X-Ray Fluorescence Is Useful for Actinide Characterization

In addition to Principal Investigator George J. Havrilla, the XRF Team includes Bill Hutchinson, Margie Moore, Lisa Colletti, Forrest Weesner, and Christopher Worley of CST-8 and Jon Schoonover of CST-4.

Although there is no "silver bullet" analytical method that can answer all questions about all actinide sample types and problems, x-ray fluorescence (XRF) spectrometry is one of the myriad of analytical methods that are available to answer such questions. X-ray fluorescence spectrometry is a mature analytical method used to determine elemental composition in a wide range of sample types.

At the left is a sample of process residue from Rocky Flats in the form of ash, placed on a piece of tape. At the right is an elemental map of the plutonium distribution within the ash sample using x-ray microfluorescence. The elemental map was acquired with a 100-micron aperture.

The fundamental process is based upon the removal of a core electron from the sample by x-rays from an x-ray tube. The resulting core electron vacancy is filled by an outer-shell electron, which emits an x-ray that is characteristic of the element. This provides the qualitative part of the analysis or answers the question, "what elements are present?" The intensity of the x-ray fluorescence detected is directly proportional to the concentration of the element in the specimen. This value gives the quantitative part of the analysis or answers the question, "how much of the element is present?" This relatively simple process is used throughout the world in analytical laboratories and plants to monitor processes, identify contaminants, and solve problems. At Los Alamos, XRF is an important tool in providing characterization information on a wide variety of samples and supporting a number of different programs.

Although the fundamental process is simple, the instrumentation used in XRF is neither simple nor inexpensive. The basic instrumentation of power supply, excitation source, and detector is universally similar, but the hardware details are sufficiently complex to push instrument base prices over $100K. There are two types of XRF instruments available, wavelength-dispersive XRF (WDXRF) and energy-dispersive XRF (EDXRF). While x-ray tubes are common to all XRF instruments, they vary in output depending upon the instrumentation. Most WDXRF systems have high-power tubes, typically 3 kW­4 kW and use crystals for diffracting the emitted x-rays before they are detected. The WDXRF instrument thus detects elements sequentially, one at a time. There are simultaneous instruments; however, these simply have separate detector channels for each element. The WDXRF instruments offer sensitivity, resolution, and both long- and short-term stability.

The EDXRF instruments utilize a solid-state detector that captures all elemental x-rays simultaneously. Their rapid analysis is offered at the expense of spectral resolution and ultimate sensitivity. In general, both instruments can handle solid and liquid samples with sensitivities in the tens of parts per million. The detection limits vary by element as well as by the matrix. The routine samples we analyze by XRF include gallium and trace uranium in plutonium and plutonium oxide samples. Although other elemental methods are capable of detecting these elements, the nature of the plutonium matrix creates additional challenges that affect the accuracy, precision, and speed of the analysis in these other methods.

The gallium and uranium analyses begin the same with sample dissolution, followed by removal of the plutonium matrix with ion exchange resins. The gallium is eluted from the ion exchange column and collected in a beaker. Then a zinc internal standard is added, and the sample is analyzed directly. This process can provide precision with values approaching ± 0.1%. The trace uranium, on the other hand, is collected and then concentrated on a resin-impregnated filter paper. This offers us sensitivities in the low parts-per-million range. While precision is not an issue, this method avoids the isotopic interference from the plutonium matrix. Both of these analyses support such programs as pit rebuilding, surveillance, and development of mixed-oxide (MOX) fuels.

Other analyses involving actinide materials are applied to samples from Hanford and Rocky Flats. In these analyses, the sludge or ash particles are analyzed directly to provide a qualitative identification of the elements present. We are currently developing a semiquantitative method that will provide both qualitative identification and relative quantitative values of the detected elements. Although this analysis will not provide absolute concentrations, the values will be referenced to known standards for lot-to-lot comparisons. This procedure takes advantage of the rapid sample analysis and minimum sample preparation requirements, keeps the per sample costs low, yet still provides the needed composition information on the sample.

The Source Term Testing Program (STTP) project is another area where XRF is providing qualitative and quantitative data. In STTP brine samples are withdrawn from test containers and filtered. The XRF team receives both the brine and filters for analysis. The XRF results give a picture of what is dissolved in the brine and what is suspended within the liquid phase of the test material. Both sample types offer challenges in analysis since calibration standards that match the matrix of the unknowns must be fabricated. Although these analyses are not absolute because of the widely varying test container compositions, the values provide a relative scale for comparing the effects of the brine interactions with the different waste forms being tested.

In addition to these routine methods, we are actively pursuing new and innovative ways to use XRF so we can meet future characterization needs. One area of research is the use of x-ray microfluorescence (XRMF), which utilizes a spatially restricted x-ray beam to excite a specific location on a specimen. The detected x-rays are localized and can provide information on heterogeneity, inclusions, thin films, and interfaces. Our current program has several thrusts that offer new approaches to actinide characterization. The dried spot method has the potential for rapid, multielemental analysis on small masses of volumes of material. The method utilizes 10-ml to 50-ml drops of solution, which are dried. The resulting dried residue, which is mere micrograms of material, is analyzed with sensitivities approaching less than 1 part per billion. This is 3 orders of magnitude better than conventional, bulk XRF. The primary advantage lies in the small sample size, which also helps to keep exposures to personnel as low as reasonably achievable in sample handling and analysis.

Joel Dahlby, CST-8, operates an x-ray microfluorescence instrument, an innovative use of XRF to excite a specific location on a specimen. The technique can be used to perform spatially resolved elemental analysis for concentrations of 10 ppm to weight percent levels.

Rapid, spatially resolved analysis of MOX surrogates is providing insights into the evolution of gallium from potential MOX reactor fuel pellets. The presence of gallium is a critical issue for disposing of weapons plutonium in MOX fuel. Although most of the gallium can be removed, the mechanism of the removal process and behavior of the residual gallium need to be studied. The capabilities of XRMF allow us to study the gallium behavior on a scale that does not require the high resolution of a scanning electron microscope.

Finally, the ultimate goal is to develop an instrument that uses chemical images to provide both elemental and molecular information simultaneously. These chemical images will rapidly transmit both qualitative and quantitative, spatially resolved information on the elemental and molecular composition of the sample. This vision is based on integrating elemental and molecular spectroscopic data from XRMF, micro-Raman, micro-FTIR (Fourier transform infrared spectroscopy) and x-ray microdiffraction. The principal advantage of this integrated approach is the nondestructive, comprehensive chemical information on either small samples or spatially resolved images of macroscale specimens. This multiplexing of spectroscopic information would improve characterization accuracy and minimize multiple sample preparations to solve analytical problems.

This is just a snapshot of the importance of only one analytical method and how analytical chemistry as a whole underpins actinide science and actinide production efforts and how analytical science impacts all of the scientific efforts at Los Alamos.

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