b'and 400C, respectively. The results of the GASR collection estimated to be 60-70% gas release. Both pins were sectioned and mounted for microscopy (Figures 1 & 2) to show radial and transverse microstruc-ture. Optical imaging revealed an unexpected closure of the annulus in FAST-007 and the more unex-pected redistribution rings within the fuel. These results are extremely notable as conventional wisdom suggests that the redistribution is driven by an transition. The irradiation centerline temperatures (Figure 3) of both fuels are approxi-mately 550C and 485C for FAST-007 and FAST-008, respectively. These temperatures are below the transition temperature of 660C and so the redistribution of Zr and U within the fuel pins is unexpected. This work casts further doubt on the accuracy of the phase diagram and the possible misrepresenta-tion of thephase after previous work within the campaign failed to observe thephase as the U-Zr samples cooled from. A further observation of the FAST pins was the closure of the annulus of the FAST-007 pin. Previous irradiations under AFC on prototypic geometry pins, albeit with lower smear densi-ties, failed to show a full closure of the inner annulus. The failure for previous AFC designs to close the annulus may be in part due to theorized swelling limitations that prompted the original metal fuel design to 75% SD as the fission gas pores driving the swelling would have reached connectivity and thus the driving force for fuel swelling diminished. Within the FAST-007Figure 1. Transverse metallography of the FAST-007 pin across the center of the pin the annulus represented thepin. Note the coloration difference caused by minor oxidation after sectioning that idealized 33% swelling fraction andis indicative of compositional differences (i.e., alloy redistribution). Secondly, tearing within the fuel can be seen along the fuel-cladding interface2023|AFC ACCOMPLISHMENTS 161'