios readily gain traction in our public discourse, but threats to our civilization from proven nuclear dangers elude us.

The public and press largely ignored other sobering news in 2010 when Wikileaks revealed that a 2009 cable from the U.S. Ambassador to Pakistan, Anne W. Patterson, warned that “our major concern is not having an Islamic militant steal an entire weapon, but rather the chance someone working in government of Pakistan facilities could gradually smuggle out enough enriched uranium to eventually make a weapon.”

By raising the specter of nuclear terrorism, the Wikileaks revelation gave concrete urgency to President Obama’s abstract concerns. But few paid attention, in part because few understand nuclear weapons risks and realities.

Passage of time and reluctance to think the unthinkable have generated complacency. No nuclear weapon has been used aggressively since the August 9, 1945, attack on Nagasaki. Despite the existence of vast numbers of nuclear weapons, Americans expect nuclear restraint because they believe Mutually Assured Destruction (MAD) is a reliable deterrent. MAD presumably ensures that a country first using nuclear weapons will be wiped out by a retaliatory blow. But if a non-state entity were to perpetrate a nuclear weapons attack, at whom and where would retaliation be directed? An act of nuclear terrorism would obviate 69 years of stability engendered by MAD, an appropriately chosen topic of satirical banter in the film “Dr. Strangelove,” yet a grim reality.

Obama’s words that nuclear terrorism is an “immediate and extreme threat” are not an exaggeration. Terrorists now have new opportunities to covertly fabricate nuclear weapons on their own, and the threat is compounded by the potential anonymity of the attackers.

The cable allegedly sent by Ambassador Patterson leads to several questions: what is enriched uranium and how available is it? How is it used in a bomb, and who could use it? Where and how could it be delivered? What would be its effect? And perhaps the first and biggest question – why would terrorists aspire to manufacture, deploy, and detonate a nuclear weapon?

The 9/11 attacks marked the beginning of a “Terrorist Era” with a capital T. That triple attack profoundly disturbed, shocked and injured the nation, psychologically as well as physically. It suggested that a goal of certain terrorists is the destruction of the United States and its allies, and nuclear capability would make that goal attainable.

In 1998, Osama bin Laden declared that it was his Islamic duty to acquire weapons of mass destruction. Because Islam deplores killing women and children, religious justification was sought for such weapons. In 2003, three Saudi clerics associated with Al Qaeda provided justification in a fatwa that stated:“One kills in a good manner only when one can.”

Thus the “why” can be revenge for the deaths of Muslim civilians or Osama bin Laden, Jihadist punishment of a decadent, anti-Islamic civilization, or retribution for western support of Israel. Still, the easily stated words “nuclear revenge” are not readily internalized. Unlike “tsunami” or “colon cancer,” the term “nuclear terrorism” does not usually evoke a visceral reaction. Such a response is unlikely until people intellectually and emotionally comprehend the potential threat of rogue nuclear weapons. And comprehension requires some understanding of the bomb itself.

In addition to understanding the weapon, people also must grasp the feasibility and consequences of an act of nuclear terrorism. A credible scenario can provide this. In his book, The Second Nuclear Age, Yale Professor Paul Bracken argues that war games based upon scenarios involving nuclear weapons played a significant role in clarifying and shaping strategic thinking during the height of the Cold War. “Scenarios set the stage for the game’s interactions,” he wrote. “Scenarios…are hypothetical plot outlines of plausible future developments. They are not forecasts or predictions…”

The Pentagon hypothesizes terrorist scenarios, but the public does not. Our opening Las Vegas scenario attempts to focus the reader’s mind on the real potential for nuclear catastrophe.

But first the Bomb.

The Bomb

Tremendous energy can be released when the nucleus or inner core of an atom undergoes a transformation. For uranium, the energy releasing transformation is the splitting apart or fission of the nucleus, producing various combinations of lighter atoms such as barium and krypton as “fission products.”

