The views and opinions expressed or implied in WBY are those of the authors and should not be construed as carrying the official sanction of the Department of Defense, Air Force, Air Education and Training Command, Air University, or other agencies or departments of the US government or their international equivalents.

Not Your Grandfather’s Nukes

  • Published
  • By Dr. Ian Kurtz

From 1950-1980, the United States (US) nuclear weapons program experienced its share of accidents with 32 on record in the opening decades of the Atomic Era. Practitioners call these events Broken Arrows. Analysts often illuminate the danger of these near misses and the trial-and-error safety environment of the nascent days of atomic weapons. Often, critics use these examples as evidence that nuclear weapons are inherently unsafe and declare that the US must eliminate them, citing the past to argue their point.[1] In reality, it is unfair to compare the weapons of yesteryear to today’s advanced arsenal. As this manuscript will highlight, the US Department of Energy (DOE) (known as the Atomic Energy Commission prior to 1977), transformed the stockpile and built a formidable program combining technical expertise and scientific capabilities to ensure a safe, secure, and reliable nuclear deterrent. Today’s generation of warheads is as safe as any previous, as evidence by zero Broken Arrows in the post-Cold War era. It is a false narrative to compare the weapon designs of the past with the advancements of today. Practitioners in the US nuclear enterprise must learn to be deterrence advocates and should understand that the level of safety and security in today’s weapons is a critical factor in maintaining a robust deterrence posture.


The design of early US nuclear weapons involved keeping the nuclear material from the rest of the weapon until right before detonation.[2] Therefore, if the material, plutonium or uranium, was not installed in the warhead, a nuclear yield was not possible. This was a primary safety practice during the early years of US development.

In the first decade–and-a-half of America’s nuclear program, there were nearly two dozen Broken Arrows. Two that were particularly noteworthy occurred in 1958 and 1961, respectively.[3] The first was the accidental dropping of a nuclear bomb from a B-47 aircraft over Mars Bluff, South Carolina.[4] The bomb landed in a rural area outside of town. The shock wave from the weapon’s high explosives ripped through a resident’s home and barn, generating enough force to create a deep crater on the property and cause various injuries to bystanders. The cause of the accident was the failure of a mounting shackle intended to hold the bomb securely in place.[5] Fortunately, the aircrew had not installed the nuclear material, and a nuclear yield was not possible.

The second incident occurred only three years later. On 24 January 1961, a B-52 aircraft broke up in mid-air over North Carolina, allowing two nuclear gravity bombs to crash to the ground. One of the bombs, a high-yield thermonuclear weapon, narrowly avoided creating a mushroom cloud twenty minutes from downtown Goldsboro. Though six safety switches on the weapon were in place, five were set off by the fall, leaving a single mechanism to prevent the detonation. As described by an Office of Naval Research official, “Only a single switch prevented the (megaton) bomb from detonating and spreading fire and destruction over a wide area.”[6] Consequently, this became the focal point of the new decade: enhance the safety of the US deterrent to prevent these “close calls” from recurring.

Unsurprisingly, concerns over these incidents triggered a safety renaissance within the US nuclear enterprise in the late 1950s and early 1960s, with US military leaders looking to establish a formal program of nuclear weapon safety and policies. They would also have to contend with an evolving security environment as the Eisenhower administration transitioned into its final years amidst the ongoing Cold War. As the USs extended deterrence mission spread throughout Europe, a more prolific footprint for US nuclear forces created even more anxiety for policymakers and raised an obvious question: What if a launch crew unintentionally employed a nuclear weapon? As Soviet premier Nikita Khrushchev warned, “An accidental bomb explosion may well trigger another world war.”[7]


By the end of the 1950s, the scientific community updated weapon designs to utilize a “wooden bomb” concept, with the fissile material permanently housed inside the weapon and therefore inaccessible to personnel. When questions were raised regarding this new technology, Navy Captain William Klee, chaired a committee to examine overlooked safety issues associated with sealed-pit weapons.[8] This committee sparked many others that recommended the development and application of several important new safety features, one of the most significant being environmental sensing devices (ESD).

