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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.

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.

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.

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.

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.

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.