[en] A series of tests designed to evaluate the high burnup irradiation performance of mixed oxide fuel rods has achieved substantial burnup in EBR-II without incidence of failure. Several stainless steel cladding alloys and fuel configurations were irradiated over a broad range of operating conditions up to 16.3 at.% burnup, 47 kW/m linear power, and 570⁰C cladding temperature. Extensive examination showed the rods to be in good condition after extended operation in both loose and tight wire wrap spaced assembly configurations. In fuel rods with typical near-term LMFBR design parameters, cladding inelastic strains in the range from 0.3 to 0.8% were commonly observed at about 10 atom % burnup. The inelastic strain appears to increase with burnup and fluence, with the possibility of threshold values for these variables. Preliminary analyses suggest that the cladding inelastic strain was not entirely caused by pressure from released fission gases. Microstructural examinations indicated good fuel-cladding chemical compatibility and comparable behaviour for both physically-mixed and coprecipitated fuel. Significant results include the observation of negligible fuel-cladding gap in low power fuel at approximately atom % burnup, and partial closure of the central void in annular fuel approximately 10 atom % burnup. (author)