Permissive Action Link

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An external PAL control unit. It originally cost $ 290,000 before unit cost could finally be reduced significantly.

Permissive Action Link ( PAL ) is the name of a US security device for nuclear weapons that is designed to prevent them from accidentally detonating in the event of an accident or misuse. A secret PAL code, which can arm the weapon, protects against ignition by plane crashes, lightning strikes, fire, explosions or by unauthorized persons such as dissidents within the armed forces or terrorists. The system also prevents the bridging of circuits and other manipulations of the nuclear weapon. The United States Department of Defense defines PALs as follows:

“An integrated or externally attached device on a nuclear weapon system that prevents arming and / or firing of it until a pre-determined single code or code combination is entered. It may contain systems and cabling that are located outside the weapon or the weapon system in order to activate components within the weapon or the weapon system. "

Technical details of the PAL systems are subject to confidentiality. As a result of further developments and general technical progress in the last few decades, there are now various approaches and implementations in construction. However, all systems are based on the principle that the weapon cannot be detonated without entering a code.

history

Starting position

The Sandia National Laboratories were instrumental in the development of the PALs from the beginning

The development of the first Permissive Action Links was a gradual process between 1945 and the early 1960s. The year 1953 formed an important point: In that year the United States Atomic Energy Commission (AEC) and the Department of Defense signed the Missiles and Rockets Agreement , an agreement that was significant for the future development of the PALs. Selected laboratories were to develop and manufacture nuclear warheads under the supervision of the AEC, while responsibility for deployment and deployment remained with the military. The laboratories were also given the option to conduct their own research in the field of weapon control and security. The idea behind this was that if the government were ever interested in such a safety device, research and development of prototypes would be advanced. In the early 1960s, the need for such a system finally grew. There were two main reasons for this.

From a technological point of view, nuclear weapons have become more and more sophisticated in construction and easier to use. Cumbersome and lengthy preparation was no longer necessary; the new warheads could be armed quickly and their number increased monthly. A new form of control was needed to prevent unauthorized use. At the beginning of the 1960s, as the Cold War continued to come to a head, the government thought less of renegade American officers who would go into business for themselves than of the commanders of the Strategic Air Command (SAC). Without a safeguard system, they had full control over most of the nuclear weapons systems, and not every general was said to have a steady hand and prudence with regard to the East-West conflict.

“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 [ie, PAL locks], SAC had the power to do a lot of things, and it was in his hands, and he knew it. "

“I was concerned about General Power. I worried that General Power wasn't crisis-proof. I worried about the fact that he was in control of so many weapons and weapon systems and could, under certain circumstances, set this force in motion. Back in those days before we had real executive control [i.e. H. 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, General Power's subordinate commander at the time

From a political point of view, another and far more pressing reason was the fact that various American nuclear weapons were stationed on foreign territory under the protection of NATO and were therefore at least partially under the control of other states ( nuclear participation ). Especially for the Congress this circumstance was very worrying, except that it violates applicable law US violated. In addition, some of the allies were viewed as potentially unstable, including Germany and Turkey . There have been significant concerns that the military in any of these countries might override the instructions of the civilian leadership. In addition, the USA assumed that in the event of war, parts of West Germany would be overrun early and nuclear weapons would fall into the hands of the Soviets .

For a long time, the US military opposed the use of PALs. It feared the loss of independence and fear of malfunctions that could put all nuclear warheads out of action in times of crisis. But the advantages outweighed the disadvantages in the end: Thanks to the PALs, the weapons could on the one hand be distributed on a larger scale in Europe in order to prevent rapid and targeted destruction or conquest by the Eastern Bloc, on the other hand control over them was still retained.

Development and dissemination

The forerunners of Permissive Action Links were simple mechanical locks that were embedded in the control systems of nuclear weapons. There they could fulfill various functions: some blocked the cavity in which the launching components had to be inserted, others blocked electrical circuits, and some simply prevented access to the control panel. As a test, these mechanisms were built into some of the nuclear weapons stationed in Europe as early as 1959.