Uranium can exist in different forms or “isotopes,” but all isotopes of uranium contain 92 positively charged protons in the nucleus with 92 negatively charged electrons dancing around the nucleus. These charged particles determine uranium’s chemical properties. However, the uranium nucleus also can contain varying numbers of uncharged neutrons. Somewhat greater than ninety-nine percent of uranium found in nature – uranium isotope 238 – has 146 neutrons in the nucleus. Thus the isotope’s 92 protons and 146 neutrons account for its atomic weight of 238.

Uranium-235, with only 143 neutrons, is the dangerous isotope –because it tends to easily fission if it absorbs a neutron. The fissioning of U-235 powered the bomb that exploded over Hiroshima. Fortunately this isotope is very rare; less than one percent of all forms of uranium found in the earth consist of U-235. Uranium can create an explosive chain reaction—only if U-235 is in highly concentrated form, which is extremely difficult to obtain as described later. But first we explain the basics of nuclear bomb design.

While modern hydrogen-plutonium bombs use a fission-fusion reaction to create the most powerful explosions (which can also use highly enriched uranium instead of plutonium to start the fission reaction), the easiest first-generation atomic bomb to construct entails only fission of highly enriched uranium (HEU), with a concentration of typically 80 percent or greater in the fissile isotope U-235. The Hiroshima bomb was essentially an artillery gun in which chemical explosives fired one 70-pound piece of HEU into another 70-pound piece of HEU, with an average enrichment of 80 percent U-235. The design was so simple that J. Robert Oppenheimer, scientific director of Los Alamos, decided that the prototype did not need to be tested. (One of the reasons not to do a proof test was due to the very limited HEU available during the Second World War.) Indeed the first test of this gun-type bomb occurred when it exploded with such devastating effect at 1,900 feet above Hiroshima on August 6, 1945, killing nearly 100,000 people. Weapons developers at Los Alamos avoided using the word bomb, instead giving euphemistic names such as gadget to their creations. The Hiroshima weapon was inaptly named “Little Boy.”

During the Manhattan Project, the great challenge was collecting and concentrating fissionable U-235. Because U-238 and U-235 have identical chemical properties, separation can only be achieved by exploiting the slight difference in mass arising from one isotope having three more neutrons than the other. One method of separation for enrichment purposes was developed using electric and magnetic forces in a modified form of the cyclotron called the calutron. A second method involved creating a compound in gaseous form of uranium and fluorine, then passing the gas through successive microscopic filters making use of the principle of diffusion.

After the Second World War, improved centrifuges emerged as a more efficient method for enriching uranium. A Dutch company, which became part of the Urenco consortium that also includes Germany and the United Kingdom, was an early adopter of the improved centrifuge technology. Commercial production of enriched uranium for nuclear power plants became a worldwide activity. A. Q. Khan, a Pakistani metallurgist working at the Dutch facility, stole the centrifuge design drawings and brought the technology to Pakistan by the mid-1970s. Throughout the 1980s to the early 2000s, he then sold enrichment equipment to Iran, North Korea, and Libya, and had a vast network of suppliers in at least a dozen other countries.

Having used these centrifuge designs to enable large-scale production of enriched uranium, today Pakistan is rapidly achieving status as having one of the largest and fastest growing stockpile of nuclear weapons after the United States, Russia and China. While some nuclear-armed countries are reducing their weapons count, Pakistan has a vigorous program of nuclear weapons development. Its stockpile is thought to have 100 to 200 nuclear weapons.

Production of nuclear weapons is of great concern, but production of highly enriched uranium is even more worrisome. Although a nuclear bomb might be stolen from a country’s stockpile, it would have anti-activation safeguards (such as access codes and electronic locks) that a thief would find very difficult to penetrate. However, highly enriched uranium for a terrorist is like flour for a baker. Each material is simply one ingredient from which something much more impressive can be concocted. Pakistan continues to produce weapons-grade U-235 at a more intense rate than any country in the world. It is reliably estimated that they have stockpiled thousands of pounds of enriched U-235 that could lead to the fabrication of dozens of new nuclear weapons.

Eyes continue to focus on Pakistan as a potential source of nuclear bomb material. Yet unsecured highly enriched uranium elsewhere has been a worry for many years. Of particular concern have been the vast amounts of weapons-grade uranium that were left relatively unguarded in Russia, Ukraine, Belarus and Kazakhstan after the break-up of the Soviet Union. These have been potential access points for terrorists. Hundreds of secret bombs could have been fabricated.

