Phase characterisation of simulated high burn-up UO2 fuel

To determine the lattice constant variation with burn-up and the oxidation behaviour at high temperatures, simulated UO2-spent fuels with burn-ups up to 200 GWd tM(-1) were prepared by cold pressing and sintering and characterised by X-ray diffraction analysis and thermogravimetry. The fcc-lattice parameter of the sintered samples determined at room temperature, showed a continuous decrease with the simulated burn-up. During oxidation the conversion of the simulated fuels to the (U,FP)(4)O-9 and (U,FP)(3)O-8-type phases was strongly delayed when the simulated burn-up exceeded 80 GWd tM(-1). Also, at high burn-ups (>100 GWd tM(-1)), large fractions of the (U,FP)(4)O-9-type phase remained untransformed to (U,FP)(3)O-8 after prolonged exposure to air at 950-1000 degrees C. This suggests a marked widening of the (U,FP)(4)O-9-phase field when the amount of substituted uranium atoms exceeds values of about 8%. (C) 1998 Elsevier Science S.A.