Quantitative morphological characterization of carbide inclusions in uranium metal

Abstract

Uranium carbides (UCs) are prevalent inclusions in U metal that form during melting operations because of interactions with crucible walls and the casting chamber atmosphere. Although UCs have been studied extensively since the beginning of U metal foundry operations, there are still unknowns regarding the effects of thermomechanical processing on their sizes and morphology. Here, we present the results of a series of controlled cooling experiments with molten uranium to elucidate the effect of cooling rate on inclusion morphology in α-U. Samples were melted using a vacuum induction melter and manually cooled at rates of 2.5, 1.7, 1.1, 0.8, and 0.3 (± 1%) K/s from ∼1600 K to < 700 K in under 1 h. Subsequent scanning electron microscopy (SEM) was performed on cross sections of the samples, revealing a complex mixture of UC morphologies that are indicative of diffusion and growth influenced by the thermal processing of the U matrix. Image analysis using the morphological analysis of materials (MAMA) software showed that UC sizes generally grew larger with slower cooling rates, and the two slowest cooling rates noticeably affected the inclusion circularity and ellipse aspect ratio. These results indicate that UC morphology is sensitive to short cooling rates (< 1 h) and could therefore be controlled in the production of metallic nuclear fuels. Additionally, inclusion speciation and morphologies could potentially provide forensic clues about the processing history of unknown metal samples. Understanding the driving forces involved in UC morphology evolution is beneficial for evaluating metal fuels for next-generation nuclear reactors and for identifying signatures for nuclear forensics.