Monolithic Fuel Fabrication Process Development at the Idaho National Laboratory

Within the Reduced Enrichment for Research and Test Reactors (RERTR) program directed by the US Department of Energy (DOE), UMo fuel-foils are being developed in an effort to realize high density monolithic fuel plates for use in high-flux research and test reactors. Namely, targeted are reactors that are not amenable to Low Enriched Uranium (LEU) fuel conversion via utilization of high density dispersion-based fuels, i.e. 8-9 gU/cc. LEU conversion of reactors having a need for >8-9 gU/cc fuel density will only be possible by way of monolithic fuel forms. The UMo fuel foils under development afford fuel meat density of ~16 gU/cc and thus have the potential to facilitate LEU conversions without any significant reactor-performance penalty. Two primary challenges have been established with respect to UMo monolithic fuel development; namely, fuel element fabrication and in-reactor fuel element performance. Both issues are being addressed concurrently at the Idaho National Laboratory. An overview is provided of the ongoing monolithic UMo fuel development effort at the Idaho National Laboratory (INL); including development of complex/graded fuel foils. Fabrication processes to be discussed include: UMo alloying and casting, foil fabrication via hot rolling, fuel-clad interlayer application via co-rolling and thermal spray processes, clad bonding via Hot Isostatic Pressing (HIP) and Friction Bonding (FB), and fuel plate finishing

Evaluation of Annealing Treatments for Producing Si-Rich Fuel/Matrix Interaction Layers in Low-Enriched U-Mo Dispersion Fuel Plates Rolled at a Low Temperature

During fabrication of U-7Mo dispersion fuels, exposure to relatively high temperatures affects the final microstructure of a fuel plate before it is inserted into a reactor. One impact of this high temperature exposure is a chemical interaction that can occur between dissimilar materials. For U-7Mo dispersion fuels, the U-7Mo particles will interact to some extent with the Al or Al alloy matrix to produce interaction products. It has been observed that the final irradiation behavior of a fuel plate can depend on the amount of interaction that occurs at the U-7Mo/matrix interface during fabrication, along with the type of phases that develop at this interface. For the case where a U-7Mo dispersion fuel has a Si-containing Al alloy matrix and is rolled at around 500°C, a Si-rich interaction product has been observed to form that can potentially have a positive impact on fuel performance during irradiation. This interaction product can exhibit stable irradiation behavior and it can act as a diffusion barrier to additional U-Mo/matrix interaction during irradiation. However, for U-7Mo dispersion fuels with softer claddings that are rolled at lower temperatures (e.g., near 425°C), a significant interaction layer has not been observed to form. As a result, the bulk of any interaction layer that develops in these fuels happens during irradiation, and the layer that forms may not exhibit as stable a behavior as one that is formed during fabrication. Therefore, it may be beneficial to add a heat treatment step during the fabrication of dispersion fuel plates with softer cladding alloys that will result in the formation of a uniform, Si-rich interaction layer that is a few microns thick around the U-Mo fuel particles. This type of layer would have characteristics like the one that has been observed in dispersion fuel plates with AA6061 cladding that are fabricated at 500°C, which may exhibit increased stability during irradiation. This report discusses the result of annealing experiments that were performed using samples from fuel plates that were fabricated at 425°C that had Alloy 5052 cladding. As part of these experiments, samples with Al-Si matrices that had different Si contents were tested. The samples had Al-2Si, Al-4Si, Al-5Si, or Al-6Si as the matrix alloy. The heat treatment temperatures and times that were investigated were 450°C (4 hours), 475°C (4 hours), and 500°C (2 hours) for all the matrix alloy compositions and 525°C (1 hour) for just the Al-4Si and Al-6Si matrix alloy compositions. The results of these experiments showed that the initial interaction layers that form around the U-7Mo particles during fabrication at 425°C continue to grow in thickness and uniformity during each of the heat treatments, though the composition of the layers remains similar to that observed in the as-fabricated samples. The Al-6Si matrix sample annealed at 450°C for 4 hours and the Al-5Si and Al-6Si matrix samples annealed at 475°C for 4 hours formed fuel/matrix interaction layers most similar to those observed in fuel plates with AA6061 cladding that are fabricated at 500°C

Potential Annealing Treatments For Tailoring The Starting Microstructure Of Low-Enriched U-Mo Dispersion Fuels To Optimize Performance During Irradiation

Low-enriched uranium-molybdenum (U-Mo) alloy particles dispersed in aluminum alloy (e.g., dispersion fuels) are being developed for application in research and test reactors. To achieve the best performance of these fuels during irradiation, optimization of the starting microstructure may be required by utilizing a heat treatment that results in the formation of uniform, Si-rich interaction layers between the U-Mo particles and Al-Si matrix. These layers behave in a stable manner under certain irradiation conditions. To identify the optimum heat treatment for producing these kinds of layers in a dispersion fuel plate, a systematic annealing study has been performed using actual dispersion fuel samples, which were fabricated at relatively low temperatures to limit the growth of any interaction layers in the samples prior to controlled heat treatment. These samples had different Al matrices with varying Si contents and were annealed between 450 and 525°C for up to 4 h. The samples were then characterized using scanning electron microscopy (SEM) to examine the thickness, composition, and uniformity of the interaction layers. Image analysis was performed to quantify various attributes of the dispersion fuel microstructures that related to the development of the interaction layers. The most uniform layers were observed to form in fuel samples that had an Al matrix with at least 4 wt.% Si and a heat treatment temperature of at least 475°C. © 2011 Elsevier B.V. All rights reserved

