A mixed carbonitride fuel irradiated to 3.9% fissions of initial metal atoms in a fast flux was examined by electron microprobe analysis. The fuel contained a large porous zone. Inclusions at the edge of the zone contained palladium and tin and were formed from the liquid phase. Within the porous region, the platinum metals occurred in the UPd3 and U(Tc0.02Ru0.78Rh0.14Pd0.06)3Cx phases, molybdenum and technetium formed inclusions of the (U,Pu)x(Mo,Tc)yCz type, and part of the rare earth elements reacted with impurity oxygen to give a sesquioxide phase. Plutonium depletion was found near the fuel center, and plutonium enrichments were measured at the outer part of the porous zone and at healed cracks in the dense fuel. Cladding carburization occurred and, at the inner cladding surface, the carbon concentration was 0.4 wt%.