In general, the performance of NPP safety-related concrete structures has been very good. However, there have been several isolated incidences that, if not remedied, could challenge the capacity ofthe contain­ment and other safety-related structures to meet future functional and performance requirements. Table 2 presents a summary of local degradation mechanisms that have been observed by one organization during condition surveys ofvarious concrete structures at both United States and foreign NPPs located in areas having several different climatic conditions.25 Some general observations derived from these results were that vir­tually all NPPs have experienced cracking of the con­crete structures that exceeds typical acceptance criteria for width and length; numerous NPPs had groundwater intrusion occurring through the power block or other subsurface structures; and aging con­cerns exist for subsurface concrete structures, as their physical condition cannot be easily verified. Collec­tively, it was concluded in this study that the general performance of the NPP concrete structures, has been quite favorable and proper evaluation and treatment of observed degradation at an early stage is both a cost — effective and necessary approach to long-term plant operations.

Initially, degradation of NPP concrete structures in the United States occurred early in their life and has been corrected.26-28 Causes were primarily related either to improper material selection and construction/design deficiencies or environmental

effects. Examples of some of the problems attributed to these deficiencies include low 28-day concrete com­pressive strengths; voids under the post-tensioning tendon-bearing plates resulting from improper con­crete placement; cracking of post-tensioning tendon anchor heads due to stress corrosion or embrittlement; and containment dome delaminations due to low- quality aggregate materials and absence of radial steel reinforcement or unbalanced prestressing forces.29-31 Other construction-related problems included occur­rence of excessive voids or honeycomb in the concrete, contaminated concrete, cold joints, cadweld (steel reinforcement connector) deficiencies, materials out of specification, higher than code allowable concrete temperatures, misplaced steel reinforcement, post­tensioning system button-head deficiencies, and water-contaminated corrosion inhibitors.26 Although continuing the service of a NPP past the initial operating license period is not expected to be limited by the concrete structures, several incidences of age — related degradation have been reported.28-33 Examples of some of these problems include corrosion of steel reinforcement in water intake structures, corrosion of post-tensioning tendon wires, leaching of tendon gal­lery concrete, low prestressing forces, and leakage of corrosion inhibitors from tendon sheaths. Other related problems include cracking and spalling of con­tainment dome concrete due to freeze-thaw damage,
low strengths of tendon wires, contamination of corro­sion inhibitors by chlorides, and corrosion of concrete containment liners. As the plants age, the incidences of degradation are expected to increase, primarily due to environmental effects. A listing of documented concrete problem areas by plant, type reactor, and degradation is available.34 Documented information on problem areas experienced with NPP concrete struc­tures in other countries has also been assembled.35 Figure 4 presents examples of occurrences of degrada­tion that have been observed at NPPs. Anchor head failure and containment dome delamination shown in the figure represent occurrences related to materials selection and design, respectively, with the remainder representing aging-related occurrences.