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 Report dateSiteEvent description
05000323/LER-2016-00128 July 2016Diablo Canyon

On May 30, 2016, at 0930 PDT, with Unit 2 in Mode 4, "Hot Shutdown," licensed operators responding to a difference greater than 12 steps between digital rod position indication (DRPI) and demand position indication in the control room, manually opened the Reactor Trip Breakers in accordance with plant procedures. The plant operators stabilized the plant and technicians identified a failure of a control rod moveable gripper fuse. At 1611 PDT, plant operators made an 8-hour, nonemergency notification in accordance with 10 CFR 50.72(b)(3)(iv)(A). Plant technicians replaced the fuse and plant operators confirmed proper operation by performance of surveillance testing.

The cause was attributed to Surveillance Test Procedure (STP) R-1C, "Digital Rod Position Indicator Functional Test," which did not explicitly specify actions to identify improper DRPI indications prior to exceeding a 12-step difference between rod demand and rod position indication.

Corrective actions to prevent recurrence include revision of STP R-1C to include guidance regarding verification of rod motion prior to exceeding 12 steps and operator training of the changes to the procedure.

This event did not adversely affect the health and safety of the public.

APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 205550001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000275/LER-2015-00111 February 2016Diablo Canyon

On December 31, 2014, while performing a walkdown as part of a surveillance test procedure, plant personnel identified through-wall seepage in a Diablo Canyon Power Plant Unit 1 socket weld inside containment that provides a flow path to a relief valve protecting a common portion of both trains of the residual heat removal system. Subsequent cleanup of the boric acid accumulation revealed active seepage of 30 drops per minute. A visual inspection identified that the source of the seepage was a circumferential crack on the socket weld.

Pacific Gas and Electric Company determined that the root cause of the cracked socket weld was containment fan cooler unit (CFCU) vibration inducing a resonant condition in the residual heat removal piping that generated stresses above the material endurance limit of the socket weld. Corrective actions included replacing two socket welds, modifying pipe supports, and correcting the condition causing the CFCU vibrations.

This condition did not have an adverse effect on the health and safety of the public.

05000275/LER-2015-0023 December 2015Diablo Canyon

As part of an apparent cause evaluation, Pacific Gas and Electric Company (PG&E) identified an incorrect insulation configuration, installed in 2010, on the thermal extension piping that houses the wires for the wide range (WR) reactor coolant system (RCS) resistance temperature detectors (RTDs). The insulation configuration, as installed, trapped heat inside the thermal extension piping and overheated the wires. The cause of the incorrect configuration of the insulation was insufficient guidance in the associated work package instructions.

An engineering analysis completed on October 5, 2015, determined that the eight WR RCS RTDs had either failed or were operating outside the environmental qualification temperature range. As a result, on October 5, 2015, PG&E determined that the required number of WR RTDs would not have been operable and therefore a violation of Technical Specification 3.3.3, "Post Accident Monitoring (PAM) Instrumentation," had occurred.

As part of the corrective actions, PG&E replaced all eight WR RTDs, restored the insulation per the design requirements, revised the drawings for Unit 1 WR RTDs to provide adequate level of detail, and revised the work order to include the correct drawing and level of details for proper installation of all WR RTDs. This event did not adversely affect the health or safety of the public.

05000323/LER-2014-0027 May 2015Diablo Canyon

On August 13, 2014, while performing scheduled maintenance on Unit 2 Emergency Diesel Generator (EDG) 2-2, Diablo Canyon Power Plant (DCPP) identified a failed Inlet-to-Fuel-Header capscrew on Engine Cylinder 1L. As part of subsequent inspections to determine whether a similar condition existed on any of the other Unit 1 or Unit 2 EDGs, a degraded capscrew was identified on EDG 2-3 Cylinder 8L.

No capscrew issues were identified on the Unit 1 EDGs or on Unit 2 EDG 2-1. EDG 2-3 was declared inoperable at 1631 on August 14, 2014, resulting in two of three EDGs being inoperable at the same time, which requires ensuring at least two EDGs are operable within 2 hours, or be in Mode 3 within the following 6 hours.

Although the capscrew on EDG 2-3 was successfully replaced within 2 hours, during fuel system fill and vent following corrective maintenance, a fuel oil leak from the belt driven fuel oil booster pump occurred. Because repairs of EDG 2-3 could not be completed within the time permitted by Technical Specification 3.8.1 for two EDGs inoperable, a Unit 2 plant shutdown commenced. On August 14, 2014, at 2351 hours, Unit 2 entered Mode 3.