Prior to 9/11, we could not imagine rogue, non-state entities having the ability to construct a nuclear weapon. Yet vast numbers of reports, documents, drawings and photographs from the Manhattan Project have been declassified. The Little Boy development work at Los Alamos is available for the world to examine. Following 9/11, government agents visited war museums in the United States to remove publicly displayed, artillery-gun components replicating Little Boy. But the proverbial horse was already out of the barn.

Conceivably, determined terrorists could acquire 140 pounds of weapons-grade uranium-235, the amount used in the Hiroshima bomb. While a person weighing 140 pounds would fully occupy a seat at a dinner table, 140 pounds of uranium, the densest of all naturally occurring elements, is less than the size of a football.With the material in hand, processing and fabrication steps for constructing a weapon rely on well-established and widely known metallurgical and manufacturing techniques. Who then might do this? Where would construction take place? And under whose leadership?

A Return to Our Scenario

The villain of our hypothetical scenario is the leader of a militant group located in the tribal territory of Northwestern Pakistan.Skeptics may doubt that our protagonist, Gulbuddin Hekmatyar, could obtain the equipment and expertise needed for weaponization of highly enriched uranium. While not on Abercrombie and Kent tourist itineraries, the frontier region of Northwest Pakistan is the site of the colorful village of Darra Adam Khel. This unique tribal enclave near the Khyber Pass, just 20 miles South of Peshawar, has been manufacturing copies of small weapons since the late 19th century using basic lathes, drills and other readily available tools. During the Mujahideen struggle with the Soviet Union in the 1980s, the United States was not unhappy to see their production of anti-aircraft weapons. Darra Adam Khel is a major source of weapons in the South Asia region. The potential to fabricate the components for a Little Boy clone clearly exists there.

Osama bin Laden and other Al Qaeda leaders have been eliminated, but other militant jihadists have the capacity and the will to engage in development and use of a nuclear weapon. We chose Gulbuddin for our scenario because he was involved with the 1993 bombing of the World Trade Center, he controls a formidable belligerent organization with the resources to actualize this sequence of events, and he is not part of Al Qaeda or the Taliban. An Afghan student of engineering at Kabul University in the early 1970s, he organized at that time what was probably the first militant Islamic organization in Afghanistan. His student group shot at and threw acid at women in Kabul who were wearing mini-skirts. In a confrontation in 1972 with the rival Maoist political group on campus, Gulbuddin shot and killed their leader. After being imprisoned, he escaped and was given refuge by Islamic fundamentalist elements of the Pakistani government in 1975. Some officials in Pakistan, which had border region disputes with Afghanistan since its founding in 1947, saw in Gulbuddin a potential ally in future conflicts with Afghanistan.

In the early 1980s, when the United States started channeling large amounts of funds and weapons to the Mujahideen for their struggle with the Soviet Union, the Pakistan intelligence agency, the ISI, provided Gulbuddin with a substantial portion of those resources. This enabled the charismatic fanatic to establish a formidable organization known as Hezb-e-Islami.

During the mid-1980s Gulbuddin was instrumental in organizing terrorist training camps in Afghanistan to which “Arab” fighters were invited. Gulbuddin welcomed Osama bin Laden, who first came to Afghanistan to fight the infidel Soviets.

Gulbuddin and his organization did not pursue the Soviets as fully as they might have, since he was waiting to use his fighting potential in a putsch to take over Afghanistan following the departure of the Red Army. He did indeed engage in a civil war for control of Afghanistan that began in 1992. But after achieving the position of Prime Minister of Afghanistan in 1993, Gulbuddin was eventually defeated by the Northern Alliance led by Massoud. With the fall of Gulbuddin, the ISI of Pakistan threw their support in 1994 to the newly organized Taliban.

