2.6.1 Crack Prevention and Limitation
The prevention of cracking due to mechanical fatigue starts in the design phase of a system or component. The lifetime of components has to be assessed to take into account the postulated loads and load cycles of the components. As a result of the VERLIFE project, a methodology has been proposed for the assessment of the lifetime of components and piping in VVER plants that takes into account the fatigue ageing mechanism and gives guidance on general provisions necessary for the calculation of residual lifetime, on the allowance of flaws and how to assess residual lifetime of components for specific degradation mechanism. In the design phase, an initiation of macro-defects of the size of 1.0 mm and above is not admissible. According to the proposed methodology in VERLIFE, areas with a cumulative damage coefficient D >= 0.3 shall be assessed. The criteria for the resistance against fatigue damage are met if the cumulative damage coefficient fulfils the relation D <= 0.8. Otherwise an assessment of allowable crack size has to be performed [VERLIFE2003].
Various projects funded by the European Coal and Steel Community (now RFCS), including 7210-MA/131, 7210-MA/823, 7210-MA/951 and 7210-PR/303, investigated design approaches to improve the fatigue performance of welds. In the preliminary projects, namely 7210-MA/131, 7210-MA/823 and 7210-MA/951, different types of joints for fatigue design of stainless steel welds (AISI 304L and S31803 duplex stainless steel) of plates or tubes have been investigated and guidance is given for the use of the design S-N curves and life reduction factors for salt water [EUR19972]. In the ECSC project 7210-PR/303, stainless steel welds (3 mm and 10 mm thick attachments) have been investigated to improve the fatigue performance of such welds [EUR22809]. Besides different welding techniques (metal active gas welding – MAG; tungsten inert gas welding – TIG; and powder plasma arc welding – PPAW), weld toe improvement techniques have also been tested for MAG welded samples, namely grinding, TIG dressing, plasma dressing, and ultrasonic impact treatment (UIT). As a result, none of the welding techniques showed any particular benefit regarding the fatigue performance. All weld toe improvement techniques achieved at least a 30 % improvement of the fatigue strength in air. Practical issues and recommendations are given for the different investigated steel types, welding techniques and weld toe improvement techniques.
Surveillance of nuclear power plant (NPP) components of over the plant lifetime (by monitoring of load history and performing Non-Destructive-Examinations/Testing) as part of an ageing management programme, is one basic principle to promote preventive measures and to guarantee the integrity of the components.
For the surveillance of load history different systems are used in NPPs to observe fatigue damage and to calculate cumulative usage factors (CUF) for equipment important to safety or critical locations (France: Systeme de Surveillance en Fatigue de la Chaudiere – SYSFAC; Germany: Fatigue Monitoring System – FAMOS and Integrity Management System – IMAS; and Russia: residual life automated control system – SACOR) [IAEA11]. Depending on national regulations, CUF offset values are defined to initiate countermeasures, e.g. reduction of load if applicable or renewal of components, or to perform additional analysis.
A technique to investigate crack formation and propagation in-situ uses the positron lifetime. It has been investigated for austenitic stainless steels in different JRC Direct Actions [JRC19983] [JRC26830] [JRC32454]. The method shows a high sensitivity but is limited to the detection of defects close to the surface.