U.S. Nuclear Weapon Enduring Stockpile

The U.S. nuclear arsenal is divided into three levels of stockpile readiness. These are:

* Operationally Deployed: These are active stockpile (fully operational) weapons and mated with delivery systems such that they are ready to be used in combat. All warheads counted under arms limitation agreements belong to this category.
* Active Stockpile: Fully operational weapons, available for immediate use, whether or not they are operationally deployed. Reasons for an active stockpile weapon to not be operationally deployed include:
o Its assigned to a delivery system is not currently operational (in particular ballistic missile submarines spend one-third of their time not on patrol),
o It is a spare for deployed warheads (should a deployed warhead require maintenance, for example), and
o It is part of the responsive force -- an inventory of warheads that are kept in operational condition (tritium reservoirs installed, etc.) to permit immediate deployment (for example to upload the number of wartheads on a ballistic missile, or reloads for bomber aircraft).
* Inactive Reserve: Weapons that are kept intact, but are not maintained in operational condition. This means that limited life components are removed from the weapons and may not be available to immediately return them to service. "Limited life components" principally mean tritium-containing components such as tritium reservoirs and neutron generator tubes. Some weapons currently in this category (e.g. the W84) will be dismantled.

At the beginning of 2007 the U.S. nuclear arsenal was composed of eight types of nuclear warheads (in thirteen variant mods) that are operationally deployed, with an estiamted count of 5,736 active stockpile warheads. For the first time the 2007 Annual Report on Implementation of the Moscow Treaty listed the aggregate number of U.S. operationally deployed strategic nuclear warheads which as of 31 December 2006 stood at 3,696. No official breakdown of this number is available, however if one assumes that ICBMs have 95% availability, 66% of SLBMs are on patrol, and 90% of bombers are on-duty with their full combat load, then this tally exactly matches the offical operational count.

One of the active warheads (the W87) is currently being redeployed (replacing the W62) after having been taken off of operational duty in the 1990s.

There are also 589 warheads of two types that are inactive, these are not kept in operational condition and one of these warheads (the W84) is slated to be completely dismantled.

The total number of warheads of all levels of readiness stands at 9,962 warheads. It should be pointed out that although precise numbers are cited here to keep tallies consistent and avoid cumulative rounding errors, they are in fact approximations. Even if exact numbers were available for one specific moment in time, continuing stockpile changes as a result of deployment shifts and inspection and maintenance actions causes actual numbers to fluctuate.

The total megatonnage of the deployed nuclear arsenal is about 1,430 Mt (but this is influenced by the choice of deployed weapons for bombers); for the entire active arsenal it is 2,330 Mt. The all-time high point in explosive yield was in 1960 when the U.S. held 20,491 Mt in its stockpile. The size of nuclear arsenals are often evaluated using "equivalent megatonnage" a scaling procedure that compensates for the fact that smaller explosions cause relatively more blast destructive for the amount of explosive energy released. An EMt value of one indicates the destructive effect of one 1 megaton bomb. Since most warheads in the U.S. arsenal are much less than one megaton this measure results in a larger value than the raw megatonnage. Using this measure the destructiveness of the deployed arsenal becomes 2,090 EMt, and the total active arsenal 3,405 EMt.

The United States has produced about 70,000 nuclear weapons of 72 major types since their invention. At the end of the Cold War in 1991 the United States had an active arsenal of some 23,000 weapons of 26 major types. Since that time actual nuclear warhead production has been completely shut down in the U.S., although warhead modification, retrofit, and maintenance activities continue. Much of the original nuclear weapons manufacturing infrastructure has been dismantled, and the focus of the remaining nuclear infrastructure has shifted to maintaining and extending the life of the remaining weapons, as well as dismantling surplus weapons.

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