EPMA of melted UO2 fuel rods irradiated to a burn-up of 23 GWd/tU

The paper reports electron probe microanalysis (EPMA) and optical microscop y results obtained on discs from the melted fuel bundle of the second Phebu s 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 2 500 degrees C. After X-ray tomography at IPSN, Cadarache, the melted fuel b undle was sent to the Institute for Transuranium Elements where it was sect ioned and selected zones examined by optical microscopy and EPMA. At mid-he ight the fuel bundle had collapsed leaving a void. Below the void, a molten pool of corium had formed. The matrix of the cerium pool was a solid solut ion of composition (U0.52Zr0.44Fe0.03Cr0.01)O2+x resulting from the fusion of the UO2 fuel and the Zircaloy cladding. In the cerium pool, oxide inclus ions rich in Fe and Cr, and metallic inclusions rich in Ni and containing s mall amounts of the fission products Mo, Tc, Ru and Pd were observed. In th e middle and upper sections of the bundle, extensive oxidation and interact ion had caused severe deformation of both the fuel and cladding.