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:
site ac power system cannot support a safe shutdown and mitigate the effects of an accident in the event of a loss of offsite power. In a single unit, this usually means that two diesel generators are inoperable. In multiple units that share onsite ac supplies, this degradation level usually means that the available onsite supplies, if any, do not have the capacity to mitigate the effects of an event in one unit and safely shut down the other unit(s). Because the offsite power system is the only source of ac power at this level of degradation, the licensee should evaluate the risk associated with continued operation against that associated with immediate shutdown. (Immediate shutdown could cause grid instability, thus resulting in a total loss of ac power.) However, because any inadvertent generator trip could potentially result in a total loss of ac power, the technical specifications severely restrict the time allowed for continued operation. In addition, all work that could potentially trip the unit should be suspended. The intent of GDC-17 is twofold: • Avoid the risk associated with immediate shutdown. • Minimize the risk associated with this level of electrical degradation by limiting the operating time (in the applicable technical specification LCO modes) and by limiting activities that could cause an inadvertent plant shutdown. DG-1244, Page 8 Licensees should make a concerted effort to restore at least one onsite ac power source during the technical-specification-allowed completion time period and during the shutdown required by the technical specifications and to coordinate with the TSO to ensure that the offsite power system can accommodate the plant shutdown in accordance with the plant-specific technical specifications. 6. The available onsite dc power sources are one less than the LCO. This degradation level means that the available dc power sources do not have the required redundancy; however, the remaining train(s) of the dc power system has