The cause of the failed capscrew on EDG 2-2, and the degraded capscrew on EDG 2-3, was determined to be high cycle fatigue. The cause of the fuel oil booster pump leak was determined to be a manufacturing defect combined with high seal annulus pressure during fuel oil system priming. Corrective actions include replacement of all capscrews with an improved material design, and incorporation of updated vendor guidance and updated fuel system priming instructions into station procedures.

This event did not adversely affect the health or safety of the public.

05000323/LER-2013-00430 March 2015Diablo Canyon

On June 8, 2013, at 08:40 PDT, with Diablo Canyon Power Plant (DCPP) Unit 2 in Mode 1 at 100 percent power, Emergency Diesel Generator (EDG) 2-3 failed to complete a scheduled surveillance run. Cyclic fatigue failure of a wire lug in the EDG 2-3 current differential protection circuit caused an automatic EDG shut down 21 hours and 42 minutes into the 24-hour load run. DCPP determined the last time EDG 2-3 would have been able to complete its 24-hour surveillance run was greater than the technical specification allowed outage time. Additionally, during the time that EDG 2-3 was unable to complete its load run, EDGs 2-1 and 2-2 also had been declared inoperable on several occasions.

DCPP determined that a vibrating terminal block cover induced cyclical fatigue in the wire lug, causing it to fail.

DCPP replaced the broken wire lug and permanently removed the cover. Additionally, DCPP will revise a procedure to periodically inspect for wear on the wires and lugs.

This condition did not adversely affect the health and safety of the public.

05000275/LER-2015-001, Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld2 March 2015Diablo Canyon

On December 31, 2014, while performing a walkdown as part of a surveillance test procedure, plant personnel identified through-wall seepage in a Diablo Canyon Power Plant Unit 1 socket weld inside residual heat removal system. Subsequent cleanup of the boric acid accumulation revealed active seepage of 30 drops per minute. A visual inspection identified that the source of the seepage was a circumferential crack on the socket weld.

This is the initial Licensee Event Report (LER) for this event. Pacific Gas & Electric will submit a supplemental LER describing event cause and corrective actions no later than May 8, 2015.

This condition did not have an adverse effect on the health and safety of the public.

05000323/LER-2013-00521 November 2013Diablo Canyon

On July 10, 2013, at 09:50 PDT, while performing the periodic hot-washing of the 500 kV insulators, a flashover of the Phase A 500 kV to ground across the Phase A lightning arrestor occurred and actuated the 500 kV differential relay. The actuation of the 500 kV differential relay opened the Unit 2 generator output breakers to isolate the generator, which then actuated a turbine trip. Since Unit 2 was operating above the 50 percent power permissive, the reactor protection system initiated a Unit 2 reactor trip. All plant equipment responded as designed.

Diablo Canyon Power Plant (DCPP) staff determined the root cause of this event to be the hot-washing of the Phase A transmission line string insulators (500 kV dead-end insulators) with inadequate controls for oversight of supplemental PG&E transmission line personnel and on-line maintenance risk analysis that resulted in a conductive overspray, which induced an external arc around the lightning arrester insulation resulting in flashover. The corrective action to prevent reoccurrence involves the development and implementation of a maintenance strategy for 500 kV dead-end insulators to ensure they remain adequately contamination free, structurally sound, and minimize risk to DCPP.

There were no personnel injuries, no offsite radiological releases, and no damage to safety-related equipment associated with this condition. This condition did not have an adverse effect on the health and safety of the public.

05000323/LER-2013-00222 August 2013Diablo Canyon

Refueling Outage Cycle 17, Unit 2 source range (SR) instrument N-32 experienced an unexpected increase in indicated counts per second (cps). Other available SR indications showed no rise in cps. At the time, SR instrument N-31 was inoperable. Since operators considered N-32 inoperable while N-31 was already inoperable, the audible count rate in the control room was no longer reliable. DCPP determined this condition constituted a loss of a safety function required to maintain the reactor in a safe shutdown condition and was reportable in accordance with 10 CFR 50.73(a)(2)(v)(A).

Following a vendor failure analysis, DCPP determined that a discontinuity in the cable insulation shield caused the N-32 high count rate readings. DCPP replaced the faulted cable. This condition did not adversely affect the health and safety of the public.

05000323/LER-2012-00226 June 2013Diablo Canyon

On October 11, 2012, at 12:08 PDT, the Diablo Canyon Power Plant (DCPP) Unit 2 500kV line differential relay actuated, resulting in a unit trip. The 500kV coupling capacitor voltage transformer (CCVT) bushing experienced a flashover to ground, resulting in a unit trip and turbine trip. With the turbine tripped and Unit 2 operating above the 50 percent power permissive, the reactor protection system initiated a reactor trip as designed. All plant equipment, including the auto-start of the auxiliary feedwater (AFW) system, responded as designed.

