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

Abstract

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 fue...