2017 | AFC ACCOMPLISHMENTS 62 Work in FY17 centered on evalua- tion of the specific microstructural mechanisms that govern failure. This work was partially prompted by observations that testing of U3Si2 in PWR water chemistries (~5 ppm H2) resulted in improved stability early in testing (negligible weight change was found through 30 days), but rapid pulverization beyond this time. Characterization of the pulver- ized material through x-ray diffrac- tion (XRD) found negligible sign of oxide compounds.This contrasted to results obtained for testing in either neutral or slightly oxidizing water chemistries, where consider- able evidence of oxidation-induced pulverization is observed. Assessment of the microstructures of U3Si2 exposed to autoclave condi- tions and following H2O oxidation at atmospheric pressure revealed that the microstructure resulting from H2O oxidation differs considerably from that found following oxidation by O2 alone. During oxidation of U3Si2 in high-oxygen environments (e.g. air), UO2 formation occurs rapidly and is followed by further oxidation of UO2 to U3O8. Above approximately 350C, oxygen diffu- sion along U3Si2 grain boundaries results in rapid pulverization of the sample. However, oxidation of U3Si2 in H2O either at atmospheric pressure or during autoclave testing drives an additional phenomenon. Figure 1 compares the microstruc- tures of U3Si2 oxidized under H2O and air.The tests were executed at different temperatures to match the kinetics at the length scale shown. Testing performed in air results in development of three surface structures: U3O8 can be observed spalling off the sample surface. A layer of UO2 appears and is more strongly adhered, and a region of USi3 can be found in the subsurface regions as formed by the Si rejected from the U3Si2 bulk as the uranium oxides form.The same structures are formed during oxidation in H2O, but at a lesser degree.This is caused by the greatly reduced oxygen activity of H2O compared with O2 at these