Work on PAL prototypes remained at a low level until 1960. The Sandia National Laboratories (SNL) had so far succeeded in developing a number of new combination locks that could be adapted to different types of weapons. In the spring of 1961, there were a series of hearings in the US Congress at which Sandia presented the prototype of a special electromechanical lock, which was then still known as the "Proscribed Action Link". However, the military leadership soon realized that this term tended to psychologically discourage officers from using weapons (proscribed = "outlawed / forbidden"), and changed the meaning from PAL to "Permissive Action Link" (permissive = "allow / allow") ).

Order on the introduction of PAL for all US nuclear weapons under NATO command

In June 1962, President John F. Kennedy signed the National Security Action Memorandum 160 (NSAM 160). It was a presidential directive that ordered the installation of PALs in all US nuclear weapons in Europe. All other American nuclear weapons were excluded for the time being. The conversion lasted until September 1962 and cost 23 million US dollars, which corresponds to a current (as of 2008) equivalent of about 164 million dollars. The Strategic Air Command in Omaha , meanwhile, was concerned that the codes would not be available in an emergency and tacitly decided to set them to "00000000". This combination remained valid until 1977.

The complete conversion to PAL systems, on the other hand, was relatively slow. In 1974, the US Secretary of Defense James Schlesinger stated that a large number of tactical nuclear weapons were still not equipped with Permissive Action Links, although the technology had now been available for some time. It took two more years for all tactical nuclear weapons to be fully equipped with PALs. In 1981, almost 20 years after the invention of the PALs, a good half of US nuclear weapons were still fitted with mechanical locks. It was not until 1987 that they were completely replaced.

Modernization and present

Over the years, the Permissive Action Links have been continuously developed and maintained. In 2002, parts of the old B61 atomic bombs were replaced with new systems in order to increase safety and reliability and to keep the weapons in service until at least 2025.

Code management system

In 1995 the code management system (CMS) began to be developed . The CMS simplified the control and logistics for the personnel and improved the flexibility and speed when maintaining and arming the weapons. Various codes could be used to reprogram PAL keys, to lock the weapon and to handle it in general, while the secrecy and validity of use codes remained ensured.

In total, the CMS consisted of fourteen new products, nine of which were software and five were hardware products. The software was developed at Sandia National Laboratories and contained approximately 160,000 lines of program code (260,000 with comments). The hardware was produced by the National Nuclear Security Administration .

The CMS was fully operational for the first time in November 2001. However, a part of the system, a special cryptography processor, was built into the weapons as early as 1997 to prevent a possible year 2000 problem . By spring 2004, all PAL systems were finally equipped with the code management system. It currently forms the general basis for future hardware and software improvements to the Permissive Action Links.

Range of functions

Permissive Action Links are powered by low-maintenance radioisotope generators . Instead of working on a chemical basis, as with conventional batteries, they work with the alpha decay of plutonium 238 , a non-fissile isotope. Although its half-life is 87.7 years, its actual lifetime is less. It is limited by the slow accumulation of helium which, as a product of the alpha decay, builds up a damaging overpressure after a few decades.

PALs are also directly or indirectly linked to a number of security measures that together form a comprehensive security package. Generally, the PAL systems are built in three dimensions; H. with components both on and deep in the weapon. This makes it almost impossible to bridge 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."

"Bypassing a PAL, as one weapons developer figuratively put it, should be about as complicated as removing tonsils, where you operate on the patient from the wrong end."

- Peter D. Zimmerman, nuclear physicist and arms controller

Two-man rule

Modern PALs use the two-man rule (Engl. Two-man rule ). This means that in the event of an order for a nuclear strike, two people are required to carry it out. For example, on a Strategic Missile Submarine (SSBN), both the Commanding Officer (CO) and the Executive Officer (XO) must confirm the attack order: Each of the two has its own safe code or key, which together give access to the authentication code release. In ICBM - missile silos of the Crew Commander and Deputy Commander crew take on this task. To start, they each have to operate two ignition keys at the same time (with both hands) (four ignition switches in total).

Strength-weakness principle

Another mechanism that prevents unintentional detonation is based on the strength-weakness principle ( strong link / weak link ). The strong part is the electrical isolation of the detonation system. It is located in an exclusive zone inside the weapon and is separated from the electrical circuit by a motorized switch. The weapon can only be armed when this bridge is closed.

