Safing, Arming, Fuzing, and Firing (SAFF)

The ability to make effective use of a nuclear weapon is limited unless the device can be handled safely, taken safely from storage when required, delivered to its intended target, and then detonated at the correct point in space and time to achieve the desired goal. Although the intended scenarios for use of its weapons will strongly influence specific weaponization concepts and approaches, functional capabilities for safing, arming, fuzing, and firing (SAFF) will be fundamental.

Nuclear weapons are particularly destructive, with immediate effects including blast and thermal radiation and delayed effects produced by ionizing radiation, neutrons, and radioactive fallout. They are expensive to build, maintain, and employ, requiring a significant fraction of the total defense resources of a small nation. In a totalitarian state the leader must always worry that they will be used against the government; in a democracy the possibility of an unauthorized or accidental use must never be discounted. A nuclear detonation as the result of an accident would be a local catastrophe.

Because of their destructiveness, nuclear weapons require precautions to prevent accidental detonation during any part of their manufacture and lifetime. And because of their value, the weapons require reliable arming and fuzing mechanisms to ensure that they explode when delivered to target. Therefore, any nuclear power is likely to pay some attention to the issues of safing and safety, arming, fuzing, and firing of its nuclear weapons. The solutions adopted depend upon the level of technology in the proliferant state, the number of weapons in its stockpile, and the political consequences of an accidental detonation.

Whether to protect their investment in nuclear arms or to deny potential access to and use of the weapons by unauthorized persons, proliferators or subnational groups will almost certainly seek special measures to ensure security and operational control of nuclear weapons. These are likely to include physical security and access control technologies at minimum and may include use control. The techniques used today by the existing western nuclear weapon states represent the culmination of a half-century of evolution in highly classified military programs, and proliferators may well choose simpler solutions, perhaps by adapting physical security, access, and operational controls used in the commercial sector for high-value/high-risk assets.

From the very first nuclear weapons built, safety was a consideration. The two bombs used in the war drops on Hiroshima and Nagasaki posed significant risk of accidental detonation if the B-29 strike aircraft had crashed on takeoff. As a result, critical components were removed from each bomb and installed only after takeoff and initial climb to altitude were completed. Both weapons used similar arming and fuzing components. Arming could be accomplished by removing a safety connector plug and replacing it with a distinctively colored arming connector. Fuzing used redundant systems including a primitive radar and a barometric switch. No provision was incorporated in the weapons themselves to prevent unauthorized use or to protect against misappropriation or theft.

In later years, the United States developed mechanical safing devices. These were later replaced with weapons designed to a goal of less than a 1 in a 1 million chance of the weapon delivering more than 4 pounds of nuclear yield if the high explosives were detonated at the single most critical possible point. Other nations have adopted different safety criteria and have achieved their safety goals in other ways.

In the 1950's, to prevent unauthorized use of U.S. weapons stored abroad, permissive action links (PALs) were developed. These began as simple combination locks and evolved into the modern systems which allow only a few tries to arm the weapon and before disabling the physics package should an intruder persist in attempts to defeat the PAL.

The first PALs were 5-digit mechanical combination locks, physically attached to almost a dozen weapon types in the mid-1960s -- they were externally fitted onto the warhead containers themselves. Since then, more sophisticated devices have been developed, integrated into the warhead itself and connected to arming control panels in the cockpit or the launch control center. PALs are now designated Category ("Cat") A, B, D, and F, in ascending order of sophistication. The most modern, electronic versions are the six-digit Cat D and 12-digit Cat F, each with a "limited try" feature permitting a specific number of crew attempts to enter the correct code -- after too many tries, the electrical circuits self-destruct, disabling the warhead.

Safing To ensure that the nuclear warhead can be stored, handled, deployed, and employed in a wide spectrum of intended and unintended environmental and threat conditions, with assurance that it will not experience a nuclear detonation. In U.S. practice, safing generally involves multiple mechanical interruptions of both power sources and pyrotechnic/explosive firing trains. The nuclear components may be designed so that an accidental detonation of the high explosives is intrinsically unable to produce a significant (>4 pounds TNT equivalent) nuclear yield; it is simpler to insert mechanical devices into the pit to prevent the assembly of a critical mass into the pit or to remove a portion of the fissile material from inside the high explosives. Mechanical safing of a gun-assembled weapon is fairly straightforward; one can simply insert a hardened steel or tungsten rod across a diameter of the gun barrel, disrupting the projectile. All U.S. weapons have been designed to be intrinsically one-point safe in the event of accidental detonation of the high explosives, but it is not anticipated that a new proliferator would take such care.

