05000331/LER-2013-001
| Duane Arnold Energy Center | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3312013001R00 - NRC Website | |
I. Description of Event:
On March 8, 2013, while operating at 100% power, the 'A' Emergency Diesel Generator (EDG) was in embedded conduit inspections. As part of post maintenance testing, the 'A' EDG was operated and a small lube oil leak was discovered on the lube oil cooler heat exchanger. After maintenance activities to verify the heat exchanger channel head bolts were properly tight, a second run of the 'A' EDG occurred.
During this run, the lube oil leak increased to an excessive amount and the 'A' EDG was manually tripped. The direct cause of the lube oil leak was inadequate gasket crush from maintenance activities that occurred during October 2012.
There were no other structures, systems or components inoperable at the start of this event that contributed to the event.
II. Assessment of Safety Consequences:
The EDG lubricating system is made up of an engine driven pump, heat exchanger, filter, reservoir, and other accessories, devices, controls, and piping required to form a complete and independent system. The purpose of the lube oil system is to deliver clean oil at proper temperature and pressure to the engine.
Emergency service water (ESW) passes through the tube side of the tube bundle to provide the cooling source for the engine's lube oil system during engine operation.
The EDG lubrication system is designed to provide sufficient lubrication to permit proper operation of its associated EDG under all loading conditions. The system is required to circulate the lube oil to the diesel engine working surfaces and to remove excess heat generated by friction during operation. The EDG Lube Oil System is designed to provide automatic lube oil makeup to the EDG crankcase for a minimum of 7 days of operation per the UFSAR.
In October 2012, during Refueling Outage 23 the 'A' EDG lube oil heat exchanger tube bundle was replaced. During post maintenance testing (PMT) a leak was observed at the joint between the heat exchanger shell and channel head. The bolt torque at the joint was increased and the leak stopped. The EDG was run (8) eight successful times between October 12, 2013 and March 8, 2013 before a leak reappeared (maintenance runs and surveillance tests).
On March 8, 2013 an oil leak of 60 drops per minute (dpm) was identified at the tube sheet flange to heat exchanger shell flange. Subsequently, a work order was written to check torque values with the engine shutdown. Torque values were verified to be no less than 22 ft-lbs. After this, a work order was written to increase torque on flange bolting. The EDG was started again to support the effort to increase torque. One to two minutes into the run the gasket failed and allowed oil to escape. Approximately 20-25 gallons of oil was discharged from the area where the gasket failed. The engine was tripped prior to overheating the EDG or the loss of a significant amount of lube oil. With the rate at which lube oil was discharged from the EDG, the engine would not have had sufficient supply to meet the 7 day unassisted operational time requirement.
Independent reviews determined that the 'A' EDG was not capable of performing its TS functions on March 8, 2013, when the gasket developed a 5 dpm lube oil leak. The failure of the gasket was a result of cyclic loading during engine starts, which means a start with no lube oil leakage would not have led to a gasket failure. Based on no lube oil leakage identified during the February 16, 2013 engine run, (last surveillance test conducted prior to the failure on March 8, 2013), it can be concluded that the 'A' EDG would not have developed a leak during an extended run at that time or at any time prior. The next start of the 'A' EDG on March 8, 2013 did exhibit a leak. Therefore, 'A' EDG was no longer reliable based on the leakage.
An extent of condition evaluation was completed to determine if there were any operability issues with the other 'A' EDG heat exchangers or the 'IV EDG heat exchangers. This evaluation determined that there were no operability concerns with either any of the other heat exchangers.
Therefore, the 'A' EDG was inoperable from February 16, 2013 to March 8, 2013. Technical Specification Section 3.8.1, AC Sources - Operating, B.5, One EDG Inoperable, requires the inoperable EDG to be restored to operable status in 7 days. This condition resulted in a Condition Prohibited by TS and is reportable to the NRC pursuant to 10CFR50.73(a)(2)(i)(B). In addition, the 'IV EDG was inoperable on three occasions during the period of time the 'A' EDG was inoperable. This occurred on February 26, 27, and 28 when the 'IV EDG was removed from service for conduit inspections. This resulted in an Event or Condition That Could Have Prevented Fulfillment of a Safety Function (Safety System Functional Failure) and is reportable to the NRC pursuant of 10CFR50.73(a)(2)(v)(D).
This event did result in a safety system functional failure.
III. Cause of Event:
A Root Cause Evaluation was completed. The evaluation identified the following root cause of this event.
Root Cause - Misalignment or flaws between flange mating surfaces of the 'A' EDG Lube Oil Heat Exchanger led to uneven compression of a gasket, leakage, and failure of the lube oil channel head connection, specifically at the 10-11 o'clock position.
IV. Corrective Actions:
On March 9, 2013, the 'A' EDG lube oil heat exchanger was repaired and the 'A' EDG was returned to operable status.
In order to address the root cause of this event, the following corrective actions will occur.
Corrective Action to Prevent Occurrence - Remove the 'A' EDG lube oil cooler tube bundle and perform the following checks to determine condition of sealing surfaces:
- Perform visual inspection of flange surfaces.
- Perform Blue-Check of stationary channel head to heat exchanger shell sealing surfaces.
- Measure flatness of shell flanges and re-machine flange surfaces if not within tolerance (specified on the plant MC drawing or specified by ASME Engineer).
- Measure and record vertical and horizontal alignment between the lube oil and jacket water channel heads.
V. Additional Information:
Previous Similar Occurrences:
A review of License Event Reports from the past 5 years did not identify any similar occurrence.
EIIS System and Component Codes:
EK - Emergency Onsite Power Supply System.
Reporting Requirements:
This event is being reported as an Operation or Condition Prohibited by TS, 10CFR50.73(a)(2)(i)(B) and an Event or Condition That Could Have Prevented Fulfillment of a Safety Function, 10CFR50.73(a)(2)(v)(D).