RELIABILITY OF STEAM-GENERATOR TUBES WITH AXIAL CRACKS

An approach for estimating the failure probability of tubes containing through-wall axial cracks has already been proposed by the authors. I t is based on probabilistic fracture mechanics and accounts for scatte r in tube geometry and material properties, scatter in residual and op erational stresses responsible for crack propagation and characteristi cs of nondestructive examination and plugging procedures (e.g., detect ion probability, sizing accuracy, human errors). Results of preliminar y tests demonstrated wide applicability of this approach and triggered some improvements. The additions to the model are extensively discuss ed in this paper. Capabilities are demonstrated by results of analysis of steam generator no. 1 in Slovenian nuclear power plant located in Krsko after the 1992 inspection and plugging campaign. First, the numb er of cracked tubes and the crack length distribution were estimated u sing data obtained by the 100-percent motorized pancake coil inspectio n. The inspection and plugging activities were simulated in the second step to estimate the efficiency of maintenance in terms of single and multiple-tube rupture probabilities. They were calculated as a functi on of maximum allowable crack length. The importance of human errors a nd some limitations of present nondestructive examination techniques w ere identified. The traditional wall thickness and crack-length-based plugging criteria are compared. The crack-length-based criterion is sh own to be more efficient and more safe, especially because of strong s uppression effect on probability of multiple-tube rupture. The results are considered to be important for safety and maintenance of existing plants and for further research.