The National Security Archive just released a documentary about the safety systems on nuclear weapons during the Cold War and beyond. At the beginning of the Cold War it became obvious that conventional forces would not be able to repel a Soviet attack in Western Europe. Nuclear deterrent, the capability to retaliate with massive nuclear power, became a primary tool of NATO to prevent war in Europe.
With so many U.S. nuclear weapons in the custody of both U.S. military and NATO partners across Europe, they had to find solutions to prevent unauthorised use or accidents. The concept called Always/Never: for an effective deterrent, you need nuclear weapons that are Always ready for use, but at the same time you need assurance that these weapons will Never be used unauthorised or accidentally.
|Early Electromechanical PAL|
The solution was the Permissive Action Link or PAL, a device inside the weapon that isolated the electronics from the detonation charge that triggers the nuclear reaction. The early PAL was a small electric motor attached to a combination lock, which in turn engaged the arming switch. The operator had to attach a control box by a cable to the weapon and enter the proper code to arm the weapon. Nowadays, they use encrypted detonation parameters, requiring the proper decryption codes to arm the warhead.
This not only prevented accidental detonation, but also shifted both decision and authority over each nuclear weapon from the military operator or commander to the U.S. President, who is the only person with the PAL codes and consequently the sole person who can initiate a nuclear war. Since then, the U.S. president is always accompanied by his military aid who carries the codes in the so-call Nuclear Football.
The system was not created overnight. It took years to develop the proper technology and procedures, but in the end it presented a major improvement of nuclear safety. The documentary Always Never, released by the Sandia National Laboratories, tells the story of the evolution of safe control over nuclear weapons. More information is found at the the Archive's Nuclear Vault. Extensive information on Permissive Action Links is also available on Steven Bellovin's Columbia page and some photos of PALs are linked at Light Blue Touchpaper.