A strong link/weak link and exclusion zone nuclear detonation mechanism is a type of safety mechanism employed in the arming and firing mechanisms of modern nuclear weapons.
The safety mechanism starts by enclosing the electronics and mechanical components used to arm and fire the nuclear weapon with a mechanical and electrical isolation barrier, the energy barrier, which encloses and defines the exclusion zone. This is insulated from mechanical, thermal, and electrical disruptions (such as static electricity, lightning, or fire).
Between the exclusion zone and the actual detonators, a normally-disconnected link mechanism is used, such as a switch which has a built-in motor to activate it. The arming system has to activate the switch in order to connect the firing circuits to the detonators in the weapon. This disconnection, which requires the arming mechanism to operate, is called the strong link.
It is possible for an accident (rocket explosion, airplane crash, accident while weapon is being moved) to disrupt the weapon and break the integrity of the exclusion zone. As a safety mechanism, a weak link is also built into the system. This is a set of components designed to fail at lower stresses (thermal, mechanical, and electrical) than the strong links, and will prevent signals from the strong links from reaching the detonators. The weak link acts to break the connection to the detonators before the strong link could be disrupted and fail by the stress of an accident: by the time the strong links fail, the weapon has already been rendered permanently inoperable. Strong links and the following weak links are intentionally co-located, so that they will experience similar environmental conditions.
The following table summarises the effects of failure modes in the strong and weak links:
Strong links
Strong links, at least in US nuclear weapons, are always implemented as electro-mechanical systems such as motor-driven switches. There are two main requirements: when functional, never to allow an invalid signals to penetrate the energy barrier, and never to fail in a way that can pass a signal though the barrier before the weak links inside the exclusion zone have also failed.
The MC2935 and MC2969 devices were two similar devices based on a rotary solenoid, acting, respectively, as "trajectory" (passing a signal only when a missile's physical movement indicated a correct launch) and "intent" (signalling that a detonation is desired by the operator) strong links.
The Mechanical Safing and Arming Device (MSAD) strong link device used a small pellet of sensitive high explosive to trigger a larger charge of insensitive high explosive. Normally, the pellet was held away from the main charge, and was physically moved into position only when the strong link was activated by a valid input and detonated by a mechanical "slapper". The MSAD also contained a weak link: the pellet would burn or explode harmlessly in a fire when it was not in position, and the insensitive explosives could not then be detonated at all.
Multiple strong links could be used in series, which, when properly designed, multiplies the safety factor.. The B61 nuclear bomb, for example, gated the trajectory strong link behind the intent strong link. Until the correct intent unique signal was sent, the trajectory unique signal would not even be presented to the trajectory strong link inputs.
Unique signals
Strong links implement a mechanism where only a single, unique form of energy may enter the exclusion zone. This energy is encoded as a unique signal: a sequence of "events" which must occur in a precise and preset pattern for the link to activate. This pattern is specifically designed to be extremely unlikely to occur by chance. The pattern is checked for validity by a discriminator. In some devices, known as single-try discriminators, an incorrect event pattern leads to the device becoming inoperable: the weapon cannot then be reset and fired remotely. "Multiple-try" discriminators could be reset remotely. A single-try strong link might have an event sequence of 24 events, whereas a multiple-try device would have more: the MC2969 had 47..
Unique signal patterns were always the same for a given strong link discriminator, and were not secret or classified: they were designed only for safety purposes and not security.. Each strong link had a different signal, so as to avoid the possibility of common mode failure.