On the probabilistic approaches applied to the maintenance of degraded steam generator tubes

This paper is divided into three main parts. The first two parts address probabilistic approaches applied to the analysis of two most important failure modes of the degraded tubing. The third part discusses some possibilities for future development. 1. Bursting of tubes. The probabilistic tackling of this failure mode is described by using the example of axial stress corrosion cracking in expansion transition zones. The influencing parameters accounted for in the analysis are: (1) a deterministic model of tube failure, (2) a deterministic model of crack propagation, (3) maintenance of tubes with special emphasis on detection probability, sizing accuracy, potential human errors and effects of sampling inspection. A numerical example evaluates the state of steam generators of Slovenian power plant at Krsko. The resulting single and multiple tube rupture probabilities are presented as a function of maximum allowable length of cracks left in operation. The relative importance of influencing parameters is showing the dominating influence of detection probability. Also, some comments are made on the impact of human errors. Comparison with results obtained by pure deterministic methods is also given. 2. Excessive leak rates through degraded tubing. This failure mode is of special concern in cases where most through-wall defects may remain in operation. A particular example is the application of alternate (bobbin coil voltage) plugging criterion to Outside Diameter Stress Corrosion Cracking at the tube support plate intersections. The aim is to discuss some possible modeling options that could be applied to solve the problem formulated as: 'the sum of all individual leak rates through degraded tubes should be less than predefined acceptable value. The failure of a steam generator is then declared when this value is exceeded'. The probabilistic approach is of course aiming at reliable and computationally bearable estimate of the failure probability. 3. Future work. Some comments on the applicability of stochastic geometry methods in the future assessment of stress corrosion cracking are given. In particular, these methods are considered able to address both failure modes and the performance of the nondestructive examination using the same geometrical model