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Images source: The Swords of Armageddon -- U.S. Nuclear Weapons Development since 1945 VOLUME VI, Chuck Hansen, ed., Release 01, October 1995.
Source: The Swords of Armageddon -- U.S. Nuclear Weapons Development since 1945 VOLUME VI, Chuck Hansen, ed., Release 01, October 1995.
[Pages 20-24.]
Stockpile Problems
For all their apparent durability and massiveness, U.S. nuclear warheads are designed and built for a "lifetime" of only about 20 years. Many things happen to a warhead during this period:
Like other, very much simpler equipment such as automobiles and home appliances, nuclear weapons deteriorate with time. For nuclear weapons, it is not necessarily a matter of "wearing out": they are required to operate only once. There is essentially no wear except that associated with their transport and handling.However, like some home appliances that have been stored unused in a family's basement, when they are closely inspected they may be found to have deteriorated into an unusable condition.
This susceptibility to deterioration tends to be much higher for nuclear weapons than for other equipment. Various features of their must be guided mainly by the desired weapon performance and by safety considerations rather than by demands for resistance to deterioration. Many of the weapon materials have a low resistance to deterioration.
Certain chemically reactive materials are inherently required in nuclear weapons, such as uranium or plutonium, high explosives, and plastics. The fissile materials, both plutonium and uranium, are subject to corrosion. Plastic-bonded high explosives and other plastics tend to decompose over extended periods of time.
Deterioration can take various forms: portions of materials can dissociate into simpler substances. Vapors given off by one material can migrate to another region of the weapon and react chemically there. One potential source of effluent is the thermonuclear fuel lithium deuteride, which can react with water vapor and release hydrogen gas.
Materials in the warhead electrical system such as insulators, batteries, capacitors, squibs, and lubricants, can produce effluents that can migrate to regions in the nuclear explosive portion of a weapon. Over an extended period of time, plastic components in the explosive portion of a weapon can be the source of effluents. These components include plastic-bonded explosives and plastic pads and cushions. The plastics may decompose, or give off gases with which they were impregnated during production processes.
As one form of deterioration, corrosion can create fissures and perforations that would have undesirable effects on weapon operation. Mechanical components can have unacceptable friction introduced by corrosion or by other chemical effects. Corrosion products can flake off the surface of some components and vibration and other movements of the weapon can distribute these flakes ("dust") to locations where they could spoil the weapon's operation.
The characteristics of high explosives can change with time. Materials can creep and become distorted. Vital electrical components can change in character; electrical circuits can open or become shorted. Some changes can be such that they will obviously cause a weapon to dud; other changes can be more subtle, and their effects on weapon performance can be difficult to predict.14
14 SOME LITTLE-PUBLICIZED DIFFICULTIES WITH A NUCLEAR FREEZE, RDA-TR-122116-001, Jack W. Rosengren, R & D Associates, Marina del Rey, California, October 1983, pp. 5, 6. Records of warhead components are maintained in "bomb books" that list the serial numbers of every component contained in a particular weapon so that problems that may arise later can be pinpointed in the production process, allowing suspect components in other weapons to be identified and inspected. In addition, in case of an accident, specific parts of weapons can be identified and catalogued. (Furman, pp. 770, 771.)
In addition, both plutonium and uranium are chemically reactive and subject to corrosion. Early uranium cores often "spalled", releasing pepper-grain sized particles spontaneously; these grains could then migrate into other parts of the weapon.15
15 SIEGELSBACH: A NUCLEAR NE'ER DO-WELL'S ATOMIC ADVENTURE, John B. White, Aglor Publishing Company, Inc., Jacksonville, Texas, p. 55.
Thermonuclear fuels, including lithium deuteride and lithium tritide, react violently with water vapor and release hydrogen gas, which can become explosive when combined with oxygen. Materials in the warhead electrical system (WES) such as insulators, batteries, capacitors, explosive squibs, and lubricants of mechanical gears can produce effluents that travel to portions of the "physics package." There is never an absolute seal between the WES and the physics package: at the very least, there is a leakage path associated with detonator cables.
Certain portions of a warhead are more likely to have deterioration problems than are others. Some major trouble sources in the past have been high explosives, fissile materials, plastic pads, various adhesives, detonator bridgewires, and mechanical safing systems.16
16 Rosengren, RDA-TR-122100-001-Rev. 1, pp. 16, 17.
One reason for many of these problems is that a nuclear weapon designer is limited as to the materials he or she can employ effectively in a design. An effective weapon based on materials selected solely for their inertness, imperviousness, chemical stability, and resistance to corrosion cannot be devised. To be reasonably optimum, current designs must include certain materials with varying degrees of stability: fissile material and plastic-bonded explosives in the fission stage and probably tritium, used as a boost gas.
If the weapon is thermonuclear, the design will likely include Li6D and uranium-235 and uranium-238. In addition to these materials, there will also be some choice of metals, plastics, and other constituents.
Some of the components that must be included -- plutonium, uranium, Li6D, and tritium -- are very reactive chemically. Plastic-bonded explosive (PBX), in some of its formulations and over a long period, tends to decompose.
To reduce the likelihood of degradation, weapon scientists usually limit the interactions between these materials and other reactive components. The plutonium, Li6D, and tritium are each separately encapsulated and hermetically sealed, remaining in contact with only one or two other materials. The uranium and PBX can interact with many more materials but are sealed in the warhead as a whole.
The primary pit and the secondary fuel capsule are sealed, but there are almost always interconnections between the various other portions of the warhead, notably via the electrical system. Usually the firing set (warhead electrical system) is somewhat isolated from the physics package (the basic nuclear explosive). However, any barrier between the firing set and the HE system must be penetrated to pass detonators cables.
The holes for the cables provide pathways along which gases and vapors can diffuse. Thus, vapors that emanate from materials in one region of the warhead are able to migrate to other regions and interact with other materials there. Another unavoidable factor that promotes deterioration is the low-level nuclear radiation associated with fissile material, U-238, and tritium. As noted above, these substances are components of most nuclear weapons, and fissile materials -- Pu or U-235 -- are components of every U.S. nuclear weapon. Some conceptual designs have fissile material that is removable in the form of insertable nuclear components, INCs, but these will not soon replace many, if any, of the more standard weapons.
The neutrons and gamma rays emitted by spontaneous fission in plutonium and uranium will pervade a weapon, and in time, can promote chemical deterioration.17
17 Rosengren, RDA-TR-122100-001-Rev. 1, pp. 89, 90.
Approximately 30 different stockpiled U.S. nuclear warhead designs out of 40 MARK-numbered systems developed since 1958 have had either unexpected surprises during developmental nuclear testing, unexpected difficulties following modification, problems with new production, or postdeployment stockpile problems. According to one source, between 1960 and 1970, approximately 91 modifications were made to stockpiled weapons, of which 60 were to rectify deficiencies discovered after stockpiling.18
18 WORLD ARMAMENTS AND DISARMAMENT, SIPRI YEARBOOK 1978, Stockholm International Peace Research Institute, Crane, Russak & Company, Inc., New York, 1978, p. 325.