In order to prevent the exclusive zone from being damaged and still causing a detonation in the event of an accident, critical components of the detonation system are intentionally designed to be weak . This means that they fail irreversibly if they are exposed to extraordinary influences (such as heat, strong acceleration, etc.). These can be capacitors, for example, that burn out at relatively low temperatures.

Critical signal detection

The B61 atomic bomb consists of 5919 individual parts, including numerous PAL components

The system only reacts to a very specific electrical voltage. This is generated by a special signal generator ( unique signal generator ) that is located outside the weapon. It delivers a precisely defined output power, so that simulated signals, background noises or interference can not activate the system. Thanks to computer technology, there are now new approaches that replace the complex analog signal with digital communication including codes.

Feature and parameter recognition

The Environmental Sensing Devices (ESD) are another safeguard . They use sensors to determine the environmental properties that are expected for this weapon. In a missile, for example, a nuclear warhead would first be subjected to strong acceleration and then to a phase of free fall. The ESD determines the external influences such as the acceleration curve, temperature and pressure and only activates the weapon if these external effects occur in the correct order and are within specific parameters. If, for example, unauthorized persons were able to steal a warhead, they would not be able to detonate it as long as the launch pad was not stolen. Apart from that, the corresponding PAL codes would still be missing.

Intentional misfire

Schematic representation of safety mechanisms on a nuclear warhead

The conventional explosives that are needed to start the chain reaction are precisely matched in terms of quantity and arrangement to the properties of the fissile material in the core. If the detonation does not occur exactly as intended, for example due to a misfire, a nuclear reaction usually does not occur - the detonation does not become larger than the amount of the chemical explosive. With the help of simulation calculations, it was possible to estimate how high the probability is that a nuclear reaction will nevertheless occur. In the event of a misfire of conventional explosives, this is around 10 −6 . The probability that a malfunction of components will lead to a complete nuclear weapon explosion is about 10 −9 under normal conditions and under exceptional conditions such as e.g. B. a plane crash, about 10 −6 .

Furthermore, there are a number of violent and non-violent mechanisms that either destroy the warhead or render it irreparably harmless when attempted to manipulate it. The latter would be e.g. B. possible through a small shaped charge that destroys the symmetry of the plutonium core. This would no longer be capable of splitting until it was machined again.

Further security measures

A number of additional security measures are integrated into all modern nuclear weapons:

Fire resistant pits (FRP)
They prevent molten plutonium from leaking out in the event of a fire. It remains in its beryllium shell, which has a very high melting point. In addition, additional heat-insulating components are installed in order to keep the influence of heat as low as possible.
Insensitive high explosives (IHE)
They use the explosive TATB , whose resistance to impact and heat is greater than that of any other known material with a comparable energy density. TATB is not detonated even by the impact of a plane crash, fire, explosion or the impact of projectiles from small arms.
Limited-try feature
The weapon is deactivated as soon as a code is entered incorrectly several times. This a wild trying to codes can be through the trial-and-error (Engl. Method of trial and error ) prevent. If the weapon is blocked, it must then be taken to a workshop for maintenance so that it can be made functional again.

PAL classifications

Various types of PAL have been used over the years, as the following table shows. It is noticeable that a category was skipped over in the course of development.

Simulation of a Peacekeeper rocket launch
Type Digits description
- 3-4 Mechanical combination locks were the first PALs ever and had three digits. Later versions had four digits to split the knowledge: two people now each had half of the key.
A. 4th Category A PALs were electromechanical switches and were designed for missiles. In order to enter the four-digit code, a portable electronic device had to be plugged into the weapon beforehand.
B. 4th Category B PALs worked in principle like those of Category A, but used fewer wires. This made wire remote control from cockpits possible, which is why the B series was used for bombs.
C. 6th Category C PALs were extended to six digits and consistently contained a function that limited code entry to a few attempts. This has already been tested on the last category B models.
D. 6th A Category D Permissive Action Link recognizes various codes, for example for deactivating the weapon or for training purposes. A number of safety functions, such as self-harm, have been added.
F. 12 Category F is similar to Category D, but accepts twelve-digit codes. In addition, these PALs can regulate the detonation strength (also known as dial-a-yield ) and have a remote shutdown.

