In the last decade, an extended power enhancement program was completed in Hungary which provided to reach an 8 % reactor heat power increase for all the four units of Paks NPP. At that time the Russian manufacturer introduces several developments in the VVER fuel construction. Increased average enrichment, modification of the lattice pitch, fuel length and diameter, profiled enrichment, application of burnable absorber, shielding the absorber assembly coupler part with Hf plates are to be mentioned in this respect.

The above mentioned modifications requiring more accurate calculations have necessitated the further development and validation of the KARATE code system, which was developed in Hungary for VVER reactors about fifteen years ago and used continuously at the Paks power plant.

This paper presents two version of the second generation Russian fuel which has been widely used in VVER-440 recently, summarizes the improvements of the new version of the KARATE code system concerning the properties of newly developed fuel types. On the basis of some international benchmark data and measurements available for these recently introduced fuels validation procedure has been performed and presented, too. Finally information for the fuel chosen for our NPP is presented.

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