05000397/LER-2009-005
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. '05000 | |
| Event date: | 11-07-2009 |
|---|---|
| Report date: | 01-04-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 45484 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3972009005R00 - NRC Website | |
Event Description
On November 7, 2009, the control room staff had completed a scheduled downpower of Columbia Generating Station (Columbia) to perform turbine governor valve [FCV] testing and other maintenance activities. After reaching the targeted power level, a rapid loss of hydraulic fluid from the main turbine digital electro-hydraulic (DEH) control system [TG] occurred. A DEH reservoir [RVR] low level alarm was received in the control room at 0718. Subsequently, a low-low level alarm was received at 0724. At 0725, the reactor was manually scrammed as required by plant procedures.
All rods fully inserted as expected in response to the manual RPS actuation and heat removal through the main condenser [SG] was maintained. Following the scram, a reactor water level 3 isolation occurred. Post trip reactor vessel water level was restored and maintained in the normal band using the feedwater [SJ] and condensate systems [SD]. Pressure was controlled in the normal band using the main steam line drains. No other safety systems actuated or were required to actuate. There was no inoperable equipment at the start of the event that contributed to the event.
Off-site power was available and all three emergency diesel generators [EK] were operable and available.
Reference Event Notification#45484 for initial reporting of this event.
Causes The direct cause for the loss of hydraulic fluid was the failure of an o-ring at the lower mounting block flange of DEH accumulator DEH-TK-1D [ACC]. This flange was determined to have been incorrectly assembled following preventative maintenance that replaced the o-ring during the previous refueling outage. Two of the four flange bolts were found to be loose and torqued improperly. The opportunity to identify this latent condition was missed when an oil leak in the general area of the accumulator was reported by an equipment operator approximately a month and a half prior to the event.
However, the leak was wiped up prior to Maintenance investigation of the leak, giving the impression that there was no equipment problem when Maintenance personnel inspected the location.
The root cause for the incorrect assembly of the block flange was determined to be that the administrative requirements associated with maintenance on the DEH accumulators did not impose special verifications, controls, or personal accountability measures to ensure that maintenance was performed appropriately, even though portions of the pressure boundary are a single point vulnerability for a turbine trip.
26158 R4 1. Energy Northwest supervision provided inadequate oversight to verify the quality of field work and failed to monitor and reinforce expectations for performing work.
2. There was no post maintenance test (PMT) performed on the DEH fluid side of the pressure boundary after the flange was reassembled.
3. Less than adequate decision making caused the work request that identified an oil leak on the accumulator issued a month and a half previous to the event to be closed based on a decision to accept DEH pressure boundary leakage as is, 4. The root cause evaluation for the failure of a DEH Swagelok fitting that occurred in August of 2008 failed to include other pressure boundary fittings in the extent of condition.
Corrective Actions Taken or Planned Immediate corrective actions include:
- The DEH-TK-1D o-ring was replaced and satisfactorily passed the PMT leak check.
- An extent of condition and technical evaluation were performed to provide a reasonable assurance of system functionality and reliability prior to plant startup. Startup was approved by the Plant Operations Committee based on this information.
Interim corrective actions include:
- Operations issued a night order to ensure condition reports and work requests are generated for all existing oil leaks.
- Operations issued a night order to establish the expectation that in the future when operators wipe up oil that they ensure a condition report, work request, and deficiency tag have been established for the leak. The Shift Manager is to ensure that all oil leaks have been identified.
Corrective actions to prevent recurrence include:
- Identify locations/joints that have experienced leaks or are of concern and perform a DEH hydraulics pressure boundary review of threaded and flanged mechanical joints on the high pressure and trip header/components. Based on vendor or industry guidance identify appropriate torque values for bolted connections and appropriate additional test/inspection requirements that should be used to validate joint pressure boundary reliability for both threaded and bolted connections. Provide information to Maintenance for inclusion into model work orders.
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- parts verification inspections sign off steps, and - quality control or peer verification sign off steps for steps that apply test/inspection criteria that validate mechanical joint pressure boundary integrity.
- Revise DEH hydraulic fluid component model work orders with the criteria and expectations developed in the first two corrective actions above.
- Identify required specifications for DEH-TK-1A through 1F o-ring application, including hardness, and provide critical characteristics information to Maintenance.
- Verify specifications of currently installed materials based on work history and generate condition reports for installed o-rings that do not meet required specifications. Identify critical characteristics for purchase of new o-rings and revise the bill of materials as necessary to include o-rings that meet specifications.
- Identify other high risk/consequence systems for a pressure boundary event and other at risk bolted connections, and perform a risk evaluation to determine which systems should have additional risk mitigation methods similar to the second corrective action above. Define appropriate risk mitigation method(s) for each system determined to be risk significant, and generate additional corrective actions to track completion.
- Evaluate personnel performance against performance gaps identified in the barrier, failure mode, and cause and effect analysis. Address individual personnel accountability issues via individual performance improvement initiatives.
Other corrective actions for addressing contributing causes have also been established that were not included in this report.
Assessment of Safety Consequences
This event did not involve an event or condition that could have prevented the fulfillment of any safety function described in 10 CFR 50.73(a)(2)(v). This event does not qualify as a reactor scram with complications; all safety systems performed as designed, and therefore, this event posed no threat to the health and safety of the public or plant personnel.
Similar Events A similar event occurred at Columbia on August 21, 2008 (reference LER 2008-001) in which a Swagelok compression fitting failed, resulting in a loss of DEH fluid. However, in contrast, the event that occurred on August 21, 2008 involved an immediate drop in DEH system trip header pressure below the reactor protection system trip setpoint resulting in an automatic initiation of a reactor scram.
Energy Industry Identification System (EllS) Information EllS codes are bracketed [ ] where applicable in the narrative.
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