Operational experience and main results from the pool side inspections up to two irradiation cycles on the advanced fuel assemblies in Loviisa

TVEL has supplied Fortum six Advanced Fuel Assemblies (AFAs) manufactured by Elemash in 1998. The advanced design features of these assemblies include profiling of the fuel rod enrichment in the cross section of the bundle, demountability of the shroud tube and fuel rods, and reduced shroud tube wall thickness. A new pellet technology leading to more stable fuel is applied in the assemblies. The six AFAs will be irradiated in Loviisa-1 reactor and undergo a pool side inspection programme and a non-destructive post irradiation examination (PIE) at the end of the planned three and four cycles of irradiation. Until now, two AFAs have been inspected in Loviisa NPP with the pool side examination equipment (ATULA), the first after one and the second after two irradiation cycles. General condition of the assemblies, fuel rods and spacer grids inspected has been very good; no abnormal observations were made during the inspections. The ATULA measurements show expected and reasonable dimensional changes of the assembly components. Assembly growth, bow, twist and distance across flats are as expected and in line with earlier measurements carried out in Loviisa. The visual inspection did not reveal any bowed fuel rods or elevated spacer grids. Measurements of the rod diameters indicate cladding creep down of 15 to 30 microns after one irradiation cycle and creep of 35 to 65 microns after two cycles in the middle parts of the rods. Oxidation of the fuel rods, as deduced from the visual inspections, is very low. As a whole it can be summarised that the experiences with the advanced TVEL fuel design after two and three cycles of irradiation are good