In the United States, the Atomic Energy Act and regulations of the USNRC limit commercial power reactor licenses to an initial 40-year period, but per­mits such licenses to be renewed. (Other countries may not have a limit set on the plant operating license period, but the utility must obtain a permanent renewal of its operating license subject to numerous and continuous justifications (e. g., periodic safety

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reevaluations).) This original 40-year term for reactor licenses was based on economic and antitrust consid­erations — not on limitations of nuclear technology. Due to this selected period, however, some structures and components may have been engineered on the basis of an expected 40-year service life.

Several nuclear power units in the United States have reached the end of their initial operating license period. To help ensure an adequate energy supply, the USNRC has established a timely license renewal process and clear requirements that are needed to ensure safe plant operation for an extended plant life. These requirements are codified in Parts 51 and 54 of Title 10, Energy, of the CFR that provides for a renewal of an operating license for an additional 20 years. In order to ensure the safe operation of NPPs, it is essential that the effects of age-related degradation of plant structures, as well as systems and components, be assessed and managed during both the current operating license period as well as subsequent license renewal periods.

As these plants mature, environmental factors are going to become increasingly important. Demonstra­tion of continued safe and reliable operation of the plants will involve implementation of a program that effectively manages aging to ensure the availability of design safety functions throughout the plant ser­vice life. Examples of considerations to be addressed by such a program for the safety-related concrete structures are identified in Figure 5 and include the following:

1. What environmental stressors or aging factors are most important with respect to impacting struc­tural reliability?

2. What in-service inspection or condition assess­ment programs are most effective in demonstrat­ing structural reliability, and how often should they be applied?

3. What material sampling and testing programs should be required, if any?

4. How effective are remedial measures in enhancing the reliability ofthe structures and extending their usable life?

5. How have the material properties changed under the influence of aging and environmental stressors?

6. What is the residual life of the structure and how might it respond to something like a design basis event?

General guidance on developing an aging manage­ment program for concrete containment buildings has been developed.35 Included in this reference is information related to practices and techniques that have been utilized by various countries for assessing the fitness for service as well as inspection, monitoring, and mitigation of aging degradation of concrete con­tainment buildings. The International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM) has held an interna­tional conference, prepared a report, and sponsored two workshops related to aging management of con­crete structures.36-39 Finally, the Nuclear Energy

Agency Committee on the Safety of Nuclear Installa­tions (NEA/CSNI) under its Integrity of Components and Structures Working Group (IAGE WG) has prepared several reports and held a series of work­shops that addressed various aspects of aging of NPP concrete structures.40-52 Also, there are a number of other documents that address aging of NPP concrete structures,32,53-56 as well as national programs.