Arming Placing the nuclear warhead in a ready operational state, such that it can be initiated under specified firing conditions. Arming generally involves mechanical restoration of the safing interrupts in response to conditions that are unique to the operational environment (launch or deployment) of the system. A further feature is that the environment typically provides the energy source to drive the arming action. If a weapon is safed by inserting mechanical devices into the pit (e.g., chains, coils of wire, bearing balls) to prevent complete implosion, arming involves removal of those devices. It may not always be possible to safe a mechanically armed device once the physical barrier to implosion has been removed.

Fuzing To ensure optimum weapon effectiveness by detecting that the desired conditions for warhead detonation have been met and to provide an appropriate command signal to the firing set to initiate nuclear detonation. Fuzing generally involves devices to detect the location of the warhead with respect to the target, signal processing and logic, and an output circuit to initiate firing.

Firing To ensure nuclear detonation by delivering a precise level of precisely timed electrical or pyrotechnic energy to one or more warhead detonating devices. A variety of techniques are used, depending on the warhead design and type of detonation devices.

Depending on the specific military operations to be carried out and the specific delivery system chosen, nuclear weapons pose special technological problems in terms of primary power and power-conditioning, overall weapon integration, and operational control and security.

Not all weapons possessors will face the same problems or opt for the same levels of confidence, particularly in the inherent security of their weapons. The operational objectives will in turn dictate the technological requirements for the SAFF subsystems. Minimal requirements could be met by surface burst (including impact fuzing of relatively slow moving warhead) or crude preset height of burst based on simple timer or barometric switch or simple radar altimeter. Modest requirements could be met by more precise HOB (height of burst) based on improved radar triggering or other methods of measuring distance above ground to maxmize radius of selected weapons effects, with point-contact salvage fuzing. Parachute delivery of bombs to allow deliberate laydown and surface burst. Substantial requirements could be met by variable HOB, including low-altitude for ensured destruction of protected strategic targets, along with possible underwater or exoatmospheric capabilities.

Virtually any country or extranational group with the resources to construct a nuclear device has sufficient capability to attain the minimum SAFF capability that would be needed to meet terrorist or minimal national aims. The requirements to achieve a "modest" or "substantial" capability level are much more demanding. Both safety and protection of investment demand very low probability of failure of safing and arming mechanisms, with very high probability of proper initiation of the warhead. All of the recognized nuclear weapons states and many other countries have (or have ready access to) both the design know-how and components required to implement a significant capability.

In terms of sophistication, safety, and reliability of design, past U.S. weapons programs provide a legacy of world leadership in SAFF and related technology. France and the UK follow closely in overall SAFF design and may actually hold slight leads in specific component technologies. SAFF technologies of other nuclear powers -- notably Russia and China -- do not compare. Japan and Germany have technological capabilities roughly on a par with the United States, UK, and France, and doubtless have the capability to design and build nuclear SAFF subsystems.

Reliable fuzing and firing systems suitable for nuclear use have been built since 1945 and do not need to incorporate any modern technology. Many kinds of mechanical safing systems have been employed, and several of these require nothing more complex than removable wires or chains or the exchanging of arming/ safing connector plugs. Safing a gun-assembled system is especially simple. Arming systems range from hand insertion of critical components in flight to extremely sophisticated instruments which detect specific events in the stockpile to target sequence (STS). Fuzing and firing systems span an equally great range of technical complexity.

Any country with the electronics capability to build aircraft radar altimeter equipment should have access to the capability for building a reasonably adequate, simple HOB fuze. China, India, Israel, Taiwan, South Korea, Brazil, Singapore, the Russian Federation and the Ukraine, and South Africa all have built conventional weapons with design features that could be adapted to more sophisticated designs, providing variable burst height and rudimentary Electronic Counter Counter Measure (ECCM) features. With regard to physical security measures and use control, the rapid growth in the availability and performance of low-cost, highly reliable microprocessing equipment has led to a proliferation of electronic lock and security devices suitable for protecting and controlling high-value/at-risk assets. Such technology may likely meet the needs of most proliferant organizations.