Few are aware of Gulbuddin’s contacts with the perpetrators of the 1993 bombing of the World Trade Center. Those connections and his meetings with the Blind Sheik were revealed in the trials of the World Trade Center bombers held in New York City. It is too often said that the weakening of Al Qaeda ensures the safety of the U.S. from terrorist attack. However, the 1993 bombing of the World Trade Center involved non-Al Qaeda players and leadership figures from that event are still active. This is just one example of a non-Al Qaeda terrorist group that might become a perpetrator of nuclear terrorism.

Today, Gulbuddin maintains a militant presence in the frontier region and has the organizational ability to engage in ambitious terrorist actions. Gulbuddin is neither a tribal “war lord” nor a narrowly educated religious militant. He is a sophisticated intellectual and political leader who completed two years of engineering education. There is every indication that he is as vicious a proponent of terrorism as was Osama bin Laden.

Nothing suggests that Gulbuddin is, in fact, plotting a nuclear attack on an American city. But he is the key player in our scenario because he has the knowledge, resources and frame of mind for implementing such an attack. If there is one such person and group, there are likely others.

The fissile materials available in Pakistan can be fabricated in the Khyber region to serve as components for a gun-type Hiroshima bomb. The amount of radiation released by the highly enriched uranium can be easily shielded and thus easily elude detection at border crossings or from aerial drones. Since suicidal jihadists probably would assemble it, many of the features that were built into Little Boy 69 years ago to ensure safe delivery and controlled detonation could be ignored.

While this scenario has focused on terrorism originating in the Afghanistan-Pakistan border area, other scenarios might be equally plausible. In May 2011, the Belfer Center for Science and International Affairs at Harvard University issued a report entitled, “The U.S.-Russia Joint Assessment on Nuclear Terrorism.” The study group analyzed possible threats from three terrorist organizations known to have systematically sought to obtain nuclear weapons: Al Qaeda, groups in the Northern Caucasus, and the Japanese cult group, Aum Shinrikyo. Each could be a suitable scenario protagonist. If Iran began producing tens of pounds of HEU, then the possibility of Hezbollah obtaining that material would also merit consideration.

Additional scenarios can be hypothesized with sociopaths who are American citizens. Two figures stand out as prototypes with the organizational and technical capacity needed for implementation. One is Timothy McVeigh, whose homemade bomb destroyed the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995. The other is Ted Kaczynski, the mathematics genius known as the Unabomber. He perpetrated seventeen explosive attacks killing three people between 1978 and his arrest in 1996.

The first public alert to the possibility of a small group of individuals in the United States building a nuclear bomb to attack society came in the early 1970s from Theodore B. Taylor, a physicist who invented highly efficient, small sized nuclear weapons at Los Alamos National Laboratory. He recognized and was obsessed by the possibility that fissile material could be stolen from commercial facilities that were enriching uranium. The U.S. government had encouraged corporations to process uranium for use in nuclear reactors. In his book, The Curve of Binding Energy, award-winning writer John McPhee documented the story of this eccentric but highly creative physicist. His account was first published in the New Yorker in 1973. Both Taylor and McPhee seemed convinced that a rogue nuclear weapon would detonate somewhere in the United States prior to the beginning of the 21st Century.

The vulnerabilities of enriched uranium supplies in the United States in the 1970’s were astonishing. We assume that such homeland dangers have been ameliorated. Yet we are aghast at the July 28, 2012, spectacle of an 82-year-old nun and two equally unlikely compatriots penetrating the innermost sanctum of the highly enriched uranium facility at Oak Ridge, Tennessee, and spraying the storage building with graffiti. The words they posted said, “Plowshares Please Isaiah.” If such lax security is exposed in Bear Creek Valley, U.S.A., what might be the case in Pakistan, Russia, North Korea or China?

Implications and Actions

A successful act of nuclear terrorism would, in a blinding flash, change the nature of civilization, as we know it. When the consequences of an action are so enormous, perhaps one should pause to reflect upon it, even if scenarios and anecdotes may not be persuasive. But our lives are frequently influenced by low probability events. We wear seat belts. We buy lottery tickets.

