a b s t r a c tThe global nuclear energy partnership (GNEP) was created in order for 'fuel-cycle supplier' nations to provide assured supplies of nuclear fuel to 'fuel-cycle customer' nations. The customer nations would utilize the fuel for electricity generation and subsequently return it to the supplier nation after it is spent. This spent fuel would then be reprocessed by the supplier nation in order to recycle the actinide constituents, mainly uranium and plutonium, in advanced nuclear power reactors, and thus reduce waste volumes [1,2]. The International Atomic Energy Agency would control the nuclear materials. One of the thrust areas for the GNEP program is the development of these actinide bearing fuels for transmutation in a fast reactor.Published by Elsevier B.V.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

In this study, actinide oxides (plutonium, uranium, neptunium, and americium oxides) were imaged to enable characterization of their morphology, particle size, cathodoluminescence (CL) signal, and trace elemental content using a scanning electron microscope equipped with a cathodoluminescence detector. Unexpectedly, neptunium oxide (NpO 2 ) powder was found to have two unique CL signals [1][2][3]. The NpO 2 powder was analyzed via energy dispersive spectrometry to identify which elements were present with the hope of determining the origin of the two CL signals. This analysis determined that magnesium (Mg), chlorine, and calcium (Ca) were the elements that caused the multiple CL signals. These samples were also analyzed in the electron microprobe. While undergoing electron probe microanalysis (EPMA), some of the particles could be seen fluorescing in the optical microscope on the microprobe, see Figure 1. An attempt was made to capture an image of the fluorescing NpO 2 particle prior to determining the distribution of neptunium via wavelength dispersive spectroscopy. This particle was damaged by the beam after a short time, see Figure 2.These initial studies have resulted in the need to expand the research project and develop the use of CL element distribution imaging. Since some materials emit CL signals, i.e. characteristic secondary electron excitation in the visual light spectrum, it is expected that CL can be used as another technique to identify trace elements [4]. Based on the initial study, it is anticipated that CL imaging will be faster than traditional EPMA mapping. However, to ensure that the CL signals have been correctly identified, the next analysis will be verification of the CL signals using elemental standards. Once the CL signals are verified, CL imaging of fuel pellets can be used to reveal the distribution of the actinide elements as well as the non-actinide elements. These analyses will confirm that CL imaging can be a viable tool and provide complementary data to supplement traditional mapping techniques.

Plutonium (Pu) is a very reactive metal that rapidly forms a thick, moderately protective surface dioxide (PuO 2 ). In dry air the corrosion rate is slow, approximately 200 nm per year [1]. But in moist air, the oxidation rate at room temperature increases by 200 times. At 100 °C in moist air, the rate is 100,000 times that of dry air. The clean metallic surface develops characteristic interference colors that are a result of the optical properties of the growing oxide and its increasing thickness. In ambient air conditions the oxidation of pure, alpha-phase Pu metal, can be observed in real-time by watching the change in color. An alloy addition of about 1%wt Gallium (Ga) stabilizes the Pu delta-phase (FCC phase stable above 300 °C) and reduces the rate of oxidation. Previously, ellipsometry was used to correlate the thickness of the surface oxide layer with color [2]. In deltastabilized Pu the color changes from gold to violet and then blue in the thickness range of 40 nm to 80 nm. This color scale is often used as a quick method to very roughly determine the thickness of surface oxide on Pu metal, but it is very qualitative in nature and the effect of subtle changes in the oxide composition or metallic surface preparations might significantly affect a thickness estimate. In this work we attempt to measure the thickness of ambient grown Pu oxides using EPMA data and STRATAGem 3.0 Thickness and Composition Thin Film Analysis software package.Six Pu-Ga alloy standards with a Ga concentration range of 0.5-2.0 wt % were prepared by arcmelting and annealed for 200 hrs. at 420 °C. These materials have been extensively evaluated for heterogeneity using EPMA, and they have been successfully utilized as microanalysis standards. The standards were permitted to age in the ambient environment for a period of several months. EPMA measurements of oxygen k-ratios relative to a PuO 2 standard were made at accelerating voltages of 5, 10, 15, 18 and 20 kV using a JEOL JSM-8200. Oxygen Kα characteristic x-rays were analyzed using synthetic multilayer crystal with 6nm lattice spacing. The STRATAGem thin film calculation assumed that all of the measured oxygen was present on the surface in the form of a PuO 2 thin film, and then calculated the apparent thickness of the oxide layer. Figure 1. is a plot of measured oxygen k-ratios as a function of accelerating voltage for the 0.7 wt % Ga alloy and a series of lines-of-constant-oxide-thickness calculated using Stratagem. The data trend qualitatively with the simulations and the apparent precision of the oxide thickness is quite good, on the order of +/-a few nanometers. The accuracy of the measurement is unknown at this time because there is no independent measure of oxide thickness other than color. Table 1 contains a summary of the measured oxide thickness for all the specimens.

Traditionally, samples taken for electron probe microanalysis (EPMA) of alpha-plutonium are tedious to prepare. The samples are mounted in thermosetting epoxy and temperatures must not exceed 125 ºC or phase transformation will occur. The mounts are then rough ground using sequential SiC grits of 320, 400, 600 and finally 15 µm (600 soft). A Trident cloth with 6 µm diamond paste is subsequently used. Final polishing is accomplished with 1 µm diamond paste on a Metcloth media [1,2].After metallographic analysis and imaging is performed using an optical microscope, the samples are then electro-polished or electro-etched in preparation for microanalysis. The metallographic mounts are carbon coated in an evaporator in order to reduce or eliminate charging of the surface. EPMA analysis involves collecting background corrected element distribution maps of 1280 × 1280 µm areas with a resolution of 256 × 256 steps at 5 µm intervals with a 5 µm beam diameter with a relative precision of 10 percent at an expected concentration level of 0.5 wt. %.Alpha plutonium is rather difficult to prepare and the entire process takes weeks to complete in a glove box environment. Even after all that work past samples submitted for EPMA analysis in the electro-polished state, though not as rough as the as-received samples, were full of scratches (see Figure 1). This study involves collecting similar EPMA maps with as-received samples in an attempt to reduce the preparation time prior to EPMA analysis, with the assumption that the results are comparable. Figure 2 is a secondary electron micrograph showing the roughness of an asreceived sample. The sample preparation involved in this case was to blow any loose particles off the sample surface, using inert gas, in this case nitrogen, and then mounting the sample on carbon tape. The sample was then carbon coated in an evaporator and then placed in the EPMA for analysis. We propose that the data collected from this sample when compared to that collected after mounting, grinding, polishing, and electro-etching should demonstrate that extensive sample preparation is not essential. References [1] A