Resolution of Generic Safety Issues: Item A-35: Adequacy of Offsite Power Systems (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
This issue was raised in NUREG-03712 following a series of events at operating plants that involved the offsite power systems. These events indicated that the reliability of the preferred source of emergency power might have been less than was expected. As a result, a study was recommended to assess the concern.
GDC-17 of Appendix A to 10 CFR 50 requires the offsite power source to be available and to have sufficient capacity and capability to assure that: (1) the fuel and reactor boundary are maintained within specified acceptable limits; and (2) core cooling, containment integrity, and other vital safety functions are maintained during accident conditions.
In the past, the NRC accepted the results of transient and steady state stability analyses documented in the SARs for license applications which indicated that offsite power sources remained stable and met the requirements of GDC-17. Issue 10 of NUREG-013832 defined the staff's concerns relating to: (1) the reliability of the offsite power system as the preferred emergency source; (2) vulnerability of safety-related equipment to sustained degraded voltage; (3) adequacy of design interfaces of offsite and onsite power sources; and (4) adequacy of testing the onsite power sources. In addition, Issue 9 of NUREG-013832 defined a concern relating to a rapid rate of frequency decay of the offsite power system. A rapid rate of frequency decay could provide an electrical braking effect on the RCP motors resulting in a flow coastdown in excess of that analyzed in the accident analysis portion of the SARs.
The power systems of operating plants and those in the licensing process were evaluated to determine the susceptibility of safety-related electric equipment to: (1) sustained degraded voltage condition on the offsite power source; (2) interaction of the offsite and onsite power sources; and (3) adequacy of existing testing requirements. Work performed by the staff resulted in a two-part resolution of the issue:
(1) In order to address the concern discussed in Issue 10 of NUREG-0138,32 new requirements were issued to some licensees116 in 1977 and to all licensees except Humboldt Bay117 in 1979. Between 1977 and 1981, these requirements were used as the interim BTP in licensing reviews. In 1981, Appendix A to SRP11 8.3.1 was revised to incorporate these requirements as BTP PSB-1, "Adequacy of Station Electric Distribution System Voltages." Thus, all licensees issued operating licenses since 1977 were in compliance with the revised Appendix A to SRP11 8.3.1.
The voltage-sensing protection stated in BTP PSB-1 conforms to a number of requirements of IEEE Std. 279-1971397; however, the single-failure criterion is not explicitly cited. Although the requirements do not include this single-failure criterion for the sensing portion of the design, independent sensing is to be provided for each electrical division. Since most plants have 2 independent 100% divisions of electrical power for all analyzed events, the overall single-failure criterion is met by providing independent sensing for each division.
Conforming to the single-failure criterion is not as straightforward at plants with a swing bus design. NRR recognized that such a design can compromise the independence of the electrical divisions and some scenarios involve such single-failure vulnerabilities as the loss of a DC power source. In some cases, explicit exemptions to the single-failure criterion were given. Although designs that would withstand a single failure in the degraded voltage sensing could be implemented, NRR/EELB concluded61 that plants with a swing bus design were not explicitly required to meet the single-failure criterion with regard to degraded grid voltage relays during LOCA conditions, a low probability event.84
Thus, new criteria resulting from the resolution of this issue were factored into the licensing process by the preparation of new or revised TS, BTPs, and SRP11 Sections. With the revision to Appendix A of SRP11 Section 8.3.1 and the DSI evaluation,83 the issue of the adequacy of offsite power systems was RESOLVED and new requirements were established.