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:
ower 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 (have) full functional capability to effect a safe shutdown and mitigate the effects of an event. Because a subsequent degradation in an onsite ac or dc system could jeopardize unit safety (e.g., a subsequent single failure could render the entire power system ineffective on a generator trip), the technical specifications restrict the time allowed for continued operation. If the affected dc source is restored during the technical-specification-allowed completion time, unrestricted operation may resume. If not, the licensee should promptly shut down the unit in an orderly manner to comply with technical specifications. In addition, the licensee should closely monitor the required functions of the dc system during the shutdown period and take corrective actions, if required, to ensure safety. The passive designs (such as the AP1000) that depend heavily on dc power systems for core cooling and containment integrity typically have batteries that last for 72 hours. The design of batteries to meet the long-duration constant current demands that are required of passive designs may have different monitoring requirements (recommended by their manufacturers) than those required in current designs. Therefore, the licensee should critically monitor the required functions of these batteries during the shutdown period and take corrective actions, if required, to ensure that these batteries will perform their functions for the long duration as designed. 7. The available inverters are one less than the LCO. This degradation level means that the available inverters do not have the required redundancy; however, the remaining trains of the inverter power system have full functional capability to prevent a reactor trip. Because a subsequent single failure of another inverter could cause a