So far, the licenses of more than 85% of US operating plants have been renewed, authorizing them to continue operations for an additional 20 years past the end of their original, 40-year operating licenses.
10 CFR §54, which governs the NRC’s renewal of plant operating licenses, defines the Current Licensing Basis (CLB), and its role in license renewal applications. The CLB includes, inter alia, compliance with all the regulations in 10 CFR §50, and its appendices, including the design-basis information presented in final safety analysis reports (FSARs). Appendix A of 10 CFR §50 specifies the General Design Criteria (GDCs), and defines the conditions under which they must be satisfied. For example, Appendix A defines anticipated operational occurrences (AOOs) as incidents that may occur during the lifetime of a particular plant. Two standards of the American Nuclear Society (ANS)   redefined AOOs as incidents that may occur during the calendar year for a particular plant, and added a class of events that may occur during the lifetime of a particular plant.
The ANS standards defined a categorization scheme that puts all the various types of incidents into four categories (i.e., Conditions: I, II, III, and IV), and specifies the GDCs and other criteria that must be met for each of the categories. AOOs, or Condition II events, for example, must not result in any fuel damage. Licensees have committed to abide by the ANS categorization system, and to comply with all of the categories’ acceptance criteria. These commitments are in their Final Safety Analysis Reports (FSARs), which are part of their CLBs.
Conditions I, and II define relatively frequently occurring incidents, and require that their consequences to be benign. Condition III events, however, are limited to only a very few incidents during the lifetime of a plant. Their consequences are not benign. (Condition IV is the most severe category. Condition IV events are not expect to occur at all.)
Since the frequency of occurrence of Condition III events is defined in terms of plant lifetime, it follows that lengthening the plant lifetime (e.g., from 40 to 60 years) will lead to the occurrence of more Condition III events. These events can result in fuel damage, or worse. The CLB of a renewed license plant, therefore, will have to account for more Condition III events.
This paper focuses upon how Condition III events can affect the CLB during an extended plant lifetime. It also discusses the concept of extending plant operating licenses by 20, or 40 years, and its potential impact upon the public health and safety.