ESDs are components inserted into the weapon that must sense its operational environment before allowing the arming sequence to commence. As it turned out, this technology served a dual purpose: to prevent accidents and provide a measure of protection against unauthorized use.[9] In addition, the development of the nuclear triad (nuclear-armed submarines, intercontinental ballistic missiles (ICBM), and bomber aircraft) during these years drove significant interest in nuclear safety and released a plethora of advances in both technology and policy. 

Also inspired during this era of safety advances was RAND researcher Fred Iklé. His report, On the Risk of an Accidental or Unauthorized Nuclear Detonation, motivated a turning point in the journey to ensure the utmost safety, security, and reliability of the US stockpile. Iklé introduced the concept of Use Control (UC), positive measures to allow the authorized use and prevent or delay the unauthorized use of nuclear weapons.[10]

A critical thinker, Iklé approached this problem from three perspectives: technical malfunction, human errors, and deliberate unauthorized action. The importance of developing locking mechanisms -- one of his landmark recommendations -- eventually became Permissive Action Link (PAL). PAL, a technology still used today in modern nuclear weapons, consists of a lock or coded device within the warhead that must receive a particular code in order to arm for use. In addition, for maintenance and security workers, Iklé recommended what practitioners today refer to as the two-person concept (TPC) by increasing opportunities to observe a rogue individual who could sabotage or damage a weapon. He reasoned, “…the presence of more than one person seems to decrease substantially the opportunities for an unauthorized act.”[11]

Today, the definition of TPC still closely mirrors Dr. Iklé’s vision from 60 years ago: “Designed to ensure that a lone individual is denied access to nuclear weapons, nuclear weapon systems or critical components, never allowing the opportunity for tampering, damage or an unauthorized act to go undetected.”[12] Furthermore, TPC “…relies on a threat to a high-value asset, implementing the practice of two-person control and preventing human compromise.”[13]

Finally, in order to provide an additional degree of safety in case of a scenario in which gunfire occurs, a concept was instituted called one-point safe. This feature continues to ensure that, if a warhead’s detonator ignited at any single point (e.g., from a bullet or a nearby explosion), there would not be a nuclear detonation.[14]

The Department of Defense (DoD) soon established a standard of safety across the department and institutionalized the DoDs first qualitative safety standards, applicable to all US nuclear weapons which “…assures that atomic weapon systems incorporate the maximum safety consistent with operational requirements.”[15] Now referred to as “surety standards”, they remain largely unchanged after nearly 60 years: 

  1. Positive measures to prevent weapons involved in accidents or incidents or jettisoned weapons from producing a yield;
  2. Positive measures to prevent deliberate arming, launching, firing, or releasing of nuclear weapons except upon execution of emergency war orders or when directed by competent authority;
  3. Positive measures to prevent inadvertent arming, launching, firing, or releasing nuclear weapons in all normal and credible abnormal environments;
  4. Positive measures to ensure adequate security of nuclear weapons.

The advancements of this period include the birth of a new program to enforce these standards of “nuclear surety”. Motivated by then-newly created DoD surety standards, the US Air Force (USAF) led the way in creating an important group dedicated to ensuring the overall safety and security of the stockpile. The Nuclear Weapons System Safety Group(NWSSG) is responsible to “review nuclear weapons systems designs and operations, including CONOPS [concepts of operations] to determine if they meet the DoD Nuclear Weapons System Surety Standards…”[16] Decades of the NWSSG’s oversight has served the USAF well as it maintains two legs of the US nuclear triad. 

This first safety renaissance did much to advance nuclear weapon safety and technology and no doubt enhanced US nuclear readiness, responsiveness, and deterrence -- attributes that the DoD still acknowledge today.[17] However, despite these advances, a pair of highly visible accidents involving US nuclear weapons with international ramifications occurred (which highlighted the need for more extensive safety measures) - first in 1966, then again in 1968.

