Appendix. Nuclear weapons, nuclear weapons complex, and stockpile stewardship program.(The Reliable Replacement Warhead Program: Background and Current Developments)

This report refers to nuclear weapons design, operation, and production throughout. This Appendix describes key terms, concepts, and facilities as an aid to readers not familiar with them.

Current strategic (long-range) and most tactical nuclear weapons are of a two-stage design. (158) The first stage, the "primary," is an atomic bomb similar in principle to the bomb dropped on Nagasaki. The primary provides the energy needed to trigger the second stage, or "secondary."

The primary has at its center a "pit," a hollow core containing fissile material (typically plutonium) and containment shells of other metals. It is surrounded by chemical explosive shaped to generate a symmetrical inward-moving (implosion) shock front. When the explosive is detonated, the implosion compresses the plutonium, increasing its density so much that it becomes supercritical and can sustain a runaway nuclear chain reaction. A neutron generator injects neutrons into the plutonium. The neutrons drive this reaction by splitting (fissioning) plutonium atoms, repeatedly doubling the number of neutrons released. But the chain reaction can last only the briefest moment before the force of the nuclear explosion drives the plutonium outward so that it becomes subcritical and can no longer support a chain reaction. To increase the fraction of plutonium that is fissioned, boosting the yield of the primary, another system injects "boost gas"--a mixture of deuterium and tritium (isotopes of hydrogen) gases--into the pit before the explosive is detonated. The intense heat and pressure of the fission chain reaction cause this gas to undergo fusion. While the fusion reaction generates energy, its purpose is to generate a great many neutrons and thus "boost" the fission chain reaction to a higher level.

A metal "radiation case" channels the energy of the primary to the secondary, which contains fission and fusion fuel. The energy ignites the secondary, which releases most of the energy of a nuclear explosion. The primary, radiation case, and secondary comprise the "nuclear explosive package." Thousands of "nonnuclear" components are also needed to make the nuclear explosive package into a militarily usable weapon, such as an arming, firing, and fuzing system, an outer case, and electrical and physical connections linking a bomb to an airplane or a warhead to a missile.

Nuclear weapons were designed, tested, and manufactured by the nuclear weapons complex, which is composed of eight government-owned contractor-operated sites: the Los Alamos National Laboratory (NM) and Lawrence Livermore National Laboratory (CA), which design nuclear explosive packages; Sandia National Laboratories (NM and CA), which designs nonnuclear components; Y-12 Plant (TN), which produces uranium components and secondaries; Kansas City Plant (MO), which produces many of the nonnuclear components; Savannah River Site (SC), which processes tritium from stockpiled weapons to remove decay products; Pantex Plant (TX), which assembles and disassembles nuclear weapons; and the Nevada Test Site, which used to conduct nuclear tests but now conducts other weapons-related experiments that do not produce a nuclear yield. These sites are now involved in disassembly, inspection, and refurbishment of existing nuclear weapons. The National Nuclear Security Administration (NNSA), a semiautonomous part of the Department of Energy, manages the nuclear weapons complex and program.

NNSA maintains nuclear weapons and associated expertise through the Stockpile Stewardship Program (SSP), which Congress created in the FY1994 National Defense Authorization Act (P.L. 103-160, section 3138). The legislation specified that the goal of SSP is "to ensure the preservation of the core intellectual and technical competencies of the United States in nuclear weapons" through "advanced computational capabilities," "above-ground experiments" (experiments not requiring nuclear testing), and construction of large experimental facilities. SSP has three main elements. Directed Stockpile Work involves work directly on nuclear weapons in the stockpile, such as monitoring their condition, maintaining them through refurbishment and modifications, R&D in support of specific warheads, and dismantlement. It includes the Life Extension Program and the RRW program. Campaigns provide focused scientific and engineering expertise in support of Directed Stockpile Work, in such areas as pit manufacturing and certification, computation, and study of the properties of materials. Readiness in Technical Base and Facilities funds infrastructure and operations at the nuclear weapons complex sites. While the legislation did not specify that SSP was not to involve nuclear testing, that goal seems clear from the history, and has become a goal of the program. NNSA does not rule out the possible need for testing, such as if a problem were to emerge in a warhead type that could not be remedied in any other way, but the United States has been able to maintain its nuclear stockpile without testing since 1992.

(1) See, for example, George Shultz, William Perry, Henry Kissinger, and Sam Nunn, "A World Free of Nuclear Weapons," Wall Street Journal, January 4, 2007, p. 15.

(2) R.J. Hemley et al., Pit …