LAWRENCE Livermore and Los Alamos—the
two national laboratories that designed the nuclear systems in
U.S. nuclear weapons—are working together to develop an improved
methodology for verifying the performance of these systems and
for presenting those data in a common format. Known as quantification
of margins and uncertainties (QMU), this methodology draws together
the latest data from simulations, experiments, and theory to quantify
confidence factors for the key potential failure modes in every
weapon system in the stockpile.
The assertion that the nuclear explosive package in a weapon performs
as specified is based on a design approach that provides an adequate
margin against known potential failure modes. Weapon experts judge
the adequacy of these margins using data from past nuclear experiments,
ground and flight tests, and material compatibility evaluations
during weapon development as well as routine stockpile surveillance,
a program of nonnuclear tests, and computer simulations.
“With QMU, we’re still examining margins against potential failure
modes,” says Charles Verdon, who leads A Program in Livermore’s Defense
and Nuclear Technologies (DNT) Directorate. “But now the assessment of
these margins relies much more heavily on surveillance and computer simulations
than in the past and therefore must be more rigorous and quantifiable.”
The Confidence Factor
factor for a component or system is defined as the performance
margin divided by the uncertainty in evaluating that margin. For
a nuclear weapon, if
the confidence factor for each potentially significant failure mode is greater
than or equal to 1, the overall system can be considered safe and reliable.
A nuclear warhead
or bomb is designed to operate successfully at a slightly lower performance level
than those defined for the worst-case scenario of potential
operating conditions. In defining the worst-case scenario,
weapon experts consider numerous events that may occur during a weapon’s
lifetime, such as extremely cold atmospheric temperatures, vibration, or tritium
decay between a gas-transfer-system exchange—any one of which could reduce
weapon performance. The difference between these two levels—minimum required
performance for successful operation versus the best estimate of the worst-case
performance—constitutes the performance margin.
variables affect how a weapon will actually perform. Some of these
variables, such as changes
that alter the structural integrity of the weapon’s outer
casing or the behavior of plutonium as it ages, give rise to uncertainties about
the best estimates of the minimum performance required for a weapon to remain
militarily useful and of the worst-case scenario for its operation. Technical
uncertainties are the root cause of such variables. For example, the equation
of state for plutonium, which is arguably the most important material in the
nuclear weapons stockpile, is not yet well understood when it is at the high-temperature
and -pressure conditions that exist in a nuclear detonation. (See S&TR,
2004, A First Look at Plutonium’s Phonons.)
According to Kent
Johnson, DNT chief of staff, reducing these technical uncertainties
drives Livermore’s continuing quest to understand the multitude of weapon
constituents through experiments and simulations. “As our understanding
increases, uncertainties may also increase for a while,” he says, “but
ultimately, we expect uncertainties to decrease considerably.”
Today, no new nuclear
weapons are being developed, and those in the current stockpile are being maintained
beyond their originally planned lifetimes. To ensure the
performance of these aging weapons, Livermore and Los Alamos take a survey–assess–refurbish
approach to evaluating the stockpile. QMU is the methodology being used in the
assessment part of this approach, to help weapon scientists identify where and
when they must refurbish a weapon system. QMU is also proving useful for deciding
whether the designed refurbishments are adequate.
of stockpiled weapons has been a feature of weapon maintenance for decades, and
it continues today. A more aggressive approach to surveillance
under the National Nuclear Security Administration’s (NNSA’s) Stockpile
Stewardship Program examines individual components to understand the aging process
and its effects, if any, on overall performance. Nonnuclear testing at the Contained
Firing Facility at Livermore’s Site 300, the JASPER gas gun at the Nevada
Test Site, and the National Ignition Facility at Livermore—all of which
were developed since nuclear testing ceased—is critical for scientists
to better understand the behavior of weapon materials under the extraordinarily
high temperatures and pressures that occur during a nuclear explosion. Livermore’s
terascale supercomputer ASCI White, one of the largest in the world, makes possible
high-resolution simulations that incorporate most of the physical interactions
that occur during a nuclear explosion.
