If properly designed and constructed, the concrete structures in NPPs generally have substantial safety margins; however, additional information for quanti­fying the available margins of degraded structures is desired. In addition, how age-related degradation may affect dynamic properties (e. g., stiffness, frequency, and dampening), structural response, structural resistance/ capacity, failure mode, and location of failure initiation is not well understood. A better knowledge of the effects of aging degradation on structures and passive components is necessary to help ensure that the current licensing basis is maintained under all loading conditions.96

Decisions as to whether to invest in maintenance and rehabilitation of structures, systems, and compo­nents as a condition for continued service and risk mitigation, and the appropriate level of investment, should consider the nature and level of uncertainties in their current condition and in future demands.107,108 Recent advances in structural reliability analysis, uncertainty quantification, and probabilistic risk assessment make it possible to perform such evalua­tions and to devise uniform risk-based criteria by which existing facilities can be evaluated to achieve a desired performance level when subjected to uncertain demands.109,110 Consideration of in situ conditions, redundancy, and uncertainties in important engineer­ing parameters often can lead to significant economic benefits when assessing the condition of an existing structure in a (possibly) degraded condition, and the maintenance or rehabilitation strategies that might be required as a condition for future service. Reliability-based approaches have been applied to the NPP concrete structures11 , 2 and in evaluation of the prestress level in concrete containments with unbonded tendons.113

Degradation effects can be quantified with fragil­ity curves developed for both undegraded and degraded components.114 Fragility analysis is a tech­nique for assessing, in probabilistic terms in the pres­ence of uncertainties, the capability of an engineered system to withstand a specified event. Fragility mod­eling requires a focus on the behavior ofthe system as a whole and specifically, on things that can go wrong with the system. The fragility modeling process leads to a median-centered (or likely) estimate of system performance, coupled with an estimate of the varia­bility or uncertainty in performance. The fragility concept has found widespread usage in the nuclear industry, where it has been used in seismic prob­abilistic safety and/or margin assessments of safety- related plant systems.115 The fragility modeling procedures applied to degraded concrete members can be used to assess the effects of degradation on plant risk and can lead to the development of probability-based degradation acceptance limits. This approach has been applied to a limited extent to degraded flexural members and shear walls.96 Additional work is desired in this area for the purpose of refining and applying the time-dependent reliabil­ity methodology for optimizing in-service inspec — tion/maintenance strategies and for developing and evaluating improved quantitative models for predicting future performance (or failure probability) of a degraded concrete structure, either at present or at some future point in time.