05000323/LER-2014-002

LER-2014-002, Unit 2 Plant Shutdown Required by Technical Specifications

Diablo Canyon Power Plant, Unit 2
Event date:	08-14-2014
Report date:	05-07-2015
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3232014002R01 - NRC Website

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I. Reportable Event Classification

This event is reportable pursuant to the following criteria:

• 10 CFR 50.73(a)(2)(i)(A), "Plant shutdown required by Technical Specifications"

• 10 CFR 50.73(a)(2)(v), "Any event or condition that could have prevented the fulfillment of a safety function.

• 10 CFR 50.73(a)(2)(vii), "Any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system.

II. Plant Conditions

Unit 2 was in Mode 1 at 100 percent reactor power, with normal operating reactor coolant temperature and pressure.

A. Background

Each Diablo Canyon Power Plant (DCPP) unit has three emergency diesel generators (EDGs) [DG] that provide vital backup power to three electrical buses [BU] to mitigate the consequences of a design basis accident (DBA) whenever normal and backup offsite power sources [EK] are unavailable. DCPP EDGs are designed to function so that a single failure of any EDG will not jeopardize the capability of the remaining EDGs to start and provide power to operate the shutdown systems required to mitigate any DBA condition.

B. Event Description

On August 13, 2014, while performing scheduled maintenance on Unit 2 EDG 2-2, Pacific Gas and Electric Company identified a failed Inlet-to-Fuel-Header capscrew on Engine Cylinder 1L. As part of the DCPP EDG design, two capscrews connect each injection pump fuel inlet to the fuel header. The fuel inlet supplies low-pressure fuel to the fuel injection pumps, which supplies high-pressure fuel to each cylinder. In the event of a capscrew failure, a potential leak path is created. 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 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, or be in Mode 3 within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

Although the capscrew on EDG 2-3 was successfully replaced within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, 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 (TS) 3.8.1 for two EDGs inoperable, a Unit 2 plant shutdown commenced. On August 14, 2014, at 2351 hours0.0272 days <br />0.653 hours <br />0.00389 weeks <br />8.945555e-4 months <br />, Unit 2 entered Mode 3.

C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event No other structure, systems, or components contributed to the event.

D. Other Systems or Secondary Functions Affected

None.

E. Method of Discovery

The EDG 2-2 failed capscrew was identified while performing scheduled maintenance. The EDG 2-3 degraded capscrew was identified during the associated extent of condition investigation performed on all the other EDGs.

F. Operator Actions

With two EDGs inoperable, a Unit 2 plant shutdown commenced in accordance with TS requirements, with Unit 2 entering Mode 3 at 2351 hours0.0272 days <br />0.653 hours <br />0.00389 weeks <br />8.945555e-4 months <br /> on August 14, 2014.

G. Safety System Responses

None.

IV. Cause of the Problem

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 high cycle fatigue was due to inadequate preloading of the capscrews. Inadequate preload can result in loosening and/or fatigue failure of the capscrews. Inadequate preloading of the bolted connection was determined to be due to the following:

The bolted connection components (capscrew, gasket, and lock washer) provided by the current EDG vendor reduced the fatigue design margin by not maintaining the required preload on the bolted connection.

EDG vendor maintenance instructions did not provide adequate details regarding maintaining the bolted connection to address high cycle fatigue concerns.

Investigation into the causes listed above revealed the following:

1. Changes to the following components were not adequately evaluated by vendor or DCPP engineering:

a. Gasket wire and filler materials and thickness b. Capscrew material strength and a reduction in as-left torque values 2. DCPP procedures did not provide sufficient detail regarding the sequence for high pressure fuel injection pump's installation (and associated make-up of the bolted connection) to ensure adequate preload was achieved and maintained to avoid capscrew loosening or fatigue failure.

