Weak link-strong link
Weak link-strong link is part of United States nuclear surety architecture, a specifically design features in weapons themselves to prevent against accident and misuse. They cooperate closely with another architectural feature, the exclusion region, which prevents any external, unintended electrical signals from reaching the firing system of a nuclear weapon. By placing two weak-link/strong-link sets in series one can achieve confidence that the 10-6 failure criterion is satisfied. In practice, the two sets employ very different designs in order to eliminate the possibility of common-mode failures"
The graphic shows a schematic of a weapon, which is surrounded by a mechanically and electrically robust exclusion region, into which flow several unique signals (UQS) recognized by designated Strong Links, probably through a cryptographic mechanism.
- Intention signal from a Permissive Action Link, which must come from higher authority than the crew
- Local arming and firing commands, coming from the crew; the crew must be of at least two people under the two-man rule, all of whom are under the Personnel Reliability Program
- Environmental signals from Environmental Sensing Devicess, especially of a predicted weapon trajectory, but also of such weapon delivery signatures such as increasing barometric pressure and radar altitude for a bomb
The exclusion system barrier also provides thermal protection to give a fire-resistant pit, and a tamper-resistant shield. These unique and operational signals arm the strong link switches within the weak link-strong link safeguard of the firing set, which are probably part of the high voltage system that charges the capacitors that power the multiple detonators. If any predesignated weak link condition comes into being, such as crushing or puncture of the exclusion barrier, fire surrounding it, or a crash if the weapon is fuzed for airburst, the weak link opens and breaks the firing circuits. For certain conditions, such as crashes, a more complex weak link sensor may initiate a self-destruct for the weapon, one which will not produce a nuclear yield.
Physically, a strong link is a set of electromechanical actuators.  A simple AC or DC signal is used to drive mechanical actuators, called drivers, in a specified sequence of events (generally 24) which are monitored by a single-try discriminator. The probability of this sequence being generated accidentally is vanishingly small. If it is matched, If the sequence is correct, the discriminator activates an energy control element, which is also mechanical and may involve electrical contacts, explosive pellets, ferrite buttons, or optical fibers and prisms.
"Stronglinks are sometimes described as mazes; the unique signal encodes the path out of the maze and controls the driver to follow that path."
Weak links are part of the protection system that, even in the presence of an enabling signal through a strong link, prevent the firing sequence from completing if inhibiting signals are present. A basic example of such a signal might be a puncture in the exclusion barrier or the presence of temperatures indicative of fire.
"The power source required to initiate detonation of the primary high explosive is isolated from the weapon detonators by a “strong-link” switch that is only closed when the weapon is intentionally armed during its delivery trajectory. This switch is also designed to remain open prior to arming, under any conceivable accident scenario, for a length of time long compared to the time required for a “weak link” to cause collapse of the power source. A high-voltage capacitor that stores the charge needed to fire the detonators is a simple example of such a weak link. It can be designed to fail predictably under high-temperature and/or energeticimpact conditions as might be experienced, for example, in an aircraft crash.
- Ashton Carter, John Steinbruner, Charles Zraket (1987), Managing nuclear operations, Brookings Institution, pp. 46-48
- Committee on Technical Issues Related to Ratification of the Comprehensive Nuclear Test Ban Treaty (2002), Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty, National Academy of Sciences, p. 24
- Grant Elliott (12 December 2005), US Nuclear Weapon Safety and Control, MIT Program in Science, Technology, and Society