Experience of TVSA fuel implementation and KASKAD code package validation at Kozloduy NPP

At the end of 2008 a transition procedure from the old Russian designed TVS-M fuel assemblies to the advanced TVSA fuel type was accomplished at Kozloduy NPP WWER-1000 Units 5 and 6. The old fuel assemblies had to be changed because of a distortion tendency observed several years before. The TVSA assembly distinguishes itself with much stronger construction and using a mixture of uranium and uniformly distributed Gd as a burnable absorber in 6 or more fuel rods. The first 12 TVSA assemblies were loaded in the reactor core of Unit 6 at the end of 2004. During the next three years, increasing the number of TVSA assemblies in the reactor core, it was found that the difference between the real fuel cycle lengths and the predicted ones was more than 7-8 % (17-20 fpd). To solve the problem a new macroscopic cross-section library for the Russian code system KASKAD has been generated in NRC Kurchatov Institute. This paper contains a presentation and discussion of the experience gained during the TVSA fuel implementation and KASKAD code system validation at Kozloduy NPP. (Authors)