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:
d if the onsite sources of ac power, independent of grid condition, are operable and can act as a substitute train of ac power in accordance with the plant-specific technical specifications. Note that some nuclear power units are designed to cause an automatic shutdown or initiate load rejection at this level of degradation. No further discussion is necessary for units designed to undergo an automatic shutdown at this level of degradation; however, those units that initiate load rejection are considered to remain operating within the context of this regulatory guide. 4. The available offsite and onsite ac power sources are each one less than the LCO. This level of power system degradation results from the loss of individual redundancy in both the offsite and onsite ac power systems. Because two different sources of power provide power system redundancy, the reliability, and thus the safety, of this degradation level appears to be slightly higher than that of the previous degradation level. However, the susceptibility of this power system to a single bus or switching failure could offset this apparent improvement. For example, the failure of an emergency power distribution bus that is energized by either the single available offsite circuit or the single available onsite ac supply could render all emergency ac power from that source ineffective. Moreover, if the offsite and onsite power were available to only one train, a bus fault could render all emergency power unavailable. Based on these considerations, the technical-specification-allowed completion time imposed at this level of degradation is more restrictive than that of the previous degradation level. 5. The available onsite ac power sources are two less than the LCO. This degradation level means that the onsite 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