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Permissive Action Link

PAL controller

A Permissive Action Link (PAL) is a security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of the nuclear weapon.[1] The United States Department of Defense definition is:

A device included in or attached to a nuclear weapon system to preclude arming and/or launching until the insertion of a prescribed discrete code or combination. It may include equipment and cabling external to the weapon or weapon system to activate components within the weapon or weapon system.

The earliest PALs were little more than locks introduced into the control and firing systems of a nuclear weapon, that would inhibit either the detonation, or the removal of safety features of the weapon. More recent innovations have included encrypted firing parameters, which must be decrypted to properly detonate the warhead, plus anti-tamper systems which intentionally mis-detonate the weapon, destroying it without giving rise to a nuclear explosion.


  • History 1
    • Background 1.1
    • Development and dissemination 1.2
    • Modernization and the present 1.3
  • Features 2
    • Two-man rule 2.1
    • Stronglinks and weaklinks 2.2
    • Critical signal detection 2.3
    • Parameter recognition 2.4
    • Misfire 2.5
    • Limited attempts 2.6
  • Versions 3
  • Usage by other states 4
  • See also 5
  • References 6
  • External links 7



Sandia National Laboratories, 1951. Sandia was instrumental from the beginning in developing PALs.

Permissive Action Links were developed in the United States over a gradual process from the first use of atomic weapons to the early 1960s. Importantly, in 1953 the United States Atomic Energy Commission and the Department of Defense signed the Missiles and Rockets Agreement, which paved the way for the development and implementation of PALs. Certain national laboratories, under the auspices of the AEC, would develop and produce nuclear weapons, while the responsibility for the use and deployment remained with the military. The laboratories were also free to conduct their own research in the field of arms control and security. The thinking behind this was that if the government would ever be interested in such a security device, the research and development of prototypes would already be well advanced. At the beginning of the 1960s, the desire for the usage of such a system grew for both political and technological reasons.

Newer nuclear weapons were simpler in their operation and were produced en masse and were less cumbersome to arm and use than previous designs. Accordingly, new controls were necessary to prevent their unauthorized use. As the Cold War came to a head in the 1960s, the government felt it best not to leave the use of nuclear weapons in the hands of possibly-renegade generals, including the commander of Strategic Air Command (SAC).[2] Without Permissive Action Links, nuclear weapons were effectively under the independent command of a number of generals.

I used to worry about General Power. I used to worry that General Power was not stable. I used to worry about the fact that he had control over so many weapons and weapon systems and could, under certain conditions, launch the force. Back in the days before we had real positive control [i.e., PAL locks], SAC had the power to do a lot of things, and it was in his hands, and he knew it.
General Horace M. Wade, (at that time subordinate of General Power), [3]

In order to protect its NATO allies, the United States had stationed various nuclear weapons overseas; these weapons were thus at least under the partial control of the hosting allied state. This was especially concerning to Congress, as this lack of control was in violation of federal law. Added to this was the fact that some of the allies were considered potentially unstable — particularly West Germany and Turkey.[4] There was considerable concern that in one of these countries the instructions of the civilian leadership of the host country could overrule the military. In addition, the U.S. realized that in the event of war, parts of West Germany would be overwhelmed early on, and nuclear weapons stationed there could fall into the hands of the Soviets.

For a long time the U.S. military resisted the use of PALs. It feared the loss of independence and feared malfunction, which could put warheads out of action in a time of crisis. But the advantages of PALs outweighed the disadvantages: thanks to the PALs weapons were able to be distributed to a greater extent in Europe, so as to prevent a rapid and selective destruction or conquest by the Soviet bloc, while still retaining U.S. control over the farther-flung weapons.[4]

Development and dissemination

The precursors of Permissive Action Links were simple mechanical combination locks that were set into the control systems of nuclear weapons, such as the Minuteman ICBM. There they could perform different functions: some blocked the cavity through which the nuclear materials were shot to create a reaction; other locks blocked circuits; and some simply prevented access to the control panel. For testing, some of these mechanisms were installed during 1959 in weapons stationed in Europe.[5]

The work on PAL prototypes remained at low levels until 1960. Sandia National Laboratories (SNL) successfully created a number of new combination locks that were adaptable to different types of weapons. In the spring of 1961, there were a series of hearings in Congress, where Sandia presented the prototype of a special electro-mechanical lock, which was then known still as a "Proscribed Action Link." The military leadership, however, soon realized that this term had negative connotations for the use of weapons by the officer corps (proscribed = "prohibited"), and changed the meaning of PAL to "Permissive Action Link" (permissive = "allowing / tolerating").

