no replacements being built for them, the nuclear weapons in the
stockpile are sitting there, steadily aging. Materials inside
them may be developing minor sags and wrinkles, just as we humans
do over time. In the mid-1990s, in the first years after nuclear
testing stopped, the Department of Energy established an Enhanced
Surveillance Campaign to determine whether those sags and wrinkles
were indeed developing and, if so, whether they would affect the
safety and ultimate performance of the weapons.
Enhanced Surveillance Campaign is a part of the Stockpile Stewardship
Program managed by DOEs National Nuclear Security Administration
(NNSA). The detection and prediction of changes in an aging stockpile
are among the most challenging and technically engaging aspects
of stockpile stewardship. In the effort to understand stockpile
aging, the Livermore and Los Alamos national laboratories joined
forces with two NNSA plants, Pantex and Kansas City, to examine
the pits inside nuclear weapons. The pits are shells of plutonium
that play a key role in the performance of a nuclear weapon. The
energy released when the plutonium atoms fission, or split, helps
to start the huge fusion explosion of a modern thermonuclear weapon.
Knowing how the pit changes as it ages is critical to predicting
the performance of weapons in the stockpile.
Most pits in the U.S. stockpile
are now 10 to 20 years old. NNSA wants to be able to project their
lifetime to 60 years so decision makers will know what to expect
as the pits age and whether they will still be safe and reliable.
In response to that challenge, Livermore and Los Alamos scientists
developed a way to spike weapons-grade plutonium to prompt it
to age as much as 16 times faster than normal. At the same time,
some of the oldest pits in the stockpile are being examined to
establish a baseline against which the accelerated aging samples
can be measured. Researchers are also developing new diagnostic
methods for examining both old pits and spiked plutonium samples.
New computational models developed through the Accelerated Strategic
Computing Initiative will provide a basic understanding of plutonium
aging, eventually leading to a prediction of the lifetime for
plutonium pits. This project, the Pits Major Technical Effort,
will continue until at least 2007.
Tom Shepp is leading the Livermore portion of the project. According
to Shepp, We are working to protect the health of the stockpile
by providing advance warning of manufacturing and aging defects.
NNSA especially wants to know whether they will need to build
a facility for manufacturing new pits. NNSAs former
pit production plant at Rocky Flats has been closed for many years,
and reopening it is not an option. If pits are aging unacceptably,
they will have to be replaced, necessitating construction of a
modern manufacturing plant.
of plutonium from the stockpile were subjected to isochronal
annealing to induce the growth of voids. This sample was annealed
at 400°C for one hour. (a) A look at 400 nanometers of
material. (b) A closer view of just 200 nanometers of plutonium,
in which minute voids are clearly visible.
Aging Process Up Close
have two major concerns about aged plutonium: corrosion reactions
and the results of self-irradiation. Most Livermore research is
concerned with self-irradiation. When plutonium decays spontaneously,
it emits an alpha particle (a helium nucleus) to become uranium.
The heavy uranium atom recoils, displacing other plutonium atoms
and disrupting the surrounding microstructure.
Says Shepp, Spontaneous
decay creates a cascade of chaos. We know that most of the helium
atoms return to their old homes, but some dont, leaving
microscopic voids behind. When they find new homes, they cause
the pit to swell ever so slightly and may change the dynamic mechanical
properties of the pit material. Over time, changes in the density,
shape, and mechanical properties of the pit may affect the overall
performance of the weapon.
machines at Livermore have been particularly useful for examining
plutonium samples from the stockpile for material changes that
result from self-irradiation. One is the new 300-kiloelectronvolt
field-emission transmission electron microscope (TEM), the best
one in the DOE complex. The new TEM is providing a better understanding
of the microstructural evolution and stability of plutonium as
a function of age and deformation. The other machine is the three-dimensional
positron microprobe, which has the highest spatial resolution
of any positron analysis system in the world. Positron annihilation
lifetime spectroscopy can detect the size, location, and concentration
of possible voids in naturally aged plutonium. (See S&TR,
Up Close...Way Close, and December
Helps to Protect Our Nuclear Stockpile.)