An example of the relationship between
the performance of a component and the overall nuclear
weapon system. Uncertainties at both ends of the performance
margin may reduce the margin. Numerical simulations, nonnuclear
tests, data from past underground experiments, and the
latest theory are combined to quantify technical uncertainties
(the sum of the magnitude of the two uncertainty arrows)
and the performance margin.
all of this information—plus the latest physics theory
and useful historic nuclear test data—to arrive at quantifiable information
with which decisions can be made about weapon certification or to answer questions
about any weapon or weapon component in the stockpile. In 2001, QMU was successfully
applied in the certification process for the W87 Alt342, the major refurbishment
of the W87 nuclear weapon that was pursued through the warhead’s life-extension
The goal is to fully
integrate QMU into the nation’s formal Annual Assessment
of the entire stockpile of nuclear weapons. Each year, the directors of Livermore,
Los Alamos, and Sandia national laboratories sign a letter to the President stating
whether the weapon systems designed by each laboratory meet all safety, reliability,
and performance requirements. By using QMU methodology, the laboratories will
have a common framework for all of the necessary evaluations that comprise the
Making QMU Work
implement the QMU process for a Livermore-designed weapon in the
stockpile, weapon experts first identify a set of components on
which to focus in-depth
analysis. Teams of experts define watch lists of credible failure modes and performance
issues. For example, they are concerned with how current weapons will perform
at extreme temperatures and whether component aging will affect performance.
They also watch for such conditions as detonator deterioration and metal corrosion.
first step in quantifying margins and uncertainties for a warhead
or bomb designed by Livermore is to identify a watch list of
potential failure modes and issues.
“The things on the
list are the ones that keep us awake at night,” says
Verdon. “We want to know what parts might be approaching the edge of the
performance margin, particularly if there are variables that could affect performance
even more. Then we know that our scientists are working on the truly sensitive
issues. During this continuing process, we must also stay vigilant for the unexpected.”
For weapons that
are being modified to prolong their life in the stockpile, new engineering features
and proposed changes receive the same scrutiny. In these
life-extension projects, weapon scientists must determine quantitative answers
to questions such as: Are the proposed changes a good idea? Does a modification
fix the problem it was designed to solve? Does the modification introduce other
Experts have developed
a taxonomy of uncertainties: known uncertainties, known unknowns, and unknown
unknowns, for which scientists are always on the lookout.
An example of a known uncertainty is the structural integrity of the weapon’s
casing. Engineering details are well known, but vibration during flight may crack
the case and cause contents to be rearranged. This known uncertainty can be accommodated
through design by building in large margins but perhaps at some weight penalty.
A known unknown is,
for example, the equation of state for plutonium at conditions critical to weapon
performance. In this instance, scientists “know what
they don’t know” and are working to fill in the gaps in their knowledge.
That way, they can use their models with confidence to address such issues as
the effects of age or manufacturing changes.
An unknown unknown
is one in which researchers “don’t know they don’t
know.” An example is an anomaly in data from past underground nuclear experiments.
Several tests of a weapon gave the same result, but another, whose parameters
appeared to be similar, provided an unexpected result. “We don’t
know why it happened,” says Johnson, “and we need to figure it out.” High-fidelity
experiments, simulations, and data from past underground tests help scientists
move the known-unknown and unknown-unknown uncertainties into the known uncertainties
category, thus reducing overall uncertainty. Confidence factors would then increase,
unless the new results indicated that margins had been overestimated.
An essential component
of this process is open and critical evaluation of results. Workshops, peer reviews,
joint evaluations with Los Alamos personnel, and senior
advisory panels are all venues for exchanging ideas and expertise. Equally important
is that the team determining the final confidence factors for a component is
not the same team that developed the original watch list for it.
second life-extension project for a warhead is under way now. The design team
responsible for refurbishing all W80 warheads in the stockpile
is using the QMU process to ensure that all credible failure modes have been
considered and properly addressed. “The goal is to demonstrate through
tests and calculations the set of confidence factors greater than one that are
needed for certification of the W80 in 2008,” says Johnson.
Into the Future
has proved to be an excellent tool for addressing a range of concerns
related to the existing stockpile. In addition, it may eventually
be applied to other
responsibilities of the NNSA’s weapons program. Pits, which include the
inner shell of plutonium in the primary part of a weapon, change slowly with
age as plutonium decays, perhaps reducing the margin for proper performance of
the primary. The U.S. does not currently have a pit production facility for replacing
existing pits. Is a dedicated production facility needed? And if so, by what
These questions cannot
be answered definitively yet, but QMU will play a role
in formulating the answers.
Key Words: quantification of margins and uncertainties (QMU),
Stockpile Stewardship Program.
For further information contact Charles Verdon
(925) 423-4449 (firstname.lastname@example.org).
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