[en] IFA-503.1 has been irradiated in the HBWR since July 1995 with the main objective being to generate representative and comparative data of standard WWER-440 fuel fabricated at the Elemash fuel plant (Russia) and conventional PWR type fuel produced at IFE Kjeller (Norway). The cladding tubes used for all test rods are a standard WWER alloy Zr-1%Nb and have been manufactured by Russia (Glazov). Fuel pellets are hollow with a bore of 1.8 mm except for the PWR rods with PFIEF instrumentations. The test rods were manufactured with two pellet-clad gaps, 0.21 and 0.27 mm. The test rig carries two clusters, each with 3 WWER rods and 3 PWR type rods. The fuel rods are instrumented with expansion thermometers (8), fuel stack elongation detectors (4) and pressure transducers (4). All se sensors have been working satisfactory. The maximal level of burnup is about 18 MWd/kgUO₂. The average burnup in the lower and upper clusters are 17.1 and 12.3 MWd/kgUO₂, respectively. The latest results from the test indicate that both fuel types have approximately the same fuel stack swelling rate. The fuel temperatures are similar despite that the WWER fuel has densified somewhat more than the PWR fuel. The gas pressure measurements show that there has been no fission gas release so far in all fuel rods. The WWER rods have slightly higher pressures than the PWR type rods at power and lower pressure at hot standby condition, obviously related to the higher gas average temperature in the hollow WWER fuel stack during operation. (author)