Mechanical Behavior Assessment of Ti-6Al-4V ELI Alloy Produced by Laser Powder Bed Fusion

The present work correlates the quasi-static, tensile mechanical properties of additively manufactured Ti-6Al-4V extra low interstitial (ELI, Grade 23) alloy to the phase constituents, microstructure, and fracture surface characteristics that changed with post-heat treatment of stress relief (670 °C for 5 h) and hot isostatic pressing (HIP with 100 MPa at 920 °C for 2 h under an Ar atmosphere). Ti-6Al-4V ELI alloy tensile specimens in both the horizontal (i.e., X and Y) and vertical (Z) directions were produced by the laser powder bed fusion (LPBF) technique. Higher yield strength (1141 MPa), higher tensile strength (1190 MPa), but lower elongation at fracture (6.9%), along with mechanical anisotropy were observed for as-stress-relieved (ASR) samples. However, after HIP, consistent and isotropic mechanical behaviors were observed with a slight reduction in yield strength (928 MPa) and tensile strength (1003 MPa), but with a significant improvement in elongation at fracture (16.1%). Phase constituents of acicular α′ phase in ASR and lamellar α + β phases in HIP samples were observed and quantified to corroborate the reduction in strength and increase in ductility. The anisotropic variation in elongation at fracture observed for the ASR samples, particularly built in the build (Z) direction, was related to the presence of “keyhole” porosity

Additive Manufacturing and Mechanical Properties of the Dense and Crack Free Zr-Modified Aluminum Alloy 6061 Fabricated by the Laser-Powder Bed Fusion

For additive manufacturing such as laser powder bed fusion (LPBF), commercial aluminum alloy (AA) 6061 is typically considered unsuitable due to formation of solidification cracking and/or excessive porosity. In this study, to improve buildability/printability of AA6061, 1 wt% of Zr was alloyed to produce Zr-modified AA6061 by LPBF. Powders of unmodified and Zr-modified AA6061 were produced by gas atomization, and utilized as a feed-stock for the LPBF to fabricate specimens for microstructural examination and mechanical testing. The as-built unmodified AA6061 exhibited poor printability due to formation of cracks and porosity in the microstructure regardless of LPBF parameters. However, the Zr-modified AA6061 exhibited near full density, with substantial reduction in porosities without any solidification crack for a certain LPBF processing window. The improved printability of Zr-modified AA6061 was attributed to a significant grain refinement, which would reduce the solidification cracking susceptibility by hampering the epitaxial growth of long columnar cracks, as observed in unmodified AA6061. Yield strength, tensile strength and strain-at-failure for the as-built Zr-modified AA6061 were determined to be 210 MPa, 268 MPa, and 26.5%, respectively. These are superior to the tensile properties of AA6061 in O-annealed condition or in as-cast condition. After T6 heat treatment, yield strength, tensile strength and strain-at-failure of Zr-modified AA6061 were determined to be 300 MPa, 327 MPa, and 14%, which were again superior to the tensile properties of wrought AA6061 in T6 heat treated condition. Effects of Zr addition on the buildability/printability improvement and mechanical properties of AA6061 were corroborated by a variety of electron microscopic characterization

Microstructural Development in Inconel 718 Nickel-Based Superalloy Additively Manufactured by Laser Powder Bed Fusion

Excellent weldability and high temperature stability make Inconel 718 (IN718) one of the most popular alloys to be produced by additive manufacturing. In this study, we investigated the effects of laser powder bed fusion (LPBF) parameters on the microstructure and relative density of IN718. The samples were fabricated with independently varied laser power (125–350 W), laser scan speed (200–2200 mm/s), and laser scan rotation (0°–90°). Archimedes’ method, optical microscopy, and scanning electron microscopy were employed to assess the influence of LPBF parameters on the relative density and microstructure. Optimal processing windows were identified for a wide range of processing parameters, and relative density greater than 99.5% was achieved using volumetric energy density between 50 and 100 J/mm3. Microstructural features including melt pool geometry, lack of fusion defect, keyhole porosity, and sub-grain cellular microstructure were examined and quantified to correlate to LPBF parameters. A simple empirical model was postulated to relate relative sample density and LPBF volumetric energy density. Melt pool dimensions were quantitatively measured and compared to estimations based on Rosenthal solution, which yielded a good agreement with the width, but underestimated the depth, particularly at high energy input, due to lack of consideration for keyhole mode. In addition, the sub-grain cellular-dendritic microstructure in the as-built samples was observed to decrease with increasing laser scan speed. Quantification of the sub-micron cellular-dendritic microstructure yielded estimated cooling rate in the order of 105–107 K/s