The Broken Arrow at Palomares, Spain on 17 January 1966 led to a complicated rescue operation after two US military aircraft (one bomber and one tanker) collided during a refueling operation over a sleepy Spanish village. Both aircraft crashed. One of the aircraft’s bombs fell into the sea about five miles off the coast.[18] The plutonium scatter from the dropped weapons posed a serious health hazard for Palomares citizens and cleanup crews alike.[19]

Barely two years later, another incident had wide-ranging effects. On 21 January 1968, a B-52 flying a 24-hour alert mission from Plattsburg Air Force Base, New York, experienced a cabin fire. The fire danger forced the aircrew to ditch the aircraft seven miles from the runway at Thule Air Base, Greenland. The fire consumed both the aircraft and the nuclear weapons it carried, igniting the latter’s conventional explosive. On this occasion, the plutonium scatter was extensive, and representatives of the Danish government closely monitored the nearly 4-month cleanup operation.[20]The Palomares and Thule accidents embarrassed the US on an international scale and had a negative impact on US nuclear deterrence.[21]

Once again, a significant change was in store for the US nuclear enterprise. After the Palomares incident, Carl Walske, the Assistant Secretary of Defense for Atomic Energy, studied the accidents and put forth the Walske Criteria, which were a set of quantitative design criteria assigned to the likelihood of unintended nuclear detonation.[22] This became the gold standard for US nuclear weapons design and inspired a second renaissance of safety advancement.

Walske specified that the possibility of a detonation while the weapon was in a normal environment, such as storage or transportation, would not exceed one in a billion per event over the course of the lifetime of the weapon. Furthermore, the chances of detonation of a nuclear weapon involved in a fire, impact, or any other abnormal environment would not exceed one in a million per accident.[23] These quantitative requirements would significantly reduce the chances of the spread of hazardous material if the high explosives in the weapons detonated in an accident.

How would the designers endeavor to accomplish these impressive requirements? In response to the Walske criteria, Sandia National Laboratories set about equipping new designs with a concept called Enhanced Nuclear Detonation Safety (ENDS). ENDS combines various features within the weapons to ensure a robust measure of redundancy, consisting of a combination of weak/strong link devices, a unique electrical signal generator, and an isolation region protecting the critical electrical components from stray electrical inputs. This concept is described below:


  1. Isolation – Critical arming components necessary for detonation are isolated from their surroundings within a physical barrier known as an exclusion region, which blocks all forms of energy such as power surges or a lightning strike – spurious electrical signals;
  2. Inoperability – During an abnormal environment such as a fire, ENDS incorporates environmental vulnerability into weak links to ensure safety;
  3. Incompatibility – Strong links serve as an electrical combination lock preventing usage until a deliberate action occurs. An operator must enter information in the form of a unique signal. An incompatible pattern will lock the weapon from use.

Other advances such as insensitive high explosives on various warheads complimented this suite of technologies and further reduced the detonation possibility in the event of an accident. These concepts achieved a goal that designers had coveted for years: to ensure that safety features were inherent in the design of US nuclear weapons.


Modern US nuclear weapons are very safe because numerous layers are in place to ensure a continued safe and secure nuclear stockpile. Thanks to today’s advanced designs, the era of the Broken Arrow has ended. The US’ “safe, secure, and reliable” approach to nuclear weapons provides untold contributions to effective deterrence of adversaries and is vital to a vigilant and credible nuclear posture.[24] To that end, the ability to sustain US nuclear expertise facilitates a strong deterrent force. Additionally, the DOE’s stockpile modernization program promises to improve on these already high standards.[25] Therefore, it is disingenuous to argue that past nuclear weapons accidents involving obsolete technology must force the reduction or elimination of US nuclear weapons. 


Ian Kurtz 
Dr. Kurtz is the Academic Director for the Air Force Nuclear College at Kirtland Air Force Base, NM. Ian retired from the USAF after 22 years in 2008, and entered into civilian service in 2010. He earned a doctorate in strategic security from National American University through AFIT’s Academic Partnerships in Nuclear Education program, and his dissertation is entitled, Nuclear Deterrence in University Education. 



