Low-enriched uranium-10 wt.% molybdenum (LEU-10wt.%Mo) is of interest for the fabrication of monolithic fuels to replace highly-enriched uranium (HEU) dispersion fuels in high performance research and test reactors around the world. In this work, depleted uranium-10 wt%Mo (DU-10wt%Mo) is used to simulate the solidification and microstructural evolution of LEU-10wt%Mo. Electron backscatter diffraction (EBSD) and complementary electron probe microanalysis (EPMA) reveal significant microsegregation present in the metastable gamma-phase after solidification. Homogenization is performed at 800 and 1000 degrees C for times ranging from 1 to 32 h to explore the time temperature combinations that will reduce the extent of microsegregation, as regions of higher and lower Mo content may influence local mechanical properties and provide preferred regions for y-phase decomposition. We show for the first time that EBSD can be used to qualitatively assess microstructural evolution in DU-10wt%Mo after homogenization treatments. Complementary EPMA is used to quantitatively confirm this finding. Homogenization at 1000 degrees C for 2-4 h may the regions that contain 8 wt% Mo or lower, whereas homogenization at 1000 degrees C for longer than 8 h effectively saturates Mo chemical homogeneity, but results in substantial grain growth. The appropriate homogenization time will depend upon additional microstructural considerations, such as grain growth and intended subsequent processing. Higher carbon LEU-10wt%Mo generally contains more inclusions within the grains and at grain boundaries after solidification. The effect of these inclusions on microstructural evolution (e.g. grain growth) during homogenization and as potential y-phase decomposition nucleation sites is unclear, but likely requires additional study. Published by Elsevier B.V.

The interface bonds are critical to in-reactor performance for a high density monolithic plate fuel system which uses low enriched uranium 10wt% molybdenum foils corolled with Zr and clad with 6061Al. Stresses induced during reactor shutdown have been identified as a source of concern for that integrity. Because of the post-reactor radioactivity, those residual stresses will have to be measured in a shielded nuclear radiation containment chamber or “hot cell” with remote handling of specimens and instrumentation, which limits measurement options. This study tested options for stress measurements by using surrogate fuel plates, such as with depleted uranium, but using only hot-cell appropriate equipment. Several measurements were performed using the incremental slitting method (a.k.a. crack compliance) but, for hot cell use, using a milling cutter instead of wire EDM for making the cut and a displacement sensor instead of a strain gauge. Measurements were also performed using incremental hole drilling using an interferometry system instead of strain gauges. For both measurement techniques, special data reduction development was required in order to handle discontinuities in the stress profiles across the layers. The results were encouraging, and the slitting method is now being implemented for use in a hot cell.

The optimization of HIP processing of aluminum cladding for LEU-10Mo monolithic fuel plates is discussed. Canned HIP process optimization focuses on reduction of materials usage and processing effort, while improving the final HIPed product. Small scale formed sheet-steel HIP cans have been modeled, designed, and produced that significantly reduce production costs and resources. A new canless HIP approach to fuelfoil production is being developed at LANL, avoiding the need for protective stainless steel canisters. This process involves hermetically sealing the outer perimeter of individual fuel foils using electron beam welding. Details of this process and discussion of the challenges encountered will be given. Grain growth across the aluminum clad-clad interface is desirable. It was characterized using electron backscatter diffraction as a function of process modifications including macroscopic grooving to enhance Al flow, changing the cleaning method, and adding cold work. The results of these initial studies will be presented.

We present the nanomechanical behavior of U-10Mo/Zr/Al fuel assemblies using nanoindentation and nanocantilever bend testing. In these assemblies, fracture is often observed near the interfaces between constituents, and may be attributed to microstructural features such as carbide inclusions, intermetallic phases, voids, etc. Nanoindentation testing gives hardness values for U-10Mo, Zr, and Aluminum in as-HIPed fuel assemblies (4.4 GPa, 2.2 GPa, and 1.2 GPa respectively). Nanocantilevers prepared via Focused Ion Beam (FIB) are tested in a nanoindenter. By scaling the yield strength of bulk material data while maintaining hardening behavior, load-displacement output from a finite element simulation of identical geometry is matched with load-displacement data from tests allowing correlation between local and higher level mechanical behavior. Our measurements show the effect of voids and chemical segregation on Al-Al HIP bonds, and the comparable strength of the U-10Mo/Zr interface with respect to Zr. Measured mechanical behavior will be discussed in terms of the influence of interfacial morphology, crystallography, and chemistry on strength and ductility of the interface.

The aim of this work was to explore the limits of polycrystalline ceramic scintillator in countering the nuclear threat. The goal was to develop a polycrystalline LaBr3:Ce, which can be processed from ceramic forming techniques and can be produced in large size scintillator panels with lower cost and high production rate. Three high purity raw powders were used as the starting materials including LaBr3, LaCl3, and CeBr3- Powder characteristics were measured. A melt spinning method was used to synthesize the nanoparticle LaBr3:Ce with stoichiometric compositions. The synthesized nanoparticles were characterized and the average particle size of the synthesized nanoparticle LaBr3:Ce was about 50 nm. The melt spun powders were consolidated using a "Nanosintering" method to achieve a high density while maintaining the stoichiometric composition. The grain size of the sintered polycrystalline is about 50 nm, which shows no grain growth during the densification process.