INTEGER: Integrity and Lifetime Evaluations of Light Water Reactors in the context of Material Ageing

The main goals of the INTEGER research program are to provide scientific contributions to the assurance of the safe and efficient long-term operation (LTO) of the existing Swiss NPPs in the context of material ageing as well as to maintain an independent expertise and to contribute to the education of young specialists in this field.

The special emphasis is hereby placed to pressure boundary components and reactor internals in the primary reactor coolant system, which are very critical components with regard to safety and lifetime. During service, these components are subjected to a variety of different potential ageing and degradation mechanism that can lead to the formation and growth of cracks (e.g., stress corrosion cracking, fatigue) and/or a reduction of the material toughness/ductility and resistance against catastrophic mechanical failure (e.g., irradiation embrittlement, thermal ageing). Potential cracks in critical components thus have to be detected by periodic non-destructive examination before they reach a critical size for safety and sufficient material toughness has to be assured and monitored during the whole lifetime.

Material ageing may seriously affect plant availability & economics, lifetime (extension) and safety. An accurate knowledge and clear mechanistic understanding on the material degradation during service and the precise knowledge of the life-limiting situations as well as a predictive capability for the safety margins, structural integrity and lifetime of ageing components under operational loads over prolonged operating times are evidently indispensable to ensure the safe and economic long-term operation in this context and to define and qualify possible cost-effective mitigation, repair and maintenance actions.

In the Swiss nuclear regulation there is currently no legal limitation of lifetime and structural integrity assessments have to be performed according to the actual and continuously evolving state-of-the-art in science & technology. With regard to the increased age of the Swiss NPP fleet (KKB-I is the oldest operating LWR world-wide), material ageing and the structural integrity of the safety barriers are thus key topics for both the Safety authority ENSI and the Swiss utilities.

• the quantitative experimental characterization of critical ageing mechanism for the safe long-term operation and the validation of their potential mitigation actions
• the identification of the underlying degradation mechanism and mechanistic modeling of the damage evolution and their validation by in-situ mechanical testing at SLS & SINQ and electron microscopy
• the reliable prediction of ageing and degradation with deterministic and probabilistic engineering structural, fracture and damage mechanics methods and their experimental validation under simulated realistic operational conditions to further reduce undue conservatism or uncertainties in lifetime prediction and safety assessments and better estimation of safety margins.
• the development and evaluation of advanced non-destructive in-service inspection and continuous monitoring techniques for the detection, characterization and evaluation of degradation and ageing, in particular in the technical pre-crack stage.