Resolution of Generic Safety Issues: Item A-35: Adequacy of Offsite Power Systems (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )

DESCRIPTION

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.

CONCLUSION

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

(2) The concern defined in Issue 9 to NUREG-0138³² was evaluated by the staff and determined to be "not a significant safety issue" in a 1981 DSI/NRR memorandum⁸³ which also resolved Item B-70.

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.

REFERENCES

0002. NUREG-0371, "Task Action Plans for Generic Activities (Category A)," U.S. Nuclear Regulatory Commission, November 1978. 0011. NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981. 0032. NUREG-0138, "Staff Discussion of Fifteen Technical Issues Listed in Attachment to November 3, 1976 Memorandum from Director, NRR to NRR Staff," U.S. Nuclear Regulatory Commission, November 1976. 0061. Memorandum for J. Murphy from B. Sheron, "Documentation of Generic Safety Issues on Degraded Voltage Protection," July 13, 1994. [9407250133] 0083. Memorandum for K. Kniel from M. Srinivasan, "Generic Issues Tracking System (GITS) B-70; Power Grid Frequency Degradation and Effect on Primary Coolant Pumps," July 31, 1981. [8109140246] 0084. Memorandum for C. Berlinger from E. Butcher, "Diagnostic Evaluation at Quad Cities Nuclear Power Station (TAC Nos. M88667/M88668)," June 8, 1994. [9406130249] 0116. Letter to D. Switzer (Northeast Nuclear Energy Company) from G. Lear (U.S. Nuclear Regulatory Commission), "Millstone Nuclear Power Station Units Nos. 1 and 2," June 2, 1977. [9507280126] 0117. Letter to All Power Reactor Licensees (Except Humboldt Bay) from U.S. Nuclear Regulatory Commission, "Adequacy of Station Electric Distribution Systems Voltages," August 8, 1979. [8005120354]