doi:10.1017/S1551929514000510 30 www.microscopy-today.com • 2014 September Nano-Scale Fission Product Phases in an Irradiated U-7Mo Alloy Nuclear Fuel D. D. Keiser, Jr.,* Brandon Miller, James Madden, Jan-Fong Jue, and Jian Gan Idaho National Laboratory, 2525 Fremont Ave., Idaho Falls, ID 83402 *dennis.keiser@inl.gov Introduction Irradiated nuclear fuel is a very difcult material to characterize. Due to the large radiation felds associated with these materials, they are hard to handle and typically have to be contained in large hot cells. Even the equipment used for performing characterization is housed in hot cells or shielded glove boxes. Te result is not only a limitation in the techniques that can be employed for characterization, but also a limitation in the size of features that can be resolved. Te most standard characterization techniques include light optical metallography (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Tese techniques are applied to samples that are typically prepared using grinding and polishing approaches that will always generate some mechanical damage on the sample surface. As a result, when performing SEM analysis, for example, the analysis is limited by the quality of the sample surface that can be prepared. However, a new approach for characterizing irradiated nuclear fuel has recently been developed at the Idaho National Laboratory (INL) in Idaho Falls, Idaho. It allows for a dramatic improvement in the quality of character- ization that can be performed when using an instrument like an SEM. Tis new approach uses a dual-beam scanning microscope, where one of the beams is a focused ion beam (FIB), which can be used to generate specimens of irradiated fuel (~10 μm × 10 μm) for microstructural characterization, and the other beam is the electron beam of an SEM. One signifcant beneft of this approach is that the specimen surface being characterized has received much less damage (and smearing) than is caused by the more traditional approaches, which enables the imaging of nanometer- sized microstructural features in the SEM. Te process details [1] are for an irradiated low-enriched uranium (LEU) U-Mo alloy fuel. Another type of irradiated fuel that has been characterized using this technique is a mixed oxide fuel [2]. With respect to irradiated U-Mo alloy fuel, the character- ization of FIB cross-section samples using SEM has revealed frst-of-a-kind images of phases present in the microstructure. Tese phases discussed in this paper are solid phases that result from the nuclear fssion process. Tese phases are comprised of constituents called “fssion products.” Overall, fssion product elements can be gases, liquids, or solids during reactor operation, and an SEM can be employed to evaluate the solid phases. Te development of these phases is important because they can infuence a very important parameter of nuclear fuel called “swelling.” Swelling is the increase in volume of the fuel as the U-235 is fssioned in a reactor. If too much dimensional change occurs, the cooling channels of a fuel assembly can be closed, and the fuel could overheat, breach, or even melt. Te size and location of solid fssion product phases in the fuel microstructure can impact the ultimate performance of the fuel in a reactor. In irradiated oxide fuel, a signifcant solid fssion product phase is the 5-metal phase that contains Pd, Rh, Ru, Tc, and Mo, which typically forms on grain boundaries [3]. In irradiated LEU U-Mo fuel, solid fssion product phases that contain Sr, Ba, Nd, Y, and Te have been observed in the fssion gas bubbles of irradiated fuel TEM samples produced using a traditional sample preparation approach [4]. In order to gain information from larger, more representative regions of a fuel sample, it is advantageous to use SEM. However, for samples produced using more conventional approaches, there is a limit as to how efectively these solid precipitate phases can be resolved, regardless of whether TEM or SEM is used, in part because of the limitation in sample preparation. Tis paper will demonstrate how one can take advantage of the negligible sample surface damage during the FIB process, especially compared to the more traditional mechanical polishing process that uses grinding papers, polishing cloths, slurries, and diamond pastes. Focus will be given to nano-scale, solid fssion product phases that have been resolved in heavily irradiated LEU U-7Mo dispersion fuel. With these never-before-reported images of fssion product phases, one can develop a much better understanding of how these solid fssion product phases may impact the swelling behavior of the fuel afer diferent irradiation times in a nuclear reactor. Materials and Methods Te U-Mo alloy interrogated in this work contained 7 wt.% Mo and was irradiated in INL’s Advanced Test Reactor (ATR) as part of the Global Treat Reduction Initiative Program’s High Performance Research Reactor Fuel Development Program. Tis is a program that is developing low-enriched uranium (U-235 < 20%) fuels for application in research and test reactors [5]. Te sample to be characterized came from a location of a fuel plate (2.5 cm wide by 10 cm long) that had been irradiated to a local fssion density of around 5.2 × 10 21 fssions/cm 3 , where this number represents the amount of U-235 that was fssioned. Afer irradiation and proper cooling at the ATR, the fuel plate was transferred to the Hot Fuel Examination Facility so that a small sample could be “punched” from the fuel plate. Tis punching process involved using a press located in the hot cell to generate a small cylinder, around 1.4 mm tall and 1.0 mm in diameter, from the irradiated dispersion fuel plate [6]. Tis type of fuel plate is comprised of dispersion fuel meat encased in Al-6061 cladding. Te fuel meat contains U-7Mo fuel particles in an Al alloy matrix. During fabrication and irradiation, an interaction zone can develop between the fuel particles the Al alloy matrix, and this zone can go amorphous during irradiation [7]. Once the punched sample was produced, it was transferred to the Electron Microscopy Laboratory, where it was mounted and polished through 1 µm diamond paste in a glove box. https://doi.org/10.1017/S1551929514000510 Published online by Cambridge University Press