In-service change in the flexural rigidity of the VVER-1000 fuel assemblies

It has been shown that an increase in the burnup causes the flexural rigidity of TVSA and TVS-2 FAs with a rigid skeleton to decrease to the minimum value of ∼5 kg f/mm, the burnup being 45-50 МW day/kg U, and then to start growing again. It has been found out that it is the fuel bundle, specifically the change in the force of the fuel rod compression in the spacer grid, which is responsible for the change in the FA flexural rigidity. The maximum TVSA and TVS-2 FA bowing is in the range of 8-11 mm whereas the burnup is 48-63 МW day/kg U. The newly adopted TVSA and TVS-2 FA designs have contributed to the safe operation of the VVER-1000 control and protection system's absorber rods.