High power ramping in commercial PHWR fuel at extended burnup

Argentinean Atucha and Embalse NPP, both PHWR with on line refuelling with approximately 68,000 fuel elements, irradiated at standard burnup, during the refuelling the fuel changes the power close to a mathematical ideal step. Recently, Atucha core started to operate fully with slightly enriched uranium (SEU, 0.85% of enrichment) increasing the burnup more than 50%, thus producing the first irradiated data base of on line refuelling at extended burnup using commercial fuel elements.The BACO code was extensively used in Argentina to model the physical behaviour of both fuel elements, natural UO2 and SEU. The hoop stress predicted by BACO at the inner surface of the cladding correlate very well with the fuel failure probability over a wide range of tests. Using the data base of present commercial extended burnup fuel, a simple criterion of fuel failure taking into account probabilistic and parametric analysis correlate very well with the present irradiation experience, in agreement with previous BACO experience which confirm the expected behaviour and the SEU fuel element design.