Structural Integrity of VVER Steam Generator Tubes
D. Stanic, INETEC, Institute for Nuclear Technology
ABSTRACT
In accordance with experience on NPP's with VVER type of steam generators, several degradation mechanisms cause tube degradation. Considering damage morphology, these degradation mechanisms cause different types of damages, depending of theirs nature. Good characterization of damages may be achivied through the development and application of advanced eddy current methods. Unfortunately, up to now, these techniques were not applied in wide range in VVER steam generator examination. In conjuction with results of metalography on pulled tubes, application of advanced techniques provides better understanding of degradation mechanisms acting. Information of damage characteristics (shape, orientation, dimensions) obtained by advanced techniques are good base for furter evaluation of damage influence on tube integrity. Namely, in many countries with VVER steam generators, plugging criteria is based only on percentage of tube wall thickness. Experience shows that it is too c onservative criteria, and may results, in some cases, with significant number of plugged tubes. Therefore, on the basis of degradation mechanisms nature and results of examination with advanced techniques, damages should be classified and structural integrity analysis have to be performed for each type of damage. Application of analytical methods provides assesment of simplier forms of damages. Valuable results may be obtained by experimental methods, but tubes with real defects from VVER steam generators are very rare. In comparison with analytical and experimental methods, numerical methods (such as finite element method) are also efficient tools for integrity assessment. Application of finite element method provides relatively simple modeling of different type of damages and simulation of various operating conditions. The stress and strain analysis may be performed in elasto-plastic area with good ability of visual presentation of results. Results obtained by numerical analysis sup plemented with experimental results are the good base for definition of less conservative plugging criteria.
Publication Source: 2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, May 24-26, 2000, New Orleans, Louisiana USA. Publisher: EPRI - [Homepage]