Experiments by Livermore
scientists on the Los Alamos gas gun as well as nonnuclear tests
of plutonium at the Nevada Test Site are supplying more data points
for the dynamic properties of stockpile pits. These measurements
help assess how aging affects mechanical properties, including
the equation of state, dynamic tensile fracture (spall), work
hardening, yield strength, and generation of defect structures.
In another set of experiments,
plutonium is cooled to near absolute zero and then cycled to higher
and higher temperatures in a process known as isochronal annealing.
The process damages plutonium and provides scientists with a fundamental
understanding of the behavior of damaged plutonium.
These data come together
to create more accurate models that can predict aging effects,
overall performance, and the safety of pits and the weapons that
contain them. Livermore has produced the first simulations of
the quantum molecular dynamics of plutonium to study what is happening
to individual atoms over very brief time periods. At the other
end of the time spectrum, Monte Carlo statistical analyses examine
a representative selection of the millions of interacting atoms
and their daughter particles over the long-term processes of void
creation and resulting swelling.
Time Machine for Plutonium
These same experiments
will also be performed on spiked plutonium alloys. If typical
weapons-grade plutonium, plutonium-239, is spiked with some plutonium-238,
which decays more quickly, the self-irradiation process dramatically
picks up speed. If 5 percent of the plutonium-239 is replaced
with plutonium-238, the sample will age 11 times faster than normal
plutonium-239. Aging can be accelerated by a factor of 16 over
normal aging processes if 7.5 percent of the sample is plutonium-238.
A useful measure of acceleration aging is defined as the number
of years required to reach a radiation dose that results in 10
displacements per atom. Weapons-grade plutonium normally takes
100 years to reach this dose but will need just 6.25 years if
it is spiked with 5-percent plutonium-238.
The first batch of 7.5-percent-spiked
plutonium was created in May 2000 at Livermores Plutonium
Facility (see S&TR, March
the Superblock). In 2004, researchers will begin to study
batches of spiked plutonium that have aged 60 years,
the lifetime NNSA hopes to achieve. In fact, this accelerated
aging process will allow studies of samples that have aged well
beyond the ages of the oldest plutonium pits in the stockpile.
better understand damage to plutonium weapon pits, Livermore
scientists integrate modeling, theory, and experimentation.
In this example, experiments with the positron microprobe
supply data for better models of defects at various time scales,
which leads to improved theory about how best to use positrons
to predict defects. Better theory in turn makes for better
experiments and thus better models, in a virtually endless
Diagnostics and More Information
the next few years, several new diagnostic tools will be deployed
at Livermore, Los Alamos, and the Pantex Plant. They include a
high-resolution computed tomography system that Livermore is enhancing
for use at the Pantex Plant. Livermore is further developing laser-shock
diagnostics for pit surveillance. And its JASPER (for Joint Actinide
Shock Physics Experimental Research) gas gun will come on line
at the Nevada Test Site in about a year for shock tests of plutonium.
Shepp says, During
the first years of the project, we were getting ready by preparing
the spiked alloys and starting the baselining process. Now were
beginning to see results. During the upcoming year, the
team will validate the accelerated aging methodology by measuring
aged samples against pit samples of comparable age from the stockpile.
At the same time, Livermore and Los Alamos will finish characterizing
the oldest pit materials of the most common pit type.
All studies to date indicate
that the U.S. nuclear arsenal is robust and shows few effects
of aging. Identifying the time scales of plutonium deterioration
is critical for maintaining the continued safety and reliability
of the stockpile.
Enhanced Surveillance Campaign, nuclear weapons, pits, plutonium.
information contact Tom Shepp (925) 422-6192 (firstname.lastname@example.org).