Feasibility of Uranium Detection Through Container Walls Using Ultrahigh-Energy X-ray Fluorescence
George J. Havrilla , 1 * Velma Lopez , 1 Kathryn McIntosh , 1 W. T. Elam , 2 and Doug Robinson 3 1 Los Alamos National Laboratory , Los Alamos , NM 87545 2 University of Washington , Applied Physics Laboratory , Box 3555640 , 1013 NE 40th St. Seattle , WA 98105 3 Argonne National Laboratory , Advanced Photon Source , Argonne , IL 60439
*
havrilla@lanl.gov
Introduction T e quantitative analysis of uranium in spent nuclear fuel is at best a challenging task, even for the specialized laboratories designed to handle such materials. T e reason this measurement is important is the need to account for all nuclear materials under the control of international safeguards agreements. Although there are existing methods employed for this measurement, eff orts to improve the accuracy of nuclear material accountability are of continuing interest. A new approach for making such measurements involves ultrahigh-energy X-ray fl uorescence (UHEXRF). UHEXRF is defi ned in this application as XRF above 80 keV. Although there have been previous eff orts exploring the use of high-energy XRF, almost all these have dealt with XRF below 80 keV [ 1 ]. T e appeal of using UHEXRF for quanti- tative analysis of uranium in spent nuclear fuel is primarily focused on the very penetrating X rays of the fl uorescent radiation [ 2 ]. In the case of uranium, the Kα line is at 98.428 keV with an absorption edge energy of 115.591 keV. At these energies, both the exciting and fl uorescent radiations can penetrate signifi cant shielding. In this particular case, the Zircaloy (zirconium metal alloy) cladding of nuclear fuel rods provides a signifi cant barrier to most spectroscopic elemental analysis methods. For excitation energies just above the uranium K edge, the calculated penetration depth is on the order of several hundred micrometers into a UO 2 nuclear fuel pellet aſt er penetrating a 600 µm thick Zircaloy cladding wall. T e ability to penetrate the cladding wall and the pellet off ers an opportunity to measure the uranium nondestruc- tively through the container walls with typical XRF accuracy and precision. T e ability to measure the U content directly through the container wall off ers a simple analytical protocol because (a) there would be no sample preparation, (b) the analyte is measured directly, and (c) the matrix and mineral- ogical eff ects are reduced. In addition, at this high energy, there are few if any line overlaps, which simplifi es the spectrometry measurements. By using a restricted or focused X-ray beam for excitation, spatially resolved elemental distributions within the fuel rod can be obtained, whether it is fresh nuclear fuel or spent nuclear fuel. T is article presents the feasibility of employing UHEXRF for qualitative and quantitative elemental analysis of uranium in nuclear fuel surrogates and demonstrates nondestructive, through- container-wall analyses.
Materials and Methods Calibration . T e samples used in this study were created in the laboratory using depleted uranium solutions from
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a stock 10,000 μ g/mL commercial standard (High Purity Standards, Charleston, SC). T e calibration samples were prepared by depositing known concentrations with a pipet, typically 1 μ L, onto a Kapton fi lm substrate. Two types of samples were prepared: one was an acidifi ed aqueous-based matrix with known uranium concentrations, whereas in the second set the uranium solution was spiked into a synthetic spent fuel (SSF) matrix. T e SSF matrix is a mixture of nearly 50 elements typically found in spent nuclear fuel, each with a mass of 100 ng. T e deposited uranium ranged from 10,000 ng to 1 ng in the dried spot residue of the 1 μ L drop. T ese samples were used to generate a calibration plot for the uranium intensity versus the known U mass. In each case the deposit was mapped by sequentially moving the probe beam over an area larger than the visible deposit. T is approach was used to ensure the entire deposit was measured. T e samples were doubly sealed in Kapton fi lm to ensure no possible leaks of radioactive material. Simulated fuel rod . A mock fuel rod sample was fabricated within an 8 mm diameter Zircaloy alloy tube, 25 mm long, with a wall thickness of 600 µm. Mock fuel pellets were made by mixing UO 2 and T O 2 in known masses to generate several diff erent UO 2 compositions within the mock fuel pellets. In this instance the uranium is a surrogate for plutonium, whereas the thorium is a surrogate for the typical uranium fuel matrix. T e pellets were mixed with stearic acid as a binder and pressed into 8 mm diameter pellets, 2 mm thick. T e pellets were loaded into the Zircaloy tube and sealed with silicone glue, then encapsulated with Kapton fi lm.
Excitation of U X rays . T e measurements were done on the 6-ID-D beam line of the Advanced Photon Source at Argonne National Laboratory. T is beam line can produce monochromatic excitation from 50 keV to 150 keV with a Δ E/E = 1.4 × 10 −4 and a photon fl ux of 1 × 10 11 photons per second at 130 keV. In this work an excitation energy of ~117 keV was used to eff ectively excite uranium above the K absorption edge of 115.591 keV. T e experimental setup is shown in Figure 1 . T e samples were mounted on multiple axis stages to accurately position the sample for single point spectra, line scans, and elemental maps. T e beam size was controlled by programmable slits. Elemental maps were acquired by stepping the beam over a selected area and recording a full spectrum at each point. T e dried spot deposits were collected using a 500 µm beam spot size with a 500 µm step size in both x and y directions with a 5-second dwell time. T e mock fuel rod was mapped using a 100 µm beam spot and 100 µm steps in x and y with a 3 second
doi: 10.1017/S1551929515000206
www.microscopy-today.com • 2015 May
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