[1.] William Perry, My Journey at the Nuclear Brink. (Stanford, CA: Stanford University Press, 2016), xiv.

[2.] Steven Younger, The Bomb: A New History (New York City, NY: Harper-Collins Publishers, 2009), 72.

[3.] Department of Energy, Unclassified Narrative Summaries of Accidents Involving US Nuclear Weapons, 1950-1980 (Homeland Security Digital Library, 1990)

[4.] Eric Schlosser, Command and Control: Nuclear Weapons, the Damascus Accident, and the Illusion of Safety. (New York, NY: The Penguin Press, 2013), 186.

[5.] Defense Threat Reduction Agency, (DTRA) Defense’s Nuclear Agency 1947-1997 (Washington DC: DTRA History Series), 148.

[6.] L. Douglas Keeney, 15 Minutes: General Curtis LeMay and the Countdown to Nuclear Annihilation (New York, NY: St. Martin’s Press, 2011), 246.

[7.] Schlosser, Command and Control, 194.

[8.] Leland Johnson, A History of Exceptional Service in the National Interest (Albuquerque, NM: Sandia National Laboratories, 1997), 72.

[9.] Peter Stein & Peter Feaver, Assuring Control of Nuclear Weapons: The Evolution of Permissive Action Links (Lanham, MD: University Press of America, 1987), 23.

[10.] United States Air Force, Air Force Nuclear Weapons Surety ProgramAir Force Instruction 91-101 (Washington, DC: U.S. Government Printing Office, 2017), 73.

[11.] Fred Iklé, (in collaboration with Aronson & Madansky), On the Risk of an Accidental or Unauthorized Nuclear Detonation. (US Air Force, Project RAND: 1957), vii.

[12.] USAF, Air Force Nuclear Weapons Surety Program, 72.

[13.] Robert Pedersen, Sandia Report: Two-Person Control: A Brief History and Modern Industry Practices (Livermore, CA: Sandia National Laboratories, 2017), 14.

[14.] Shaun Gregory, The Hidden Cost of Deterrence: Nuclear Weapons Accidents (London, UK: Brassey’s UK, 1990), 19-20.

[15.] Johnson, History of Exceptional Service, 88.

[16.] United States Air Force, Nuclear Weapons System Safety Studies, Operational Safety Reviews, and Safety Rules: Air Force Instruction 91-102 (Washington, DC: U.S. Government Printing Office, 2017), 6.

[17.] C. Donald Alston, Deterrence and the ICBM: A Practitioner’s PerspectiveThe Potomac Foundation. (W. Cross, editor, 2016),

[18.] Charles Loeber, Building the Bombs: A History of the Nuclear Weapons Complex (Albuquerque, NM: Sandia National Laboratories, 2002), 153.

[19.] Barbara Moran, The Day We Lost the H-Bomb: Cold War, Hot Nukes, and the Worst Nuclear Weapons Disaster in History (New York, NY: Ballantine Books, 2009), 155.

[20.] Charles Hansen, The Swords of Armageddon (Sunnyvale, CA: Chukelea Publications, 2010), 7:285-287.

[21.] Sandia National Laboratories, Always/Never: The Quest for Safety, Control & Survivability, DVD. (Albuquerque NM: Sandia National Laboratories, 2010), 26:30.

[22.] Johnson, History of Exceptional Service, 146.

[23.] Office of the Deputy Assistant Secretary of Defense for Nuclear Matters, Nuclear Matters Handbook 2020 [Revised], 91.

[24.] Kevin Chilton, Defending the Record on US Nuclear Deterrence. Strategic Studies Quarterly, 12(1), 12-22. Spring 2018.

[25.] Department of Energy, FY21 Stockpile Stewardship and Management Plan, (Washington, DC: U.S. Government Printing Office, 2020), 1-11,


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