PHASE CHARACTERIZATION OF SIMULATED HIGH BURN-UP UO2 FUEL

To determine the lattice constant variation with bum-up and the oxidat ion behaviour at high temperatures, simulated UO2-spent fuels with bur n-ups up to 200 GWd tM(-1) were prepared by cold pressing and sinterin g 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 simul ated 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 untra nsformed to (U,FP)(3)O-8 after prolonged exposure to air at 950-1000 d egrees C. This suggests a marked widening of the (U,FP)(4)O-9-phase fi eld when the amount of substituted uranium atoms exceeds values of abo ut 8%. (C) 1998 Elsevier Science S.A.