Perhaps we should be more proactive in supporting our government’s actions to ameliorate potential risks.  The international community is currently discussing at least three treaties. One is to create uniform legal frameworks for prosecuting terrorists who seek to use nuclear materials, another is to develop uniformly effective security procedures for safeguarding nuclear materials, and the third is the Fissile Material Cut-off Treaty. It is likely that these issues are far more important and could have much greater consequences than some of the actions that might be taken to thwart nuclear weapons development in Iran or North Korea. Nuclear policy priorities need to emphasize non-state weapons proliferation at least at the same level as state-centered weapons proliferation.

In the non-government sector, non-state weapons proliferation also should be as central in public forums, the press, blogs, general discourse and academic discussions as the continuing crises in North Korea and in Iran.

The fact that the Federal Emergency Management Agency (FEMA) and the Department of Homeland Security are engaging in detailed analyses of emergency responses to a Hiroshima-type bomb detonation in central Washington D.C. means that they are taking that possibility seriously. While the Gulbuddin scenario chose Las Vegas as a symbolic target, another team of jihadists might choose to focus on our nation’s capital.

Today, the news media continually reports about the potential for North Korea to attack the United States with nuclear-tipped intercontinental ballistic missiles. But an attack using a crate holding a lead-shielded, twelve-foot long artillery gun, delivered by sea to one of America’s busiest container ports, such as ports in New Jersey, New York, or California, is a more likely mode of attack and would be equally effective and deadly.

“Nuclear Fuel Memos Expose Wary Dance With Pakistan,” New York Times, November 30, 2010.

Nasir Bin Hamad Al-Fahd, “A Treatise on the Legal Status of Using Weapons of Mass Destruction Against Infidels,” 2003, available at

Paul Bracken, “The Second Nuclear Age: Strategy, Danger, and the New Power Politics” (Times Books, 2012)

See FAS’s world nuclear forces chart: /issues/nuclear-weapons/status-world-nuclear-forces/ . This estimate shows that Pakistan has 120-199 nuclear weapons, and the UK and France have about 225-300. But the Pakistani nuclear weapons program is estimated to be growing while the UK and France have stopped producing new nuclear weapons.

See the video at–472252.htm, accessed on May 14, 2014. This YouTube video shows the light manufacturing equipment in Darra Adam Khel with capacity to manufacture a Hiroshima firing unit.

For example, the profile by Michael Crowley, “Our Man in Kabul?” New Republic, March 9, 2010,

Belfer Center study is available at

John McPhee, The Curve of Binding Energy (Farrar, Straus and Giroux, New York, 1973) “He (Ted Taylor) did say he thought it was already too late to prevent the making of a few bombs (by terrorists), here and there, now and then. Society would just have to take that, and go on. None of this was said with the least trace of cynicism or despair.” (p. 196)

Matthew L. Wald and William J. Broad, “Security Questions are Raised by Break-In at a Nuclear Site,” New York Times, August 7, 2012, and see the following website for the image the protestors made:

National Capital Region Key Response Planning Factors for the Aftermath of Nuclear Terrorim – November 2011 – FEMA, Homeland Security and Lawrence Livermore National Laboratory, available at /irp/agency/dhs/fema/ncr.pdf

Edward A. Friedman is Professor Emeritus of Technology Management at Stevens Institute of Technology in Hoboken, N.J. He holds a B.S. in Physics from MIT and a Ph.D. in Physics from Columbia University.  He was director of a USAID program to develop an indigenous college of engineering in Afghanistan (1970-73) when Gulbuddin Hekmatyar was arrested for murder of a political rival at Kabul University. Dr. Friedman was a founder and senior vice president of the Afghanistan Relief Committee (1979-1995). In 2012 he developed and taught a graduate course on Nuclear Weapons in International Relations as an Adjunct Professor at The John C. Whitehead School of Diplomacy and International Relations at Seton Hall University.

Roger K. Lewis is an Architect and Planner. He has been a long-term columnist for the Washington Post’s “Shaping the City.” He is Planning and Preservation Trustee for the National Children’s Museum and President and Director of the Peace Corps Commemorative Foundation. His book, “Architect? A Candid Guide to the Profession” is known as the best basic introduction to the profession. He is Professor Emeritus of Architecture at the University of Maryland School of Architecture.

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