RIS 00-024: Concerns about Offsite Power Voltage Inadequacies and Grid Reliability Challenges Due to Industry Deregulation

December 21, 2000


All holders of operating licenses for nuclear power reactors, except those that have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.

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The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS) to inform addressees of concerns about grid reliability challenges which industry deregulation might pose and potential voltage inadequacies of offsite power sources, and of actions the industry has committed to take to address this issue. The NRC is continuing to work with the nuclear power industry to address this matter; this RIS presents the current status of the NRC/industry effort. This RIS does not transmit any new requirements or staff positions. No specific action or written response is required.

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On August 11, 1999, the Callaway plant experienced a rupture of a reheater drain tank line. As a result, the plant operators initiated a manual reactor trip. Once the plant was shut down, offsite power was required to supply the plant equipment loads. During this period, the grid conditions were such that a substantial power flow was observed to occur from north to south through the local Callaway grid. The licensee has stated that the deregulated wholesale market has contributed to conditions in which higher grid power flows are likely to occur. This power flow, coupled with a high local demand and the loss of the Callaway generator, resulted in switchyard voltage at the site dropping below the minimum requirements for greater than 12 hours. Although offsite power remained available during the reactor trip transient, the post-trip analysis indicated that if there had been additional onsite loads at the time of the event, 4.16 kV distribution voltage would have dropped below the setpoint of the second-level undervoltage relays separating the loads from offsite power. The NRC conducted a special inspection at Callaway from November 29 to December 3, 1999, on the circumstances surrounding the event.

The inspectors found that similar conditions prevailed in 1995 that went undetected by the licensee (Licensee Event Report 50-483/99-005 (Accession No. 9909200074); NRC Inspection Report (IR) 50-483/99-15 (Accession No. ML003684343), dated February 15, 2000). NRC Information Notice (IN) 2000-06 (attached) documents several events in which additional combinations of main generator unavailability, line outages, transformer unavailability, high system demand, unavailability of other local voltage support, and high plant load could have resulted in inadequate voltages. Common among all events is the inability to predict the inadequate voltages through direct readings of plant switchyard or safety bus voltages, without also considering grid and plant conditions and their associated analyses.

NRC IN 98-07 discussed the possibility that the deregulation of the electric utility industry could affect the reliability of the offsite power systems in nuclear power plants. NRC IN 95-37 alerted licensees to circumstances that could result in inadequate offsite power system voltages during design basis events. NRC IN 93-17 identified the potential for the loss of plant generation following a loss-of-coolant-accident causing degraded switchyard voltage and leading to a delayed loss of offsite power condition.

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Under the process described in SECY 99-143, "Revisions to Generic Communications Program," the NRC held a meeting with representatives from the nuclear power industry on May 18, 2000, to discuss what assistance industry could provide in addressing the voltage adequacy issue. A summary of this meeting is in the public record under ADAMS Accession No. ML003722320. At this meeting, the staff proposed that the Nuclear Energy Institute (NEI) take the following steps as an industry initiative:

  (1) Provide guidance to utilities on the need for and acceptable techniques available to ensure adequate post-trip voltages;
  (2) Establish provisions to log and evaluate unplanned post-trip switchyard voltages to help verify and validate that the intent stated in Item (1) is met;
  (3) Determine plant-specific risks of degraded voltage/double sequencing scenarios.

Item 1 is based on the Callaway inspection finding in IR 50-483/99-15 that the licensee did not have adequate provisions in place to ensure the functionality of the offsite power system following a trip of the reactor and main generator. The inspection report concluded that this condition was caused, in part, by a failure to consider potential changes in the reliability and electrical characteristics (e.g., voltage levels) of the electrical grid caused by power market deregulation. Reduction in the reliability of the electrical grid from that assumed in the design of plant power systems could result in a failure of the offsite power sources to be able to meet their design capacity and capability, thereby resulting in a failure to meet General Design Criterion (GDC) 17 of Appendix A to 10 CFR Part 50. Licensees must continue to comply with GDC 17 despite any reductions in grid reliability and electrical characteristics.

