Irradiation performance of WWER and PWR fuel tested in IFA-503.2 to a burnup of 20 MWd/kg UO2

IFA-503.2 has been irradiated since March 1999 with main objective to study the thermal and mechanical behaviour of different types of modified WWER-440 fuel compared with PWR reference fuel. 10 rods with three types of WWER-440 UO2 fuel pellets produced by MSZ (Electrostal, Russia) and 2 reference rods with PWR pellets produced by IFE (Kjeller) are instrumented with ET, EF and PF detectors providing comparative information on irradiation performance of these fuel types. The results obtained during first stage of the test have indicated that all types of the modified WWER fuels exhibit improved densification properties compared to earlier tested WWER fuel. One of these fuels (denominated WWER-1) has shown best thermal and mechanical performance which is similar PWR reference fuel. As of February 2002 the average burnup achieved in the higher power rating rods was around 20 MWd/kg UO2 .The updated results from the in-pile measurements and analysis are presented together with some fuel microstructural examinations carried out to increase the existent data base on fuel porosity distribution (in particular for WWER fuel) associated with irradiation-induced densification. Quantitative analysis of these data confirmed the effect of sub-micron porosity on in-pile fuel densification. (Author)