DCPP determined that the bushing failed because the insulator minimum creepage distance was not consistent with industry codes and standards for its operating environment. When the bushing was replaced in 2011, DCPP staff made unvalidated assumptions, and over-relied on industry experts. DCPP will move CCVT metering to an area with lower contamination levels, and reinforce expectations to review current industry codes and standards.

Additionally, an unintended AFW pump restart occurred following this event as a result of a human performance error that resulted in a procedure not being revised following a plant modification. DCPP will revise procedure supporting documents and train procedure writing staff on use of the supporting documents to identify all changes required by a plant modification.

05000275/LER-2013-0014 April 2013Diablo Canyon

On January 3, 2013, at 19:32 PST, with Units 1 and 2 in Mode 1 and at 100 percent power, Diablo Canyon Power Plant (DCPP) determined that the Limiting Condition for Operation of Technical Specification (TS) 3.4.12, low temperature overpressure protection system, was not met during Unit 1 and Unit 2 refueling outages over the past 3 years.

Specifically, when TS 3.4.12 was applicable, DCPP operated with more than one centrifugal charging pump (CCP) capable of injecting into the reactor coolant system. DCPP determined this condition was reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). The noncompliance was identified based on a Nuclear Regulatory Commission TS Interpretation letter dated January 3, 2013, to Wolf Creek Nuclear Operating Company.

DCPP concluded that it had not complied with TS 3.4.12 since it replaced the positive displacement pump (PDP) with a CCP in Unit 1 (2005) and in Unit 2 (2007).

Immediate corrective actions in response to this event included revising the affected procedures to ensure compliance with TS 3.4.12.

The apparent cause for this event includes a deficiency in DCPP's 10 CFR 50.59 procedure and human error. The procedure did not provide guidance regarding proposed design changes that may maintain the original intent but create new literal compliance issues. The human error occurred when DCPP staff interpreted the operability requirements outlined in TS 3.4.12 as being equivalent with respect to the PDP to CCP design change.

Corrective actions included revising the associated 10 CFR 50.59 procedure, revising the Current Licensing Basis Determination Procedure and providing a lessons-learned discussion to the staff.

This event did not adversely affect the health and safety of the public.

05000275/LER-2010-00224 September 2010Diablo Canyon

On March 9, 2010, while analyzing the consequences of a postulated sustained degraded grid voltage, Pacific Gas and Electric (PG&E) concluded both Units 1 and 2 were in an unanalyzed condition. On March 9, 2010, at 23:39 (EST), PG&E reported this unanalyzed condition to the NRC in accordance with 50.72(b)(3)(ii)(B) (reference NRC Event Notification Number 45754).

The postulated sustained degraded voltage condition could have resulted in multiple safety-related pump motors, tripping overcurrent relays. As a result, these pumps would not be immediately available to mitigate a postulated accident and is considered a safety system functional failure.

PG&E had not considered this postulated worst case degraded voltage condition credible, and had therefore not analyzed for it. As a result, technical specification (TS) surveillance requirement (SR) 3.3.5.3 values are nonconservative, and several TS Actions were not met.

The immediate compensatory measure was to raise the first level undervoltage relay setpoints on the vital buses of both Units 1 and 2, thus transferring loads to emergency diesel generators prior to tripping motors on overcurrent. PG&E will submit a license amendment request to establish conservative TS SR 3.3.5.3 undervoltage relay settings.

The apparent cause was misinterpretation of the undervoltage relay design criteria in the 1970's.

05000275/LER-2009-00228 August 2009Diablo Canyon

On June 29, 2009, at 06:47 PDT, with Unit 1 in Mode 1 (Power Operation) the Eagle 21 Protection Set II, Rack 8, alarmed in the control room due to a Loop Calculation Processor (LCP) card failure. Plant operators declared motor-driven auxiliary feedwater (AFW) Pumps 1-2 and 1-3 inoperable in accordance with TS 3.7.5 Limiting Condition for Operation (LCO).

Plant operators entered Operating Procedure (OP) Abnormal Procedure AP-5, "Malfunction of Eagle 21 Protection or Control Channel," and took manual control actions in the control room.

On June 29, 2009, at 07:14 PDT, Technical Specification (TS) 3.7.5, Condition C, was exited when the first level control valve (LCV) was placed in manual and a dedicated licensed plant operator was assigned to the AFW level controller. TS 3.7.5 LCO was exited when the second LCV was placed in manual at 07:17 PDT.

The cause of the TS 3.7.5 entry was determined to be the result of intended design response to an Eagle 21 LCP failure, i.e., to lockup the control output in a "fail-as-is" status to minimize a plant transient as a result of a single failure. On June 30, 2009, at 08:26 PDT, the failed Eagle 21 LCP card was replaced and the LCVs returned to automatic level control.