created a new, separate entity within DOE—the National Nuclear Security Administration (NNSA)—whose primary task is to maintain the nuclear weapons stockpile.

Every year, the SSP must assess the safety, reliability, performance, and effectiveness of the nuclear weapons stockpile in the absence of nuclear testing. The directors of the three national security laboratories—Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), and Sandia National Laboratories (SNL)—are required to submit letters each year to the Secretary of Energy with their assessment of whether the stockpile is safe, reliable, and effective and expressing their opinions on whether nuclear testing needs to be resumed in the subsequent year to assure those conditions.

In 2001, the three national security labs began using a framework called quantification of margins and uncertainties (QMU) to help with the assessment process. QMU is a decision-support framework that provides a means for quantifying the laboratories’ confidence that the critical stages of a nuclear weapon will operate as intended. In general terms, its purpose is to provide a systematic means to apply—using sophisticated simulation models—the varied output of the science base of the SSP to the assessment of the nuclear weapons stockpile. This output includes the aboveground nonnuclear and subcritical experiments, data from past underground nuclear tests, and expert judgments of the weapons scientists.

QMU is an important part of the assessment process and one that is growing in significance. It is also used to help set priorities for SSP research and engineering activities. And it helps to identify those components or operating characteristics of the various nuclear weapons in the stockpile that put them most at risk.

Recently, the NNSA reported that extending the life of the existing stockpile would become increasingly difficult over time.3,4 It has raised concerns about how the need for continual refurbishments of existing warheads could affect the reliability of the stockpile. Over time, it argued, there would be a buildup of small changes that would cause the warhead to become more and more removed from the tested design.5 To counter


U.S. Congress, Congressional Research Service, The Reliable Replacement Warhead Program: Background and Current Developments, RL32929 (updated November 8, 2007), p. 1.


This concern relates not so much to replacement of control systems and electronics that can be fully tested but more to the nuclear explosive package itself. Thus, the nuclear explosive package is the primary target of the QMU process. It is important to clearly document changes already made or expected to be made to the nuclear explosive package that would result in the warhead becoming more and more removed from the tested design.


U.S. Congress, Testimony by NNSA Acting Administrator Thomas D’Agostino before the House Armed Services Committee, March 20, 2007.

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