In-service change in the flexural rigidity of the VVER-1000 fuel assemblies
In-service dimensional stability of the VVER-1000 reactor fuel assemblies (FA) depends to a great extent on their flexural rigidity. A decrease in the flexural rigidity in the process of the FA operation in the reactor may lead to such FA bowing as will make it difficult for the absorber rods of the reactor control and protection system to move in the FA guide channels. This is not admissible from the point of view of the reactor operation safety. This paper describes a method and a facility for the hot cell testing of the irradiated VVER-1000 FA flexural rigidity. The method is based on measurements of the FA bowing induced by cross-sectional loading. The load applied to the spacer grids is perpendicular to the grid ribs, and the FA bowing is measured optically using a TV camera. The facility can also be used to test the flexural rigidity of the FA skeleton after all of the fuel rods are removed. Several tens of VVER-1000 FAs with a burnup of ~ 4 to ~ 65 МW-day/kgU were tested by Dimitrovgrad Research Institute of Atomic Reactors. The generalization and an analysis of the test results have made it possible to identify the major factors that contribute to the in-service change in the flexural rigidity of the VVER-1000 FAs and to determine the experimental dependence of its change on burnup. 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 kgf/mm, the burnup being 45 to 50 МW-day/kgU, 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, that is responsible for the change in the FA flexural rigidity. The maximum TVSA and TVS-2 FA bowing is in the range of 8 to 11 mm while the burnup is 48 to 63 МW-day/kgU. 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.