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On January 10, 2011, at 1321 PST, Diablo Canyon Power Plant, Unit 2, entered Technical Specification Limiting Condition of Operation (TS LCO) 3.0.3, when both trains of Auxiliary Building Ventilation System (ABVS) became inoperable following closure of damper M-4 and the ensuing loss of both exhaust fans E-1 and E-2. TS LCO 3.0.3 was exited on January 10, 2011, at 1342 following a status reset and selection of fan E-2. This provided a ventilation flowpath and use of both exhaust fans in the Safeguards mode. Both trains of Auxiliary Building Ventilation are operable. This 8-hour non-emergency report is made pursuant to 10 CFR 50.72(b)(3)(v)(D). The unit is not in a TS LCO. All 3 unit EDG's are operable and offsite power is in the normal lineup. The NRC Resident Inspector has been notified.

• * * UPDATE FROM WES FIANT TO DONALD NORWOOD AT 0021 EST ON 1/14/2011 * * *

On January 13, 2011, at 1603 PST, engineering determined that a single failure design vulnerability exists at Diablo Canyon Power Plant Units 1 and 2. Engineering review of the control logic of the ABVS determined that while in Buildings Only (non-safeguards) alignment or during system realignment from Safeguards Only to Buildings Only alignment, failure of damper M-4A or M-4B (series dampers) could result in the control logic securing both ABVS exhaust fans. This would prevent ABVS actuation on receipt of a valid safeguards actuation signal. When this occurs, a control room alarm is actuated, which requires the operators to reset the control logic from the control room, thereby re-enabling the capability of the ABVS to respond to a safeguards actuation signal. The Unit 2 event on January 10, 2011, at 1321, occurred due to this single failure vulnerability when the control system attempted to restore the ABVS alignment from Safeguards Only to the Buildings Only alignment. This design vulnerability is currently mitigated by maintaining the ABVS in either of the two safeguards alignments (Safeguards Only or Buildings and Safeguards). This single failure design vulnerability is an 8-hour non-emergency report made pursuant to 10CFR50.72(b)(3)(ii)(B) for an event that resulted in the nuclear power plant being in an unanalyzed condition for both Unit 1 and Unit 2. The licensee will notify the NRC Resident Inspector. Notified R4DO (Hagar).

On September 12, 2011, at 1745 PDT operators declared the control room envelope (CRE) inoperable and entered Technical Specification (TS) 3.7.10 Action B. This was due to discovery of inadequately documented CRE in-leakage test data. On September 12, 2011, DCPP (Diablo Canyon Power Plant) personnel reviewing the CRE testing dated February 3, 2005 determined that the test report provided inadequate information to conclude that the most limiting alignment for control room pressurization would result in zero cubic feet per minute (CFM) in-leakage into the CRE, contrary to the Final Safety Analysis Report (FSAR) accident analysis for the most limiting design basis accident. Three of the four ventilation alignments tested had reported values of in-leakage greater than zero CFM. Plant staff implemented compensatory measures by placing the control room ventilation system into its pressurization accident alignment at 1828 PDT using the alignment from the test which had a reported value of zero CFM in-leakage. Additionally, administrative controls are being established to maintain post-Loss of Coolant Accident Emergency Core Cooling System leakage at a rate that would ensure operator doses are maintained less than the FSAR accident analysis results for the highest in-leakage rate reported by the test. Plant personnel notified the NRC Resident Inspector.

• * * UPDATE FROM MICHAEL KENNEDY TO JOHN KNOKE AT 1816 EDT ON 09/16/2011 * * *

On 9/13/11 procedure revisions were approved with reduced limits for post-Loss of Coolant Accident Emergency Core Cooling System (ECCS) leakage. These reduced limits ensure operator doses are maintained less than the FSAR accident analysis results for the highest in-leakage rate reported by the CRE in-leakage test. Plant staff have since determined that the potential benefit of operating the control room ventilation system in its pressurization alignment was unnecessary with the ECCS leakage restriction and on 9/16/11 operators restored the control room ventilation system into its normal operating alignment. The licensee has notified the NRC Resident Inspector. Notified R4DO (Greg Pick)

• * * UPDATE FROM SHANE GUESS TO DONALD NORWOOD AT 0042 EDT ON 10/19/2011 * * *

This is an update to EN #47258 reported on 9/13/11 where it was reported that operators had declared the Control Room Envelope inoperable. (This report was subsequently) updated on 9/16/2011. On 10/18/11 at 16:45 PDT, plant staff determined that the CRE testing dated February 3, 2005 was not performed using a bounding configuration which would result in greatest consequence to the control room operators. The recorded in-leakage from the test was therefore considered to be non-bounding. (As a result of this determination, plant staff have) implemented additional compensatory measures by issuing a shift order requiring the use of self-contained breathing apparatus and potassium iodide tablets under certain accident conditions in accordance with Regulatory Guide 1.196 and NEI 99-03. Plant personnel notified the NRC Resident Inspector. Notified R4DO (Campbell).