The cause of EDG 2-3 developing a fuel oil leak at the Fuel Oil Booster Pump (FOBP) seal was due to the original manufacturing assembly process which failed to remove an excessive amount of silicone sealant from the pump seal. This, in conjunction with the use of a hand priming pump to prime the fuel oil system, which resulted in a higher than normal FOBP seal annulus pressure than during operating or start conditions, led to the fuel oil leak.

V. Assessment of Safety Consequences

In accordance with the TS, Unit 2 was safely shutdown.

An analysis was performed regarding the as-found condition of the capscrews. This analysis concluded that the degraded capscrews would not have prevented the diesels from performing their specified safety function.

A risk assessment performed for this event has concluded that the calculated incremental core damage probability (ICDP) is less than 1.0E-06 and the calculated incremental large early release probability (ILERP) is less than 1.0E-07, therefore the quantitative risk significance can be characterized as Green for the Unit 2 shutdown due to EDGs 2-2 and 2-3 being simultaneously unavailable.

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

VI. Corrective Actions

A. Immediate Corrective Actions

1. The degraded capscrew on EDG 2-3 was replaced, the fuel oil booster pump leak was repaired, and the EDG was successfully restored to Operable condition on August 15, 2014 at 1831 hours0.0212 days <br />0.509 hours <br />0.00303 weeks <br />6.966955e-4 months <br />.

2. The failed capscrew on EDG 2-2 was replaced as part of its scheduled maintenance window.

B. Additional Corrective Actions The following corrective actions have been or will be taken to address capscrew high-cycle fatigue:

On all three Unit 2 EDGs, DCPP has installed Grade 8 special capscrews, which were tightened to vendor- approved torque values, capscrew positive locking mechanisms, and new gaskets to achieve metal-to- metal bolting connections. On all three Unit 1 EDGs, DCPP has installed Grade 8 special capscrews, which were tightened to vendor-approved torque values, and new gaskets to achieve metal-to-metal bolting connections. The capscrew positive locking mechanisms will be installed on all three Unit 1 EDGs during maintenance outage windows in July and August of 2015.

Updated vendor maintenance instructions and/or engineering evaluations have been obtained for the Grade 8 capscrews, including associated torque values, installation sequence, and necessity of metal-to- metal contact with adequate preload, and corresponding guidance has been incorporated into DCPP maintenance procedures.

Additionally, plant procedures have been revised to ensure hand priming instructions minimize the effects of increased pressure in the FOBP seal annulus during system refill and restoration activities.

All of the corrective actions identified in this LER will be implemented in accordance with the Diablo Canyon Power Plant Corrective Action Program.

A. Previous Similar Events

On February 9, 2011, a fuel leak was found on EDG 2-1 during a routine test run. The disassembly investigation determined that the leak had originated from the Cylinder 9L special capscrews. Further disassembly determined that one of the special capscrews was fully cracked across the entire cross-section of the threaded portion of the bolt. Metallurgic analysis identified that the failure mechanism was high-cycle fatigue with a slow propagation of the crack due to a low stress in the joint. It was concluded that without proper special capscrew preloading, these special capscrews would be susceptible to high-cycle fatigue failure.

As an extent of condition corrective action, all Original Equipment Manufacturer ALCO special capscrews were removed and replaced with FM-G5E special capscrews in the Unit 1 and Unit 2 EDGs. These removed special capscrews were magnetic particle tested to determine if any crack initiation existed. Two hundred eight special capscrews tested satisfactory, satisfying the initial sampling population with no unsatisfactory findings.

In determining the apparent cause of the failure, a system engineer from Salem Nuclear Generating Station was contacted about a similar event. Salem had two failures of the special capscrews and it was concluded to be caused by the special capscrew being inadequately tensioned before reaching the end of the threads in the blind tapped hole. Measurements of the EDG 2-1 Cylinder 9L capscrew length, flange thickness, and tapped blind hole thread depth determined that less-than-adequate thread depth was available to ensure proper special capscrew preloading and flange gasket crush. For this reason, special capscrew blind holes were bottom-tapped on all six EDGs in the fuel header to ensure sufficient thread engagement as a corrective action.