Stewardship, the Department of Energy's program for assuring the
long-term safety and performance of the nuclear weapons stockpile
without underground testing, has created a heightened focus on better
At Lawrence Livermore, a
number of experiments are under way to measure the structural, electrical,
and chemical properties of plutonium and its alloys and to determine
how these materials change over time. The measurements will enable
scientists to better model and predict plutonium's long-term behavior
in the aging stockpile (see the article in this issue entitled "Inside the Superblock").
"Plutonium is a complex and
perplexing element," notes metallurgist Adam Schwartz. "For instance,
plutonium has seven temperature-dependent solid phases—more than
any other element in the periodic table. Each phase possesses a
different density and volume and has its own characteristics. Alloys
are even more complex; you can have multiple phases present in a
sample at any given time."
Because plutonium is so complex,
surrogate materials cannot give a complete picture of plutonium's
characteristics. With the importance of stockpile stewardship, the
Laboratory has seen a resurgence of interest and research in plutonium
and the other actinide elements (see S&TR, June
the Secret of Actinides"). One area that Schwartz, microscopist
Mark Wall, and physicist Bill Wolfer are pursuing as part of their
stockpile stewardship responsibilities is the evolution of damage
to plutonium's structure. As with the atoms of all metals, plutonium
atoms form structures on scales as small as a billionth of a meter.
These microstructures are constantly changing because of plutonium's
radioactive nature. When an atom of plutonium-239 (the isotope of
plutonium used in nuclear weapons) decays, it splits into an alpha
particle--a helium nucleus with two protons and two neutrons--and
an atom of uranium-235. The heavy uranium atom recoils, displacing
other plutonium atoms and disrupting the surrounding microstructure.
Scientists are concerned that the buildup of gaseous helium atoms
combined with other elements in the weapon's environment might gradually
change the properties of the plutonium metal.
An atomic resolution image of plutonium. Such an image was created
for the first time ever by the team studying plutonium properties
with a transmission electron microscope. (b) A high-resolution
computed image of plutonium's atomic structure.
Inside Scoop with the Transmission Electron Microscope
to Mark Wall, the new 300-kiloelectronvolt transmission electron
microscope (TEM) leased by the Laboratory is the best of its
kind in DOE's weapon complex. "Having a high accelerating
voltage allows us to see through thicker specimens, facilitating
more microstructural observations and better image resolution,"
says Wall. The TEM is used to characterize the internal structure
of a wide variety of materials, not just plutonium. It not
only can image the microstructure directly, but can also identify
the phases present in a specimen. The TEM characterization
techniques are cataloged here under headings that describe
what they do (although there is some overlap among the techniques):
of Atomic Structure
Atomic Structure Imaging: Directly resolves the atomic
structure of crystalline materials down to individual columns
of Microstructure, Defects, and Phases
Bright Field: Images the internal microstructure of
materials, including grain and defect structures such as dislocations
and voids. Can also be used to observe precipitates or inclusions.
Dark Field and Weak Beam: Allows researchers to link
diffraction information with specific phase regions in the
sample. Weak-beam imaging is dark-field imaging at higher
resolution and is primarily used for imaging closely spaced
defect structures on the nanometer scale.
Electron Diffraction (Selected Area Diffraction) and Microdiffraction:
Both techniques help researchers identify internal crystal
structures. Selected area diffraction allows researchers to
view and record the electron diffraction pattern from selected
areas as small as 0.5 micrometer. Microdiffraction allows
analysis of regions as small as 1 nanometer.
Convergent Beam Electron Diffraction: Reveals diffraction
details that provide additional three-dimensional crystallographic
and symmetry information.
Lorentz Microscopy: Images directional variations in
the magnetic field within thin samples.
In Situ Microscopy: Allows researchers to record the
evolution of a material's microstructure during heating, cooling,
and mechanical deformation.
of Chemical Composition and Impurities
Energy-Dispersive Spectroscopy: Produces x-ray spectra
that reveal the presence and amount of elements (for carbon
and heavier elements).
Parallel Electron Energy Loss Spectroscopy: Complements
energy dispersive spectroscopy, in that it is more sensitive
to light elements, including lithium and heavier elements.
Energy-Filtered Transmission Electron Microscope: Acquires
real-time, quantitative chemical "maps" of a specific region
with a resolution as small as 1 nanometer.
Beneath the Surface
To better understand the
basic nature of this complex metal and search out the long-term
effects of the weapon environment, scientists must know what goes
on at the atomic level. To aid this endeavor, the Laboratory acquired
a 300-kiloelectronvolt, field-emission transmission electron microscope
(TEM) about one year ago. This remarkable instrument uses electrons
instead of light waves to "see," so features can be resolved, or
viewed at the atomic scale. Where most microscopes can only probe
the surface of materials, a TEM looks directly at the internal structure
of materials, explains Wall.
The primary strength of the
instrument is that it can provide detailed characterization simultaneously
over many length scales and at high resolution—from hundreds of
micrometers to nanometers—and do this in either imaging, spectroscopic,
or diffraction modes (see box above). "In principle, we can observe
and measure the defects and composition of microstructural features
in these materials down to the nanometer level," says Wall.
Schwartz and Wall start with
plutonium samples measuring less than 3 millimeters in diameter
and 150 micrometers thick. They then use special sample preparation
techniques to thin each sample until it is transparent to high-energy
electrons, that is, to between 10 to 100 nanometers in thickness.
The specimens are then vacuum-transferred to the TEM for characterization
experiments. The resulting electron micrographs reveal in unprecedented
detail the nature of the material and any defects in it. During
this work, Schwartz and Wall produced the first-ever image of plutonium
at the atomic level.
Using samples of plutonium
from old, disassembled nuclear warheads and comparing their resulting
micrographs to those from newly cast plutonium, the researchers
can better determine the kinds and amounts of defects and changes
that occur over time. In particular, they look for voids or bubbles
created by recoiling uranium nuclei and the gaseous helium from
alpha particles. An example from an old material annealed to intentionally
form bubbles is shown in the image directly below. Dislocations—which
can be described as an extra half plane of atoms—can create sinks
or sources for radiation damage (see image at bottom).
or bubbles could be created by recoiling uranium nuclei and
gaseous helium from alpha particles that result from plutonium
decay. Here, an aged sample has been intentionally annealed
to create bubbles.
dislocation--an extra half plane of atoms--in the plutonium structure can create
sinks or sources for radiation damage.
Far, So Good
To date, the news for the
stockpile is encouraging. Schwartz sums up the results as "So far,
so good. We haven't seen any issues or surprises with the pit samples
we've viewed." Last year, the team began another project, looking
at special plutonium alloys that have been prepared to accelerate
the rate of aging. For Livermore's Enhanced Surveillance Program
(see S&TR, September
1999, "A Better
Picture of Aging Materials"), scientists have made several alloys
spiked with plutonium-238, which decays much faster than plutonium-239,
to try to understand what will happen with stockpiled plutonium
as it ages.
Schwartz and Wall also plan
to conduct in situ microscopy of plutonium. Heating plutonium samples
up to 400 degrees Celsius will allow researchers to see helium bubbles nucleate
and for the first time see the early stages of nucleation. "In essence,
we'll be speeding up the kinetics of the material and increasing
the diffusion rate," said Schwartz.
plutonium research, stockpile stewardship, transmission electron
information contact Adam Schwartz (925) 423-3454 (email@example.com).