National Security Action Memorandum 160: introduction of PAL to all U.S. nuclear weapons under NATO command

In June 1962, President John F. Kennedy signed the National Security Action Memorandum number 160. This presidential directive ordered the installation of PALs in all U.S. nuclear weapons in Europe. All other U.S. nuclear weapons were excluded at that time. The conversion was completed September 1962 and cost $23 million ($179 million in 2015 dollars[6]).

For the Minuteman ICBM force, the US Air Force's Strategic Air Command worried that in times of need the codes would not be available, so they quietly decided to set them to 00000000. The missile launch checklists included an item confirming this combination until 1977.[7]

The complete conversion to PAL systems was relatively slow. In 1974, U.S. Defense Secretary James Schlesinger found that a variety of tactical nuclear weapons were still not fitted with Permissive Action Links, even though the technology had been available for some time.[8] It took another two years until all the tactical nuclear weapons were fully equipped with PALs. In 1981, almost 20 years after the invention of PALs, just over half of U.S. nuclear weapons were still equipped only with mechanical locks.[4] It took until 1987 until these were completely replaced.

Modernization and the present

Over the years the Permissive Action Links have been continuously maintained and upgraded. In 2002, PALs on older B61 nuclear bombs were replaced and upgraded with new systems to improve reliability and security, as a part of extending the weapons' service life to at least 2025.[9]

Code Management System

1995 saw the development of the Code Management System (CMS). The CMS has simplified the control and logistics for staff and improved the flexibility and speed in deploying and arming weapons. New codes can be used to recode, lock, and manage the weapons, while the secrecy and validity of the possible launch orders is still ensured. In total, CMS consists of fourteen custom products (nine software and five hardware products).[10] The software products were developed by Sandia National Laboratories while the hardware was created by the National Nuclear Security Administration.

The CMS was fully operational for the first time in November 2001. A part of the system, a special cryptographic processor fitted into the weapons in 1997, had a potential Year 2000 problem. By the spring of 2004, all PAL systems were equipped with the Code Management System. It is thus currently the general foundation for future hardware and software improvements to Permissive Action Links.


Permissive Action Links are powered by low-maintenance radioisotope generators. Instead of a conventional battery, these generators produce electricity based on the heat evolved from the radioactive decay of plutonium-238. Although half-life of 238Pu is 87.7 years, these generators have shorter lifespans. This is due to the pressurization of the generator from helium produced by the alpha decay of the plutonium fuel.[11]

PALs are also linked directly or indirectly with a number of security measures, which together form a comprehensive security package. In general, elements of PAL systems are located deep within the nuclear device. This makes it almost impossible to bypass the system.

Bypassing a PAL should be, as one weapons designer graphically put it, about as complex as performing a tonsillectomy while entering the patient from the wrong end.
—Peter D. Zimmerman, nuclear physicist and weapons inspector[12]

PAL devices have been installed on all or nearly all nuclear devices in the US arsenal, including the Minuteman ICBM, MGM-13A Mace Tactical Ballistic Missile, CGM-13B Mace Tactical Ballistic Missile, Sergeant, Pershing, and WAC Corporal missiles as well as the Nike-Hercules, Honest John rockets, the Davy Crockett system, and artillery-launched howitzer rounds.

Two-man rule

These two locks are part of the implementation of the two-man-rule in a Minuteman ICBM launch control capsule.

Modern PALs use the two-man rule, which is designed to prevent accidental or malicious launch of nuclear weapons by a single individual.

