EPMA and X-Ray Diffraction of the Degraded Fuel Bundle from the Phebus FPT1 Test

 The degraded fuel rod bundle from the second Phebus test (FPT1) was examined at ITU, Karlsruhe. Metallographic and microprobe analysis of the degraded fuel pieces were carried out. The fuel samples from the upper bundle and cavity edge were porous and had remnants of thick oxidised cladding adhering to them. Electron microprobe analysis line scans across the cladding-fuel interface showed interdiffusion of U and Zr, with U diffusing down the grain boundaries of the oxidised cladding, while point analyses revealed noticeable amounts of Zr (1.5–4.2 wt%) in the UO₂ fuel. EPMA oxygen measurements revealed in the upper part of the bundle a superstoichiometry of x = 0.3–0.4 in UO _2 + x , indicating that fuel fragments in this position had undergone considerable oxidation. X-ray diffraction of the corium pool disclosed a deformed cubic fluorite lattice of UO₂. The lattice parameter of a = 5.2984 ? was considerably reduced compared with pure, stoichiometric UO₂ and was consistent with a lattice containing approximately 45 mol% ZrO₂ that had undergone little oxidation. The corium’s nominal composition of (U_0.5Zr_0.5) O₂ also corresponded to its observed single phase microstructure.