DAMAGE MODELING OF THE LOAD-BEARING CAPACITY OF DAMAGED STEAM GENERATOR TUBES IN NUCLEAR POWER PLANTS

In austenitic steam generator (SG) tubes installed in nuclear power plants (NPP), wall thinning and crack formation caused by stress corrosion cracking have been observed. These cracks are manly orientated in longitudinal direction of the tube but also in a few cases circumferential oriented cracks nave been found. Because of the small crack depths and wall thickness, the application of fracture mechanics assessments of the load-bearing capacity of these cracks is very limited. Equally it is almost impossible to determine the necessary fracture mechanics parameters for such component geometries. Damage mechanics models are able to describe the experimentally observed processes in smooth and cracked specimens. They have no restrictions like fracture mechanics models. In this paper, the damage model of Rousselier was used to describe the failure behavior and load-bearing capacity of cracked steam generator tubes. The operation temperature of these tubes is about 300 degrees C. The Rousselier parameters for the austenitic steel X 2 NiCrAlTi 32-20 (1.4558) of the tubes were determined by using tensile tests and burst tests at temperatures of 20 degrees C and 300 degrees C. With this damage model it is possible to evaluate the existing limit values for burst pressures and necessary safety factors for the plugging of steam generator tubes in nuclear power plants. These results can also contribute to the safety evaluation of stress corrosion cracking in steam generator tubes.