On December 31, 2011 at 0200 PST, a plant employee indentified the fire barrier material in each unit was potentially not qualified as a 3 hour rated fire barrier as specified by the Diablo Canyon power plant fire protection program. The fire barrier material in question is installed between the block wall portion of the barrier and the door frames in the walls separating three of the 480 Volt switchgear buses and separating the three vital instrument AC inverters. The condition was discovered by a plant employee who questioned the configuration while performing a routine tour of that area. Operators confirmed existing fire watches were in place and established additional fire watches as compensatory measures as required by the Diablo Canyon fire protection program. A review of the Appendix R fire protection analysis is in progress to confirm the rating of the installed configuration. The licensee has notified the NRC Resident Inspector.

• * * RETRACTION FROM CRAIG HARVEY TO DONALD NORWOOD At 1955 EST on 1/4/2012 * * *

This is an update to EN 47564 reported on 12/31/11 where it was reported that plant personnel had identified rated fire barriers were not qualified as a three hour rated barrier as required by the DCPP Fire Protection Program. On 1/3/12 plant personnel provided barrier design drawings to the manufacturer of the DCPP fireproofing material. The manufacturer reviewed the design detail and confirmed that the barrier design depicted on the drawing is qualified as a 3 hour barrier. Therefore no unanalyzed condition that significantly degraded plant safety existed. This event is being retracted. The licensee notified the NRC Resident Inspector. Notified R4DO (Vasquez).

On 10/9/12, at 16:30 PDT control room operators were questioned whether recently identified fire protection program deficiencies should have been reported to the NRC. On October 3 and October 8, 2012, DCPP (Diablo Canyon Power Plant) staff reviewing NFPA 805 Nuclear Safety Capability Assessment (NSCA) Variance From Deterministic Requirements (VFDRs) identified fire areas that neither conformed to Appendix R requirements nor had established, proceduralized and practiced compensatory measures in place. The issues were identified in the DCPP corrective action program and compensatory measures were established in accordance with the DCPP fire protection program requirements. Event: 10/3/12 Fire areas containing cables associated with startup transformers 1-2 and 2-2 could result in loss of startup power and also prevent the emergency diesels from performing their Appendix R safe shutdown function. Event: 10/8/12 Fire areas containing reactor coolant pump (RCP) breakers could result in loss of RCP seal cooling and prevent the credited local manual trip of the RCPs, contrary to the specified method of performing the Appendix R safe shutdown function. Event: 10/8/12 Fire areas containing cables for the ventilation systems of the 480V switchgear, DC panels, and battery chargers could require unproceduralized use of portable fans to maintain adequate cooling of the electrical equipment necessary to perform the Appendix R safe shutdown function. Operators established fire watches as compensatory measures as required by the DCPP fire protection program requirements. The above late notification of discovery of the unanalyzed conditions has been entered into the DCPP corrective action program. NRC Resident Inspector has been notified.

• • • • • UPDATE AT 0028 EDT ON 11/01/12 FROM GLENN GOELZER TO S. SANDIN*****

This is an update to EN #48395 reported on October 10, 2012, at 0032 EDT. During the NRC's Fire Protection Triennial Inspection the NRC identified that several Alternate Compensatory Measures (ACMs) were not in the current post-fire procedure CP M-10. The ACMs address potential Appendix R non-conformance issues identified via the initiative to convert the DCPP fire protection program to NFPA 805. PG&E has established compensatory measures for all the identified areas in accordance with the DCPP fire protection program requirements. (The) NRC Resident Inspector has been notified. Notified R4DO (Deese).

• * * UPDATE FROM D. BAHNER TO V. KLCO ON 11/30/12 AT 1618 EST * * *

This is an update to EN #48395 reported on October 9, 2012, at 2132 PST. During the ongoing evaluation of the issues previously identified in this event notification, PG&E (Pacific Gas & Electric) has concluded that a fire in the fire areas containing cables associated with startup transformers 1-2 and 2-2 would not result in loss of startup power or prevent the emergency diesels from performing their Appendix R safe shutdown function. This issue was reported as an unanalyzed condition. However, it is analyzed and controlled in plant procedure CP M-10, 'Fire Protection of Safe Shutdown Equipment.' PG&E retracts the initial event documented on October 3, 2012, thus making the date on which the first unanalyzed condition was discovered October 8, 2012. Therefore, PG&E will submit the 60-day Licensee Event Report by December 7, 2012. The NRC Resident Inspector has been notified. Notified the R4DO(Whitten).

