EPMA of Melted UO2 Fuel Rods Irradiated to a Burn-up of 23 GWd/tU

The paper reports electron probe micro-analysis (EPMA) and optical microscopy results obtained on discs from the melted fuel bundle of the second Phebus test (FPT1). In this test 18 irradiated UO2 fuel rods along with two non-irradiated instrumented rods were heated by fission power to an estimated 2500 °C. After X-ray tomography at IPSN, Cadarache, the melted fuel bundle was sent to the Institute for Transuranium Elements where it was sectioned and selected zones examined by optical microscopy and EPMA. At mid-height the fuel bundle had collapsed leaving a void. Below the void, a molten pool of corium had formed. The matrix of the corium pool was a solid solution of composition (U0.52Zr0.44Fe0.03Cr0.01)O2+x resulting from the fusion of the UO2 fuel and the Zircaloy cladding. In the corium pool, oxide inclusions rich in Fe and Cr, and metallic inclusions rich in Ni and containing small amounts of the fission products Mo, Tc, Ru and Pd were observed. In the middle and upper sections of the bundle, extensive oxidation and interaction had caused severe deformation of both the fuel and cladding.