Background and Diablo Canyon Initiatives Open Phase Conditions Background and Diablo Canyon Initiatives Yves Nembo Supervisor, Design Engineering PG&E June 21, 2016

Overview Open Phase Conditions (OPC) Global Operating Experience NRC Actions Industry Follow-up Actions DCPP Initiatives Schedule / Path Forward Introduce yourself! Lead by saying we have not experienced an OPC;

What is an Open Phase Condition (OPC)? Loss of one or two of the three phases (with or without ground), on the high voltage side of a transformer connecting a General Design Criterion 17 (GDC 17) offsite power circuit to the transmission system.

Open Phase Condition ` Connections Of Interest IEEE Std 765-2012 – IEEE Standard for Preferred Power Supply (PPS) for Nuclear Power Generating Stations (NPGS). Figure 4 – Example of an enhanced PPS design Source: IEEE Std 765-2012

Why Do OPCs Matter? An Open Phase Condition creates unbalances (sequence voltages and currents) in the plant AC electrical distribution system. If undetected for an extended period of time, an OPC could lead to: Overheating running motors and serious damages to rotating machines Protective device actuation and lock out

Global Operating Experience Thirteen open phase events between 2001-2016 6 Internationally 7 in United States Open Phase Conditions at Byron Nuclear Generating Station, Illinois: Byron Unit 2 – January 30, 2012 Byron Unit 1 – February 28, 2012 US: 6 Europe: 4 Canada:1 Other: 1 OPC OEs due to either a failure of insulators & switchyard connections or malfunctions of breakers Oconee Nuclear Station, Units 1 and 3 – December 2015 Dungeness B, UK - April 2014 Forsmark, Unit 3, Sweden – May 30, 2013 Bruce Power, Unit 1, Canada – December 22, 2012 Byron Station, Unit 1 – February 28, 2012 Byron Station, Unit 2 – January 30, 2012 Beaver Valley, Unit 1, US – November 1, 2007 Dungeness A, UK – May 14, 2007 Vandellos, Spain – August 9, 2006 Fitzpatrick/and Nine Mile Point, US – December 19, 2005 Koeberg, South Africa – November 11, 2005 South Texas Project Unit 2, US – March 1, 2001

Byron OPC Events Initiated by a failed inverted/underhung porcelain insulator Rendered offsite power supply circuits inoperable Resulted in automatic reactor trip or electrical transients Revealed design vulnerability of offsite power circuit to OPCs Potential noncompliance with General Design Criteria 17 (GDC 17) OHIO Brass Insulator (Underhung type)

NRC Actions Information Notice 2012-03: Design Vulnerability in Electric Power System - Informs licensees of the Byron OE as well as other similar OPC events at other plants Bulletin 2012-01: Design Vulnerability in Electric Power System - Requires licensees to confirm compliance with 10CFR50.55a(h)(2), 10CFR50.55a(h)(3), and GDC 17 BTP 8-9: Open Phase Conditions in Electric Power System - Provides design criteria and guidance to NRC staff in reviewing various actions related to OPC design vulnerability Will be used for future licensing actions to verify compliance with applicable regulations related to electric power systems Pending NRC Actions: OPC Petition: Interim Enforcement Policy for OPC

Industry Stakeholders: INPO Actions INPO IER L2 12-14 (Rev.0 & Rev.1): Automatic Reactor Scram Resulting from a Design Vulnerability in the 4.16-kV Bus Undervoltage Protection Scheme Distributed February 16, 2012 Required prompt interim compensatory actions to detect OPCs Required implementation of procedural guidelines to help operators promptly diagnose and mitigate OPC symptoms Required identification of long-term solutions to provide automatic protection from OPCs

Industry Stakeholders: NEI Actions NEI 13-12: Open Phase Condition Industry Guidance Document Established industry-led OPC Task force Organized various OPC Working Group Workshops Issued a new industry guidance document for OPC Coordinated with the industry’s Chief Nuclear Officers to commit to address OPC and ensure safety margins

OPC Initiatives at DCPP Active engagement in various industry task forces Comprehensive design comparison: Different insulators in the switchyards/offsite power circuits to the plant. Key differences in the physical arrangement of vulnerable conductors and structures Minimal configuration-based risk as compared to Byron configuration Adopted compensatory measures: Operating procedure revised to provide guidance in diagnosing OPCs Daily inspections of equipment Safety is the highest priority at Diablo Canyon; we took immediate action to assess conditions and implement mitigations to guard against OPC. Reviews and analyses of offsite power circuits and existing protection schemes concluded current design is susceptible to undetected OPCs. - PG&E does not utilize Ohio Brass insulators in the switchyards or the offsite power circuits to the plant. - Except for 230 kV disconnect switches; the DCPP switchyard design does not employ suspended post insulators. - The use of rigid bus bar in the DCPP switchyards is limited to straight line sections only (i.e. no "S" configurations). Expansion joints are also used to minimize growth / shrink affects. - These key differences in the physical arrangement of vulnerable conductors and structures continue to demonstrate minimized configuration-based risk as compared to the Byron configuration.

Initiatives at DCPP Immediate Interim Corrective Actions: Operating procedure revised to provide guidance in diagnosing OPCs Implemented switchyards monitoring via daily operator rounds Long Term Corrective Actions: Committed to develop and implement OPC Detection Design Changes on start-up transformers and generator step-up transformers NRC Commitment DCL-14-118 DCPP is committed to the following Open phase Detection implementation schedule: Unit 1 – 1R20 forecasted to be completed in June 2017. Trip functions will be enabled by June 30, 2018. Unit 2 – 2R20 forecasted April 2018. Trip functions will be enabled by December 31, 2018.

Path Forward & Schedule Develop and Implement an OPC Detection and Protection Design Change Phase 1: Alarm Only – To verify the dependability of the system settings. Phase 2: Trip Functions Enabled Implementation Schedule Unit 1 – 1R20 completion forecast. Trip functions enabled by mid-2018. Unit 2 – 2R20 completion forecast. Trip functions enabled by Dec. 2018. NRC Commitment DCL-14-118 DCPP is committed to the following Open phase Detection implementation schedule: Unit 1 – 1R20 forecasted to be completed in June 2017 (Alarm Only). Trip functions will be enabled by June 30, 2018. * Unit 1 Mod ~ 40% complete * No adverse schedule issues at the moment. Unit 2 – 2R20 forecasted April 2018 (Alarm Only). Trip functions will be enabled by December 31, 2018. * Unit 2 scoping meeting scheduled for 11/10/16. * Timing selected to allow incorporation of Unit 1 lessons learned.

NRC Reference Documents Ref. No. NRC ADAMS Accession No. Document Title 1 ML120480170 NRC Information Notice 2012-03, “Design Vulnerability in Electric Power System,” dated March 1, 2012 2 ML12074A115 NRC Bulletin 2012-01, “Design Vulnerability in Electric Power System,” dated July 27, 2012 3 ML15057A085 Branch Technical Position 8-9, dated July 2015 4 ML113050583 RIS 2011-12, Revision 1, “Adequacy of Station Electric Distribution System Voltages” 5 ML16050A212 2.206 Petition on Current Operating Nuclear Power Plants - Open Phase Conditions in Electronic Power System which Lead to Loss of Safety Functions of both Offsite and Onsite Power Systems (NRC Bulletin 2012-01) 6 ML16069A214 NRR initial response to OPC petition (ML16050A212) NRC Reference Documents

Thank You Yves Nembo Safety is the highest priority at Diablo Canyon; we took immediate action to assess conditions and implement mitigations to guard against OPC.