Performance tests of mechanical decladding technology for estimating the feeding portions of the recovered fuel fragments to an electrolytic reduction process were conducted in terms of the fuel rod burnups of 27.3 to 65.7 GWd/tonne uranium (tU) for the used pressurized water reactor nuclear fuel. The decladding efficiencies with fuel burnups were quantitatively obtained from slitting decladding tests. Based on the average fuel rod burnups, fuel rods with an average burnup of up to 52.3 GWd/tU showed above 99%, but higher burnup fuels of above 54.9 GWd/tU were below 97.52% in the decladding efficiency. It was interpreted that variations in decladding efficiency with fuel burnups were closely linked to the opening characteristics of the gap between the pellets and cladding. However, the fuel fragment size distribution after slitting decladding has little difference in fuel burnup changes between 34.8 and 55.4 GWd/tU. Hence, feeding portions of the fuel fragments from an assembly basis by using the decladding efficiency and recovered fragment size distribution data were estimated with burnup variations of 35 to 52.5 GWd/tU.