For example, on a ballistic missile submarine (SSBN), both the commanding officer (CO) and executive officer (XO) must agree that the order to launch is valid, and then mutually authorize the launch with their operations personnel. Instead of another party confirming a missile launch as in the case of land-based ICBMs, the set of keys is distributed among the key personnel on the submarine and are kept in safes (each of these crew members has access only to his keys), some of which are locked by combination locks. Nobody on board has the combination to open these safes - the unlock key comes as a part of the launch order from the higher authority.[13]

In the case of Minuteman missile launch crews, both operators must agree that the launch order is valid by comparing the order's authorization code against a code from a Sealed Authenticator (a special sealed envelope that holds a code). The Sealed Authenticators are stored in a safe that has two separate locks so a single crew member cannot open the safe alone. Both crew members must simultaneously turn the 4 launch keys. An additional safeguard is provided by requiring the crew in another Launch Control Center to do the same for the missiles to be launched.

The two-man rule also applies when the President of the United States orders the launch of nuclear weapons. Although only the President can order the launch, the order must be confirmed by the Secretary of Defense.

Stronglinks and weaklinks

Another part of the PAL design is the inclusion of "stronglinks" and "weaklinks." These words, which come from the proverb "A chain is only as strong as its weakest link" ensure resilience to accidental activation through damage. The stronglinks include the ruggedization of some components and inclusion of insensitive munitions so that they will not be circumvented by fire, vibration, or magnetic fields, leaving the PAL vulnerable to bypass after such damage. On the other hand, activation-critical electronics within the weapon, such as capacitors, are selected so that they will fail before the safety device in the event of damage, ensuring that the weapon fails safe.[14]

Critical signal detection

The B-61 bomb contains 5919 parts, including its PAL

Nuclear weapons will only respond to a specific arming signal. This is passed to the weapon by a unique signal generator located outside the weapon. This output is specific and well-defined, precluding approximation, emulation, noise, or interference from being accepted as a false positive.[15]

Parameter recognition

A further safeguard is the Environmental Sensing Device (ESD). It determines through environmental sensors whether the weapon is operating under expected parameters. For example, on an ICBM, a nuclear warhead would first be exposed to a strong acceleration, and then a period of free fall. The ESD determines the external parameters such as acceleration curve, temperature and pressure and only activates the weapon when these external effects occur in the correct order and must fall within specific parameters. So, should unauthorized personnel succeed in stealing a weapon, they could not activate it as long as a functional launch mechanism was not also stolen along with it. Of course, without the PAL codes they would still not succeed.


Simplified illustration of some nuclear weapon safety mechanisms

The conventional explosives needed to start the chain reaction are tailored to the characteristics of the fissile material in the core of the weapon. If the detonation does not occur exactly as planned, such as in the case of a misfire, a nuclear reaction is unlikely to occur - the explosion will be no greater than the amount of conventional explosive (although radioactive material—the unreacted nuclear fuel—may be dispersed). Computer simulations have assisted in calculating the likelihood of a chain reaction still occurring in a misfire. The odds of a nuclear chain reaction after a misfire of the conventional explosives is estimated to be one in one million.[16] The likelihood of a full-yield nuclear explosion is estimated to be even lower, at one in one billion.

Limited attempts

The weapon renders itself inoperable if an activation code is entered incorrectly multiple times. This precludes attempts to bypass the PAL codes through trial and error. The weapon must be returned to a maintenance facility in order to work again.


Over the years the design and feature set of PALs has increased, as has the length of the access code. US-manufactured PALs are divided into five categories; however, the earliest PALs were never assigned a category letter.