Item 2 is based on a concern similar to that stemming from the Callaway event, i.e., inadequate voltage to safety loads. This concern was articulated in two NRC generic letters, one dated June 2, 1977 ("Millstone Nuclear Power Station Degraded Grid Voltage") and the other dated August 8, 1979 (Generic Letter 79-36, "Adequacy of Station Electric Distribution Systems Voltages"). These generic letters were followed by Branch Technical Position (BTP) PSB-1 in the NRC standard review plan, which provided the original basis for analyses and commitments on the degraded voltage issue. In addressing this issue, licensees were to establish an anticipated range of normal offsite grid voltages over which they were to demonstrate that adequate voltages would be provided to the terminals of all safety-related equipment for all design-basis events. Typical plant technical specifications (TS) require that written procedures be established, implemented and maintained covering the activities specified in Appendix A of Regulatory Guide 1.33, "Quality Assurance Program Requirements (Operations)," dated February 1978. Appendix A, Section 8.b of Regulatory Guide 1.33, sets forth acceptable positions for implementing procedures for each surveillance test, inspection, or calibration listed in the TS. IR 50-483/99-15 found that the Callaway licensee committed to satisfy the positions of Regulatory Guide 1.33; however, the licensee did not provide adequate instructions to assure reliable indication of switchyard voltages or establish adequate procedures for the evaluation of potentially inoperable offsite power supplies.

Item 3 relates to staff commitments documented in a letter from the Executive Director for Operations to the Advisory Committee on Reactor Safeguards, dated December 28, 1998 (Accession No. 9902120319), which are that the staff will (1) pursue concerns relating to the functional capability of the emergency core cooling system under degraded voltage conditions and (2) determine the regulatory process to be used to ensure that no plant has an electrical configuration that would result in an unacceptable core damage frequency.

The staff also discussed the need for NEI to move expeditiously to develop a schedule for the resolution of this issue and to establish milestones.

NEI informed the NRC in a letter dated June 26, 2000 (ADAMS Accession No. ML003727547), that it had committed to take the following actions:

  • Communicate to licensee chief nuclear officers the issues associated with degraded grid voltage.

  • Monitor industry progress in addressing the industry action plan for addressing grid voltage adequacy concerns.

  • Obtain insights from recently initiated industry efforts to assess the risk significance of potential grid voltage sequences.

  • Conduct an industry workshop early next year on degraded grid voltage to facilitate a broader sharing of licensee practices, insights and activities.

On October 27, 2000, the NRC held a follow-up meeting with representatives from NEI, the Institute of Nuclear Power Operations (INPO), North American Electric Reliability Council (NERC) and Electric Power Research Institute (EPRI) to discuss further the related grid voltage adequacy issues and to provide the status of industry efforts in this area. A summary of this meeting is in the public record under ADAMS Accession No. ML003770544.

NEI discussed plans for an industry workshop on nuclear power plant grid reliability to be held in April 2001. Representatives from INPO and EPRI presented discussions on their initiatives and experience related to ensuring adequate grid voltage to nuclear plants. INPO reported partial results from the nuclear power plant owners implementing Significant Operating Experience Report (SOER) 99-01, "Loss of Grid." INPO reported that 14 stations (27 nuclear units) have been evaluated for implementation of the SOER recommendations. SOER 99-01 contains five recommendations for ensuring that barriers to protect nuclear power plants from grid loss or degradation are in place. INPO will complete its review of SOER 99-01 at all nuclear power plants by June 2002. EPRI provided an overview of the Power Delivery Reliability Initiative, a 5 million dollar program with a goal of reducing the North American Grid vulnerability to disruption of service.

The industry provided information during the October 27, 2000, meeting with the NRC that showed there are initiatives in several areas. These initiatives, combined with the low probability of the initiating event, provide reasonable confidence that nuclear power plants will be protected from the detrimental effects of potential degraded grid voltage situations. The staff will continue to monitor plant events and/or industry trends as described in SECY-99-129, "Effects of Electric Power Industry Deregulation on Electric Grid Reliability and Reactor Safety," dated May 11, 1999.

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This RIS requests no action or written response. Consequently, the staff did not perform a backfit analysis.

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A notice of opportunity for public comment was not published in the Federal Register because this RIS is informational, and the public was afforded opportunities to comment on this issue at the meetings held on May 18, 2000 and October 27, 2000. The NRC intends to work with NEI, industry representatives, members of the public, and other stakeholders to resolve this issue.

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This RIS does not request any information collection.

If there are any questions concerning this RIS, please contact the person listed below.

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David B. Matthews, Director
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation

Technical contact: Ronaldo V. Jenkins
E-Mail: Ronaldo.Jenkins@nrc.gov
  1. NRC Information Notice 2000-06, "Offsite Power Voltage Inadequacies"
  2. List of Recently Issued NRC Regulatory Issue Summaries

(ADAMS Accession Number ML003695551)

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Page Last Reviewed/Updated Wednesday, March 24, 2021