• * * UPDATE AT 1944 EST ON 1/30/13 FROM KLINE TO TEAL* * *

This is an update to EN #48395 reported on October 9, 2012, at 2132 PDT. On October 31, 2012, at 2128 PDT, PG&E provided an update to this event notification identifying that several alternate compensatory measures (ACMs) were not in plant procedure, CP M-10, 'Fire Protection of Safe Shutdown Equipment.' PG&E has evaluated this further and concluded that ACMs had been adequately implemented in CP M-10, and therefore retracts the EN update of (November 1, 2012 at 0028 EDT). NRC resident inspector has been notified. Notified R4DO (Walker).

At 1503 PST on December 14, 2013, Pacific Gas and Electric Company identified that, if atmospheric conditions were to develop that had both sustained high winds exceeding 60 miles per hour from the NW to NNE direction, and ambient air temperature exceeding 97 degrees Fahrenheit, the combination of these conditions could result in inadequate heat removal to support continuous operation of the Unit 1 emergency diesel generators. Upon identification of this condition, shift orders were issued requiring implementation of existing procedural guidance to open plant doors to allow additional air flow that would provide adequate emergency diesel generator cooling to support continuous operation of the U1 emergency diesel generators.

This report addresses a condition as described in 10 CFR 50.72(b)(3)(ii)(B).

The NRC Resident Inspector has been notified of this condition. This condition was discovered during a license basis verification review.

The condition described below is being reported as an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) and per the guidance of NUREG-1022, Rev. 3. On 03/06/2014 at 0906 PST, Diablo Canyon Power Plant (DCPP) identified a nonconforming condition involving the Emergency Diesel Generator (EDG) ventilation exhaust plenums installed in Unit 1 and Unit 2. Specifically, the radiator exhaust plenums and exhaust piping need to be re-evaluated to ensure adequate protection against flying debris that could be generated by a tornado. The occurrence of such an event is highly unlikely and there is no imminent concern regarding severe weather involving tornados. The EDGs are located inside the power plant structure and are capable of performing their safety function. Compensatory measures are being developed to address the associated nonconformance. This event does not adversely affect the health and safety of the public. The licensee informed the NRC Resident Inspector."

• * * UPDATE PROVIDED BY RUSS CRUZEN TO JEFF ROTTON AT 2245 EDT ON 09/09/2014 * * *

This condition does not adversely affect the health and safety of the public. Based on an extent of condition review being performed for this event, the issue identified in the original event notification 49879 has also been determined to similarly affect the ventilation systems associated with the Unit 1 and 2 Vital 480 volt AC switchgear and battery/inverter equipment. The condition described in this update is being reported as an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) and as an event or condition that could have prevented the fulfillment of a safety function per 10 CFR 50.72(b)(3)(v)(A). Compensatory measures are being developed to address the associated condition. The licensee informed the NRC Resident Inspector. Notified R4DO (Azua)

On 9/20/2016, a coupling sleeve on a pipe whip restraint located on the 119 foot elevation of the turbine building associated with Unit 2 main steam line 4 was found to be not engaged. As a result of the detached coupling, the restraint was not capable of performing its restraint function for a postulated pipe whip event on the main steam line. The coupling was reconnected on 9/20/2016, restoring its functionality. An extent of condition walkdown was subsequently performed for the other Unit 1 and Unit 2 steam line restraints and no similar issues were identified. This concern did not result in any adverse effect on the radiological health and safety of the public. The purpose of this whip restraint is to restrain the steam line for a postulated loss at the G-line anchor (east side of Turbine Building above the 104 foot elevation). The restraint protects the floor slab at the 104 foot elevation, which extends over the Unit 2 component cooling water heat exchangers. With the detached coupling, equipment in the area may have been vulnerable to damage if a pipe whip event occurred. Further analysis is needed to conclude whether the heat exchangers and other equipment would have remained protected in such an event and whether this would have significantly affected the designed plant response to a pipe event. Based on the need for further analysis, this event is being reported as an unanalyzed condition that may have significantly degraded plant safety in accordance with 10 CFR 50.72(b)(3)(ii)(B). The NRC Senior Resident Inspector was notified.

• * * RETRACTION FROM FRANK LEE TO DONALD NORWOOD AT 1953 EST ON 11/18/2016 * * *

The purpose of this notification is to retract a previous report made on EN #52260, reported 9/23/2016. NRC notification was initially made as a result of a condition that required further analysis to determine whether the condition would have significantly degraded plant safety in accordance with 10 CFR 50.72(b)(3)(ii)(B). Further analysis of the condition concluded that PG&E Design Class I equipment located inside the Turbine Building would have remained undamaged and capable of performing their safety functions. The Turbine Building would not have experienced failure of major structural elements or adverse impact to the overall building stability. Therefore, the coupling sleeve on a pipe whip restraint located on the 119 foot elevation of the Turbine Building associated with Unit 2 main steam line 4 that was found to be not engaged did not constitute an unanalyzed condition that may have significantly degraded plant safety in accordance with 10 CFR 50.72(b)(3)(ii)(B). The NRC Resident Inspector has been notified. Notified R4DO (Azua)