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

摘要

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 500DGC, 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 425DGC), 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 500DGC, 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 425DGC 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 450DGC (4 hours), 475DGC (4 hours), and 500DGC (2 hours) for all the matrix alloy compositions and 525DGC (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 425DGC 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 450DGC for 4 hours and the Al-5Si and Al-6Si matrix samples annealed at 475DGC for 4 hours formed fuel/matrix interaction layers most similar to those observed in fuel plates with AA6061 cladding that are fabricated at 500DGC.