Event Notification Report for October 11, 2012

PART 21 - POTENTIAL GROUNDING PATHWAY BETWEEN HEAT SINK AND CHASSIS OF CONTROL MODULES

The following information was obtained from the vendor via facsimile:

"This report notifies the Nuclear Regulatory Commission of a defect in CMM830 and TMD830 [modules] that could result in an unintended ground loop that could lead to a low impedance connection between the module output circuitry and chassis.

"There is a mechanical interference between the sheet metal housing and the heat sink for transistor Q1 on the master board assembly. The heat sink of transistor Q1 is connected to the drain of Q1, which is the output of the current loop circuit. Should the heat sink of transistor Q1 short to chassis because of the interference, then an unintended ground loop could lead to a low impedance connection between the module output circuitry and chassis. The effect of grounding Q1 to chassis will vary with the application.

"If Q1 is grounded to the chassis, but no ground connection exists in the output loop, the incidental ground may produce no observable effect on the performance of the module. In that case, any subsequent independent event which introduces a ground in the output loop will cause a loop fault, potentially affecting the module output current.

"Alternatively, if Q1 and chassis are initially isolated, the defect may not manifest unless vibration causes mechanical wearing of the oxide coating of the heat sink. It is possible that a seismic event could cause sufficient wearing to initiate a loop fault, again potentially affecting the module output current."

The affected facilities are HB Robinson and Turkey Point.

TECHNICAL SUPPORT CENTER OUT OF SERVICE DUE TO MAINTENANCE

"At approximately 1428 EDT on October 11, 2012, preplanned maintenance will be performed that will affect the Technical Support Center (TSC) ventilation system. This maintenance will be performed as part of an electric heater control panel inspection (DH-1). This maintenance is expected to be performed and completed within 4 hours. If an emergency condition should occur, the power will be restored, but potentially not within the time required for activation of the TSC. In addition, the Harris Nuclear Plant (HNP) emergency plan has procedural guidance for use of an alternate location in the event the TSC is not habitable.

"This event is reportable per 10CFR50.72(b)(3)(xiii) as described in NUREG-1022, Rev. 2 since this work activity affects an emergency response facility for the duration of the maintenance."

The licensee notified the NRC Resident Inspector.

UNIT 2 EXPERIENCED AN AUTOMATIC REACTOR TRIP ON A 500KV LINE DIFFERENTIAL RELAY ACTUATION

"On October 11, 2012, at 1208 PDT, the Diablo Canyon Power Plant Unit 2 500kV line differential relay actuated, resulting in a unit trip. The unit trip actuated the turbine trip. With the turbine tripped and Unit 2 operating above the P-9 50% power permissive, a reactor trip was initiated from the reactor protection system. All plant equipment responded as designed.

"Unit 2 is stable at normal operating temperature and pressure. All power transferred to the plant startup source without incident. Condenser vacuum was maintained. The cause of the differential relay actuation is under investigation. Based on personnel observation it appears to have been initiated due to a flashover to ground across the phase 'A' main bank transformer capacitive coupled voltage transformer.

"PG&E will issue a press release regarding the Unit 2 trip.

"NRC Resident Inspectors have been informed of this event."

* * * UPDATE FROM DAN STERMER TO DONG PARK AT 2259 EDT ON 10/11/12 * * *

"This update to clarify the status of the auxiliary feedwater system during the above noted transient. Both motor driven pumps and the steam driven pump automatically started as expected.

"In addition, on October 11, 2012, at 12:26 PDT, the Diablo Canyon Power Plant Unit 2 turbine driven auxiliary feedwater pump was secured by plant operators manually closing the steam supply valve to the pump turbine. This action was performed in accordance with plant operating procedures after the operators had verified that the indicated steam generator levels were greater than the procedural requirement of 16%. However, because the steam generator low level bistables associated with the auxiliary feedwater actuation circuits had not yet cleared, the emergency safeguards actuation signal drove the steam supply valve back open, restarting the turbine driven auxiliary feedwater pump.

"Operators increased steam generator levels to clear the bistables and successfully reclosed the steam supply valve.

"The NRC Resident Inspector has been informed of this event."

Notified R4DO (Hagar).

OFFSITE NOTIFICATION DUE TO ONSITE SIRENS ALARMING DURING A DRILL

"On 10/11/12 the Prairie Island Nuclear Generating Plant (PINGP) Security Force was conducting scheduled force on force security drills from 2000 to 2330 CDT. At 2107 CDT the Licensee was notified by the Wisconsin State Radiological Coordinator (SRC) that an intern had contacted the Wisconsin State Patrol Duty Officer to say they had taken cover at the plant and that there were sirens going off in the background. The intern did state that he did not know whether this was a real event or a drill.

"The Wisconsin State Patrol contacted the office of Wisconsin Emergency Management and the SRC. The SRC then contacted the PINGP Control Room and was informed by control room personnel that this was a security force-on-force drill. Xcel Energy Communications has notified other government agencies of this event via an email. No press release is planned at this time. This event is reportable per 10 CFR 50.72(b)(2)(xi)."

The licensee has notified the NRC Resident Inspector.