PALs in other states

Due to the increasing number of nuclear powers and their weapons stocks, there were repeated security concerns in the USA. Since the 1960s, it has been considered several times to make parts of the PAL technology available to other countries. The US saw this step as necessary: ​​preventing a nuclear war was only half as effective if one could only prevent an accidental first strike by the US, while all other nuclear powers had no such security technology.

In the early 1970s, for example, France was one of the first countries to receive US assistance on key issues relating to nuclear safety. However, the Nuclear Non-Proliferation Treaty (NPT) turned out to be an obstacle. In addition to the further armament and proliferation of nuclear weapons, it also forbade cooperation between states in nuclear weapons technology. To work around this problem legally, a trick was used: The system of "negative guidance" (English. Negative guidance ). The US scientists were regularly given a description of the French advances in warhead technology, while they gave the French advice in an advisory capacity as to whether or not they were on the right track with their solutions.

Despite, or perhaps because of, the Cold War, the USA also offered PAL technology to the Soviet Union in 1971 . However, the latter turned down the offer and preferred to rely on "people who are supervised by people who are supervised by people". The Soviets relied on triple control by the military , the KGB and political officers . After the collapse of the USSR , an exchange took place with the legal successor Russia , which had taken over the nuclear weapons stocks, for the safety of which the United States was very concerned at the time.

In the early 1990s, the People's Republic of China voluntarily asked for information about the Permissive Action Links. However, the Clinton administration feared that passing on the technology would reveal too much about American arms manufacture and improve China's warheads in many ways, and declined to answer.

After the September 11, 2001 attacks , the Bush administration debated whether to make PAL technology accessible to Pakistan . In the end, she decided against it due to legal restrictions. In addition, the Pakistanis were suspicious that American technology might include kill switches hidden in their warheads that would enable Americans to turn off their weapons.

  • Nuclear powers in the Nuclear Non-Proliferation Treaty (China, France, Russia, UK, USA)
  • Nuclear powers outside the NPT (India, North Korea, Pakistan)
  • unexplained nuclear powers outside the Non-Proliferation Treaty (Israel)
  • Member States of Nuclear Participation
  • Former nuclear powers

    • Many nuclear technology experts in the government endorsed the release of the PAL system because they viewed Pakistan's arsenal as the world's most vulnerable to abuse by terrorist groups. Other members of the government, however, had the same concerns that kept the Clinton administration from sharing the technology with China. Furthermore, Pakistan, along with India and Israel, was one of the only three states that had not signed the Nuclear Non-Proliferation Treaty.

    “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. "

    “Whether it's India or Pakistan or China or Iran, the most important thing is that you want to make sure that there is no unauthorized use [of nuclear weapons]. You want to make sure the guys who have their hands on guns can't use them without proper authorization. "

    - Harold M. Agnew, former director of the Los Alamos weapons laboratory

    In November 2007, it was announced that the US had invested over $ 100 million since 2001 in a secret program to make Pakistani nuclear weapons safer. However, this apparently did not include the PAL technology itself, but instead involved the training of Pakistani personnel in the United States and the provision of large amounts of equipment such as helicopters, night vision devices and nuclear detectors to protect nuclear material, warheads and the laboratories.

    In the same year, the British government admitted that until the end of the 1990s their nuclear weapons were not equipped with Permissive Action Links, but could be armed using a simple cylinder lock.

    PALs in popular culture

    Over time, PALs and PAL-like systems have also been featured in numerous publications in the entertainment industry, mostly without mentioning them by name. Examples of this can be found in films such as Straße der Verdammnis (1977), WarGames - Kriegsspiele (1983), Crimson Tide (1995) or The attack (2002), the video game Metal Gear Solid (1998) or the television series 24 ( fourth season ) .