Simulated Peacekeeper missile launch
CLASS Code Length Description
3–4 The first PALs were combination locks with a three-number sequence. Later versions used four numbers, so that the access code could be divided between two people, each of whom would only know half of the sequence.
A 4 Class A PALs were electromechanical switches designed for ballistic missiles. The four-digit code was entered into the weapon using a portable electronic device.
B 4 Category B PALs were essentially identical in function to Category A, but designed with newer technology. Additionally, they could be activated via a wired remote, and were thus used on weapons launched by aircraft.
C 6 These PALs featured a 6-digit switch, and allowed for only limited code attempts before lockout. Such behavior was pioneered in some late model Category B PALs.
D 6 Permissive Action Links in Category D had all the features of the previous generation, but also allowed for the input of multiple types of codes, including ones that could set the device to a training mode, or disable the weapon entirely.
F 12 Category F PALs expand the code length to 12 digits, and disable the weapon in addition to lockout after a series of failed code entry attempts. They also include the ability to control the magnitude of the nuclear reaction (the so-called Dial-A-Yield feature) and an emergency stop.[17]

Usage by other states

The increase in the number of nuclear-armed states was a similar cause for concern for the United States government for reasons similar to the original impetus for PALs. Thus, since the 1960s, the US has offered its own PAL technologies to other nuclear powers. The US considered this a necessary step: if the technology were kept secret, it would only be half as effective as possible, since the other power in a conflict might not have such safety measures.

     Nuclear-Weapon-Free Zones      Nuclear weapons states      Nuclear sharing      Other NPT signatory

In the early 1970s, France was an early recipient of United States assistance on this critical element of nuclear security. The Nuclear Non-Proliferation Treaty (NPT) went into effect in 1970 and precluded treaty members (including the US) from directly disseminating technology related to nuclear weapons development or enhancement. In order to get around this prohibition, the US developed a legal trick: "negative guidance". French nuclear scientists would regularly brief US scientists on French developments in the field of PALs, and the US scientists would tell their French counterparts whether they were on the right track. In 1971, the US also offered its technology to the Soviet Union, which developed a similar system.

In the early 1990s the People's Republic of China requested information to develop its own PALs.[18] The Clinton administration believed that to do so would give too much information to the Chinese about American weapon design, and therefore, refused the request.

In 2007 the UK Government revealed that its nuclear weapons were not equipped with Permissive Action Links. Instead, the UK's nuclear bombs to be dropped by aircraft were armed by just inserting a key into a simple lock similar to those used to protect bicycles from theft. The UK withdrew all air-launched bombs in 1998. The current UK Trident warheads can also be launched by a submarine commander with the support of his crew without any code being transmitted from the chain of command.[19]

Pakistani truck-mounted IRBMs on display at the IDEAS 2008 defense exhibition in Karachi, Pakistan.

Detailed information about PAL systems design and their use is classified, although these mechanisms have been offered to Pakistan[20] for protection of their nuclear weapons.[21] In the end, the US decided that it could not do so for legal reasons; the Pakistanis were also concerned that such technology would be sabotaged by a "kill-switch" that the US could operate. However, many experts in the field of nuclear technology in the US government supported the publication of the PAL system because they considered Pakistan's arsenal as the world's most vulnerable to abuse by terrorist groups.

Whether it’s India or Pakistan or China or Iran, the most important thing is that you want to make sure there is no unauthorized use. You want to make sure that the guys who have their hands on the weapons can’t use them without proper authorization.
—Harold M. Agnew, former director, Los Alamos National Laboratory

In November 2007, the New York Times revealed that the US has invested 100 million USD since 2001 in a secret program to protect Pakistan's nuclear arsenal. Instead of transferring PAL technology, the US provided helicopters, night vision and nuclear detection devices, as well as training to Pakistani personnel in order to prevent the theft or misuse of Pakistan's nuclear material, warheads, and laboratories.[21]

