A typical modern U.S. nuclear weapon consists of approximately 6,000 individual components. Of these, only the so-called nuclear subsystem, which includes the weapon’s “primary” and “secondary,” would be subject to the restrictions of a CTBT. The remaining components of a nuclear weapon and its delivery system could be tested at will under a CTBT.

It is assumed explicitly that the enduring stockpile will not depend on the introduction of new nuclear subsystem designs that would require nuclear testing for performance validation. However, the SSP is required to maintain the capability for executing new designs should a compelling need ever arise for such. In the shorter term it is conceivable that nuclear subsystems of established nuclear-test pedigree might be incorporated into new weapons to maintain compatibility with evolving strategic and tactical military delivery systems. This should not introduce uncertainties in weapon performance provided that none of the modifications intrude on the nuclear subsystem beyond established design practices.

The expectation that the United States will ratify the CTBT at some point in the future has fueled concerns in some quarters that confidence in the stockpile will inevitably erode over time in the absence of nuclear testing, notwithstanding the SSP. The fact that the stockpile has declined both in total numbers as well as in numbers of weapon types has added to this concern. In the remainder of this chapter we address these issues in four steps: a historical perspective on nuclear testing; a discussion of the factors affecting nuclear-weapon safety and reliability in the context of a CTBT; an analysis of five elements of an effective stockpile stewardship program; and a treatment of some issues in priority-setting in stockpile stewardship. We do not offer here a comprehensive or detailed assessment of the initiatives and facilities of the SSP—this would have been beyond our mandate—but confine ourselves mainly to the effects, on stewardship requirements and effectiveness, of a cessation of nuclear-explosive tests.

Nuclear Testing: Historical Perspective

Since 1945 the United States has amassed an extensive data, knowledge, and experience base derived from over 1,000 individual test explosions. The first 20 years were marked by rapid progress as measured by increasing yields, increasing yield-to-weight ratios, and advances in other militarily significant features. During the 1960s and 1970s tailored output concepts were explored, such as reduced residual-radiation, enhanced neutron, and hot X-ray devices. None of these advanced concepts, however, attracted lasting support from the military services, and none is part of today’s enduring stockpile. During the 1970s and early 1980s the advances in nuclear-explosive technology reached a plateau as the nuclear designs of interest to the services approached performance limits set by the laws of physics. It is during this same period that the current stockpile designs evolved.

From a knowledge standpoint, nuclear-test data obtained during the past 25 years are of greatest relevance since many of these bear directly on the designs in the enduring stockpile and were obtained from well-diagnosed tests. Most nuclear tests were focused on the development of new designs—ranging from exploratory concepts to designs aimed at specific requirements promulgated by the military—although only a small fraction of the warhead designs subjected to nuclear testing were identical to the warhead designs that actually entered the stockpile. Other tests centered on weapon-physics issues not related to a specific weapon-development program. Some pursued novel ideas, such as the X-ray laser. Many primaries utilized in nuclear tests conducted for a variety of these purposes, however, were taken from the stockpile production line, which enabled these tests to contribute something to stockpile confidence. Certainly, the totality

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