1 Open Phase Conditions Background and Diablo Canyon

1 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 2

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. 3

Open Phase Condition ` 4 Connections Of Interest 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 5

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 6

Byron OPC Events 7 • 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)

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 10 CFR 50. 55 a(h)(2), 10 CFR 50. 55 a(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 8

Industry Stakeholders: INPO Actions 9 INPO IER L 2 12 -14 (Rev. 0 & Rev. 1): Automatic Reactor Scram Resulting from a Design Vulnerability in the 4. 16 -k. V 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 10 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 11

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 12

13 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 – 1 R 20 completion forecast. Trip functions enabled by mid-2018. • Unit 2 – 2 R 20 completion forecast. Trip functions enabled by Dec. 2018.

14 Ref. No. NRC ADAMS Accession No. Document Title NRC Reference Documents 1 ML 120480170 NRC Information Notice 2012 -03, “Design Vulnerability in Electric Power System, ” dated March 1, 2012 2 ML 12074 A 115 NRC Bulletin 2012 -01, “Design Vulnerability in Electric Power System, ” dated July 27, 2012 3 ML 15057 A 085 Branch Technical Position 8 -9, dated July 2015 4 ML 113050583 RIS 2011 -12, Revision 1, “Adequacy of Station Electric Distribution System Voltages” ML 16050 A 212 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) 5 6 ML 16069 A 214 NRR initial response to OPC petition (ML 16050 A 212)

15 Thank You Yves Nembo