See also


  1. ^ "Nuclear Command and Control". Security Engineering: A Guide to Building Dependable Distributed Systems. Ross Anderson, University of Cambridge Computing Laboratory. Retrieved April 29, 2010. 
  2. ^ Richard Rhodes: Dark Sun: The Making of the Hydrogen Bomb. Simon & Schuster, New York 1996, ISBN 978-0-684-81690-6.
  3. ^ Peter D. Feaver: Armed Servants: Agency, Oversight, and Civil-Military Relations. Harvard University Press, Cambridge 2005, ISBN 978-0-674-01761-0, S. 151.
  4. ^ a b c Peter Stein, Peter Feaver: Assuring Control of Nuclear Weapons: The Evolution of Permissive Action Links. University Press of America, Lanham 1989, ISBN 978-0-8191-6337-0
  5. ^ Weapon Dispersal without Fear of Unauthorized Use. In: Sandia Lab News, Family Day Special Edition, Bd. 38 Nr. 20, 1986, S. 4.
  6. ^ Consumer Price Index (estimate) 1800–2014. Federal Reserve Bank of Minneapolis. Retrieved February 27, 2014.
  7. ^ "Keeping Presidents in the Nuclear Dark (Episode #1: The Case of the Missing "Permissive Action Links") - Bruce G. Blair, Ph.D". February 11, 2004. Retrieved April 29, 2010. 
  8. ^ Thomas C. Reed: At the Abyss: An Insider’s History of the Cold War. Presidio Press, New York 2005, ISBN 978-0-89141-837-5.
  9. ^ Grossman, Elaine M. (September 26, 2008). "U.S. Air Force Might Modify Nuclear Bomb". 
  10. ^ Hans M. Kristensen: U.S. Nuclear Weapons in Europe. Natural Resources Defense Council, New York 2005, S. 20–21. (PDF; 4,9 MB, accessed February 4, 2009).
  11. ^ Milliwatt Surveillance Program Ensures RTG Safety and Reliability. In: The Actinide Research Quarterly, Winter 1994. accessed February 4, 2009.
  12. ^ Dan Caldwell, Peter D. Zimmerman: Reducing the Risk of Nuclear War with Permissive Action Links. In: Barry M. Blechman, David K. Boren (Eds.): Technology and the Limitation of International Conflict. Johns Hopkins Foreign Policy Institute, Washington, D.C. 2000, ISBN 978-0-941700-42-9.
  13. ^ Douglas C. Waller: Practicing For Doomsday
  14. ^ David W. Plummer, William H. Greenwood: History of Nuclear Weapon Safety Devices. Sandia National Laboratories, Albuquerque 1998. Presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, July 1998. (PDF; 1,3 MB, accessed September 23, 2010).
  15. ^ Donald R. Cotter: Peacetime Operations: Safety and Security. In: Ashton B. Carter, John D. Steinbruner, Charles A. Zraket (Eds.): Managing Nuclear Operations. Brookings Institution Press, Washington, D.C. 1987, ISBN 978-0-8157-1313-5.
  16. ^ Sidney D. Drell: Addendum on Nuclear Warhead Safety. In: In the Shadow of the Bomb: Physics and Arms Control. American Institute of Physics, New York 1993, ISBN 978-1-56396-058-1.
  17. ^ Thomas B. Cochran, William M. Arkin, Milton M. Hoenig: Nuclear Weapons Databook: Volume I - U.S. Nuclear Forces and Capabilities. Ballinger Publishing Company, Pensacola 1984, ISBN 978-0-88410-173-4.
  18. ^ Steven M. Bellovin: Permissive Action Links, Nuclear Weapons, and the Prehistory of Public Key Cryptography. Department of Computer Science, Columbia University, April 2006. (PDF; 0,1 MB, retrieved on February 4, 2009).
  19. ^ "Programmes | Newsnight | British nukes were protected by bike locks". BBC News. November 15, 2007. Retrieved April 29, 2010. 
  20. ^ Sanger, David E. (2009). The Inheritance. London, UK: Bantam Press. p. 224.  
  21. ^ a b New York Times: U.S. Secretly Aids Pakistan in Guarding Nuclear Arms, Accessed on February 4, 2009.

External links

  • "Principles of Nuclear Weapons Security and Safety". October 1, 1997. Retrieved April 29, 2010. 
  • "Permissive Action Links". Retrieved April 29, 2010. 
  • The JCAE and the Development of the Permissive Action Link
  • Keeping Presidents in the Nuclear Dark
  • Security Engineering, Chapter 11: Nuclear Command and Control, Ross Anderson, ISBN 0-471-38922-6
  • "BBC News (video)". Retrieved April 29, 2010. 
  • - a BBCThe Dr. Strangelove scenario Newsnight report featuring the lack of security features on the WE.177 (streaming video)
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