Document: NRC Regulatory Guide
Document ID: 9a8db182-3194-4663-aeb3-64f2e4edd6aa
Document Type: regulatory_guide
Title: Availability of Electric Power Sources (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1008/ML100840581.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.93
CFR Part: 
CFR Title: 

Content:
system degradation in order of increasing severity: 1. The available offsite ac power sources are one less than the LCO. This degradation level means that one of the required offsite ac sources is not available and that, therefore, the offsite ac power system has no redundancy. Operation could, therefore, safely continue if the availability of the remaining power sources is verified; however, because the system is degraded below the LCO, the technical specifications identify a time limit on continued operation. Operating experience indicates that availability and reliability are higher for typical offsite ac power sources than those of a typical onsite ac power source. Thus, if risk is evaluated in terms of availability and capability, the risk associated with the loss of an offsite power source (the source with higher availability) would appear to be more severe. However, this apparent difference in severity is usually offset by maintainability considerations—that is, the time required to detect and restore an unavailable offsite source is generally much less, especially when the grid operator uses real-time contingency analysis. However, if the loss of an offsite source occurs because of an event associated with extensive consequences, such as a severe ice storm or forest fire, the event has severe implications. The risks associated with such an offsite loss would be compounded because of the following: • The ability to quickly restore the offsite sources would likely be lost. • The remaining offsite circuit could be susceptible to the same cause. • The consequential trip probability of a number of units would be higher because of the potential loss of loads. • The availability and capability of the offsite power system might be affected. DG-1244, Page 6 Thus, if the first offsite source is lost (because of a fire or ice storm), then the licensee should evaluate the susceptibility of the second source for a potential common-cause failure and continued