Mark Wall of Lawrence Livermore National Laboratory uses Livermore's powerful transmission electron microscope to image a sample. Los Alamos researchers are collaborating with Livermore and the Atomic Weapons Establishment at Aldermaston, United Kingdom, to study the microstructural effects of the buildup of helium in aging plutonium. Recent research has revealed the existence of minute bubbles too tiny to be seen with conventional TEM instruments.
Photo courtesy of Lawrence Livermore National Laboratory
The ability to directly image self-irradiation damage accumulation in plutonium is critical to understanding aging. Scientists know that helium is building up in plutonium metal during self-irradiation. What they don't know is what is happening to that helium over time, and how it ultimately affects the behavior of plutonium over long periods.
Researchers from Los Alamos and Lawrence Livermore national laboratories and the Atomic Weapons Establishment at Aldermaston in the United Kingdom are collaborating on a study to observe the microstructural effects caused by the formation of helium atoms and vacancies during self-irradiation.
Recent studies using Livermore's state-of-the-art Transmission Electron Microscopy (TEM) facility have revealed the existence of minuteÑapproximately 1 nanometer in diameter-spherically shaped bubbles, which are too tiny to be seen with conventional TEM instruments. Scientists presume the tiny bubbles formed from the migration and coalescence of many helium-filled vacancy clusters, which occur as plutonium ages.
The existence of bubbles in plutonium has been seen before in heated samples. The fact that researchers saw bubbles in materials under approximately room-temperature storage conditions is somewhat surprising, according to Los Alamos researcher Tom Zocco of Manufacturing Systems (NMT-6).
"It implies that helium and helium vacancy clusters are mobile at room temperature and can cluster, forming the bubbles," said Zocco. "The formation of bubbles can have a variety of effects on the mechanical and physical properties of plutonium metals and alloys, which can possibly affect the long-term aging of our stockpile."
This schematic illustrates the radioactive decay process of a
plutonium-239 atom. The alpha particle releases and the uranium-235 atom recoils.
Zocco was one of the first researchers to successfully use TEM for the microstructural analysis of plutonium metal and alloys. As part of this collaboration, he has developed a sample matrix and supplied prepared and aged plutonium materials for examination. Livermore is finishing the sample preparation, performing the TEM operations, and providing image simulations. Researchers from Aldermaston also are providing material and expertise.
Radioactive materials are made up of atoms that are inherently unstable and decay over varying periods of time to form more stable atomic elements. For example, the unstable plutonium-239 isotope decays by the process of alpha emission. When the alpha particle is emitted, the loss of protons and neutrons from the plutonium-239 atom transmutes it to a uranium-235 ion. This uranium ion rapidly recoils during the alpha release, as in Newton's third law: For every action there is an equal and opposite reaction. This movement may cause significant damage to the surrounding atomic arrangement.
This two-dimensional representation shows the crystalline lattice and two types of damage caused by the radioactive decay process. Each red dot represents an atom. In the figure on the left, an interstitial atom (the black dot) is displaced and squeezed between other atoms. In the figure on the right, a vacancy is created by the alpha release and uranium-235 recoil.
Imagine a three-dimensional, periodic atomic arrangement-a lattice-of plutonium atoms in a crystalline structure. When any atom radioactively decays, the resulting uranium atom and helium nucleus fly apart, hitting other atoms as they travel through the lattice. Both the uranium and helium atoms generate substantial damage within the atomic arrangement of the crystalline structure, which results in defects or discontinuities in this normally periodic arrangement. The defects are primarily of two forms: vacancies or missing atoms in the lattice, or interstitial atoms, which are atoms squeezed between other regularly spaced atoms.
The amount of damage produced is directly related to the mass and energy of each moving particle. The uranium atom is large and does not travel far and deposits its kinetic energy over a short distance. This causes significant damage to the lattice and creates thousands of displaced plutonium atoms.
The alpha particle (or helium ion), on the other hand, is very energetic and travels farther through the lattice. But because the alpha particle is relatively small, it creates a lower number of lattice defects and less overall damage.
Because of the high local stresses created from squeezing the atoms into abnormal positions, most of them quickly return to the vacancies created when they were displaced from their original positions in the lattice. This "self-healing" process returns most of the atoms to their original, uniformly spaced positions.
However, the defects that do not self-heal ultimately result in the buildup of excess damage in the material. It is this lattice damage and its long-term accumulation that is of interest to researchers investigating the aging or self- irradiation damage phenomena in radioactive materials. (For more details on the aging effects in plutonium, see The Actinide Research Quarterly, 4th Quarter 1999.)
After the alpha particle comes to rest in the lattice, it rapidly attracts free electrons from its surroundings to become a helium atom. This process occurs at a pace that creates approximately 29 helium atoms per year for every 1 million atoms of plutonium. This may not seem like a significant amount, but over a period of years the accumulation of helium becomes substantial and potentially can bring about significant changes in macroscopic physical properties.
At the left is a raw (as-captured) digital Transmission Electron Microscopy (TEM) image. The image on the right has been processed and shows identified and measured bubbles. The existence of bubbles in plutonium has been seen before in heated samples. The fact that bubbles were seen in materials under approximately room-temperature storage conditions is somewhat surprising, according to researchers.
The remaining defects (vacancies and interstitials) that survive the self-healing process coexist with the helium atoms, forming complex interactive relationships. Helium atoms may readily combine with nearby vacancies to form helium-filled vacancies, which diffuse randomly until they meet and bind with other similar species, creating a bubble nucleus. The bubble nucleus grows as it captures additional helium-filled vacancies moving through the lattice.
Larger voids or bubbles, and/or those having associated strain fields, are readily observable in conventional TEM. However, the imaging and observation of very small voids (less than 2 nanometers in diameter) or small bubbles that are in equilibrium (no strain) with the surrounding lattice are difficult and require the use of a TEM with a highly coherent source of electrons and relatively high resolving ability.
The technique for imaging these small voids is called the defocus or "Fresnel fringe" imaging technique. Depending on the amount and direction of defocus, the small voids or bubbles will visually appear as small white or black spots with surrounding black or white fringes, respectively. The diameter of these circular fringes will vary with defocus, and is not easily related to the true diameter of the voids or bubbles.
For example, when measuring small (less than 1 nanometer) voids or bubbles, the diameter of the central bright or dark spots may be in error as much as 50 percent from the true diameter. Therefore, it is necessary to perform image simulations to correctly interpret the actual size.
After the under- or over-focused images are collected, they can be processed and analyzed. Through careful control of the processing parameters, image-processing software quickly identifies and measures the bubbles in a variety of ways, such as bubble density, mean diameter, area, aspect ratio, and roundness. By measuring or estimating the thickness of the TEM specimen and counting the number of bubbles in each image, researchers can calculate the true bubble density.
Through the use of complex image simulation techniques, Livermore researchers are determining how Fresnel contrast images of bubbles appear and change as a function of defocus and bubble position in the TEM sample. This may require correction factors for bubble size, to account for distortions produced from the many imaging effects.
The researchers also are modeling helium bubble nucleation and growth. By coupling experiments and modeling, they hope to develop a good correlation between bubble formation and age.
Contributors to this article are: Thomas Zocco (NMT-6); Mark Wall, Adam Schwartz and Bill Wolfer (Lawrence Livermore National Laboratory); and Paul Roussel (Atomic Weapons Establishment, Aldermaston, United Kingdom). Also contributing to this project are: Mary Esther Lucero and Michael Ramos (NMT-16). Figures: L. Kim Nguyen Gunderson (IM-1)
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