The most effective structural condition assessment programs are those that focus on the components most important to safety and at risk due to environ­mental stressor effects. Aging assessment methodolo­gies have been developed to provide a logical basis for identifying the critical concrete structural ele­ments and degradation factors that can potentially impact the performance of these structures.83

An evaluation of the impact on plant risk due to structural aging can also be used in the selection of structural components for evaluation.84 Probabilistic risk assessments conducted to date indicate that the structural systems generally play a passive role in miti­gating design basis (or larger) internal initiating events: a notable exception being the pressure-retaining func­tion of the containment following a degraded core incident involving failure of the reactor pressure vessel. The structural components play an essential role in mitigating extreme events initiated by earthquake, wind, and other extreme influences, and their failure probabilities due to external events can be higher. Moreover, failure of major structural components may impact the operation of a number of mechanical and electrical systems and lead to so-called common cause failures. Thus, deterioration of structural com­ponents and systems due to aging and other aggressive environmental influences may be more serious in terms of overall plant risk than might be evident from a cursory examination of their role in accident mitiga­tion. The significance of structural aging and deterio­ration to plant risk can be evaluated by considering the impact that they have on risk associated with external initiating events, especially earthquakes. It is in miti­gating the effects of strong ground motion due to earthquakes that structural systems play a particularly significant role. The apparent impact of structural aging can be investigated using a margins analysis to assess suitability for continued service. Sensitivity analysis can help to identify the structures of impor­tance that should warrant particular attention.

A third approach involves the combination of finite-element analysis and nondestructive testing methods for evaluation of aging and degradation of concrete containments.85 The CONMOD Project objective was to find a practical means to determine the condition of a containment structure as well as how this condition can be expected to change with time under the influence of various loading condi­tions, including aging. Applications of the approach developed include (1) identification of the critical parts of a structure for nondestructive evaluation including critical parameters, (2) updated structural analyses using input from nondestructive evaluations, (3) prediction of nondestructive responses for a known condition at a given time using finite-element method modeling techniques, and (4) prediction of the nondestructive evaluation responses using finite — element modeling techniques based on a known con­dition and how this will change because of aging processes. One of the conclusions of this study was that development ofnew containment designs should focus on establishing rules, designs, and novel ideas on how to significantly improve the accessibility of the concrete structures for diagnostic investigations.