    See also

    literature

    • Hendricus J. Neumann: Nuclear weapons in Europe: NATO double resolution, arms control, glossary. Osang Verlag, Bonn 1982, ISBN 3-7894-0085-8 .
    • Christian Tuschhoff: Germany, nuclear weapons and NATO 1949–1967. Nomos Verlag, Baden-Baden 2003, ISBN 978-3-7890-8274-0 .
    • William M. Arkin, Richard W. Fieldhouse: Nuclear Battlefields. The nuclear weapons report. Athenaeum Verlag, Bodenheim 1986, ISBN 978-3-7610-8391-8 .
    • Ashton B. Carter, John D. Steinbruner, Charles A. Zraket (Eds.): Managing Nuclear Operations. Brookings Institution Press, Washington, DC 1987, ISBN 978-0-8157-1313-5 .
    • Chuck Hansen: US Nuclear Weapons: The Secret History. Crown Publishing Group, New York 1988, ISBN 978-0-517-56740-1 .
    • 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 .
    • Ross J. Anderson: Nuclear Command and Control. In: Security Engineering: A Guide to Building Dependable Distributed Systems. 2nd Edition. John Wiley & Sons, Hoboken 2008, ISBN 978-0-470-06852-6 , pp. 415-430.
    • Thomas B. Cochran, William M. Arkin, Milton M. Hoenig: Nuclear Weapons Databook: Volume I - US Nuclear Forces and Capabilities. Ballinger Publishing Company, Pensacola 1984, ISBN 978-0-88410-173-4 .

    Web links

    Commons : Nuclear Weapons  - Collection of images, videos and audio files

    Individual evidence

    1. ^ A b Hans M. Kristensen: US Nuclear Weapons in Europe . Natural Resources Defense Council, New York 2005, pp. 20-21. ( nrdc.org PDF; 4.9 MB, accessed February 4, 2009).
    2. ↑ Quoting from the Department of Defense Dictionary of Military and Associated Terms . United States Government Printing Office , Washington, DC 1999, ISBN 978-0-16-049783-4 . Retrieved on February 4, 2009 (own translation).
    3. ^ Richard Rhodes: Dark Sun: The Making of the Hydrogen Bomb. Simon & Schuster, New York 1996, ISBN 978-0-684-81690-6 .
    4. ^ Peter D. Feaver: Armed Servants: Agency, Oversight, and Civil-Military Relations. Harvard University Press, Cambridge 2005, ISBN 978-0-674-01761-0 , p. 151 (own translation).
    5. ^ A b 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 .
    6. ^ Weapon Dispersal without Fear of Unauthorized Use. In: Sandia Lab News , Family Day Special Edition, Vol. 38 No. 20, 1986, p. 4.
    7. ↑ Bruce G. Blair, Ph.D: The Case of the Missing "Permissive Action Links". Archived from the original on February 14, 2004 ; accessed on September 1, 2014 . In: Bruce Blair's Nuclear Column , February 11, 2004.
    8. ^ A b 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. ↑ milliwatts Surveillance Program Ensures RTG Safety and Reliability . In: The Actinide Research Quarterly , Winter 1994. Retrieved February 4, 2009.
    10. ^ 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, DC 2000, ISBN 978-0-941700-42-9 (own translation).
    11. ^ David W. Plummer, William H. Greenwood, History of Nuclear Weapon Safety Devices . Sandia National Laboratories, Albuquerque 1998. Contribution to the 34th AIAA / ASME / SAE / ASEE Joint Propulsion Conference, Cleveland, July 1998. ( osti.gov PDF; 1.3 MB, accessed February 4, 2009).
    12. ^ A b 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, DC 1987, ISBN 978-0-8157-1313-5 .
    13. ^ 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 .
    14. ↑ Thomas B. Cochran, William M. Arkin, Milton M. Hoenig: Nuclear Weapons Databook: Volume I - US Nuclear Forces and Capabilities. Ballinger Publishing Company, Pensacola 1984, ISBN 978-0-88410-173-4 .
    15. ↑ Steven M. Bellovin: Permissive Action Links, Nuclear Weapons, and the Prehistory of Public Key Cryptography. Department of Computer Science, Columbia University, April 2006. ( usenix.org PDF; 0.1 MB, accessed February 4, 2009).
    16. ^ A b New York Times: US Secretly Aids Pakistan in Guarding Nuclear Arms , accessed February 4, 2009.
    17. ↑ Meirion Jones: British nukes were protected by bike locks . In: BBC NEWS | Newsnight , November 15, 2007. Retrieved February 4, 2009.
    This article was added to the list of excellent articles on February 26, 2009 in this version .