WOLF CREEK GENERATING STATION - NRC SPECIAL INSPECTION

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by Bob Meyer

The following special inspection report is a great lesson learned that should be shared during OE for running diesels after maintenance. The use of OE was a contributor to this event. The NRC found that Wolf Creek had 19 violations for inadequate procedures / work instructions and six licensee event reports related to procedural issues for emergency diesel generators over the last 4 years. The industry has been living with huge procedure backlogs for too long. Latent errors that contribute to events present themselves during critical work and event responses. The events that have happened due to latent errors are becoming more costly to the utilities.

March 1, 2012 the report was issued to Matthew W. Sunseri, President and Chief Executive Officer, Wolf Creek Nuclear Operating Corporation.

On September 1, 2011, while performing a surveillance run on Emergency Diesel Generator A, operators noted load swings of 400 to 500 kW. The amplitude of these oscillations was greater than the acceptance criterion and caused the emergency diesel generator to be declared inoperable. Your staff was aware that as load on the Emergency Diesel Generators increased, the load swings increased, but they still attempted to test the unit at the new, higher design basis accident loads. As a result, the load swings on Emergency Diesel Generator A increased to the point that caused the emergency diesel generator to become inoperable.

The result of the inspection was a violation of Technical Specification 5.4.1.a, “Procedures,” was identified for the failure to include essential information needed to correctly adjust the emergency diesel generator governor actuator compensation potentiometer in Work Order 10-327976-000. Specifically, on May 23, 2011, maintenance personnel adjusted the actuator compensation potentiometer by following instructions from the system engineer per Work Order 10-327976-000. Work Order 10-327976-000 did not contain the cautionary note from Procedure MPE NE-003, “Governor Adjustments for Emergency Diesel Generator NE01,” which stated, “DO NOT set actuator compensation adjustor below 1.5.” The maintenance personnel set the potentiometer to 1.0. This improper adjustment resulted in Emergency Diesel Generator A being declared inoperable due to excessive load swings on September 1, 2011.

The finding is more than minor because it was associated with the procedure quality attribute of the Mitigating Systems Cornerstone and adversely affected the cornerstone objective to ensure the availability, reliability, and capability of systems that respond to initiating events to prevent undesirable consequences.

This finding was determined to have a crosscutting aspect in the Resources component of human performance because the licensee did not provide complete, accurate and up-to-date procedures/work orders to plant personnel because the licensee had not developed procedure guidance sufficiently detailed to ensure maintenance personnel properly adjusted the compensating actuator potentiometer for the electronic governor.

Sequence of Events

During a Component Design Bases Inspection in July 2007, the NRC identified that the licensee had not correctly calculated the worst case loading for the emergency diesel generators during a design basis accident. The licensee made corrections to Design Drawing E-11005, “List of Loads Supplied by Emergency Diesel Generator” to correct the discrepancy, but failed to identify that Procedure STS KJ-005A, “Manual/Auto Start, Sync & Loading of EDG NE01” needed to be updated to the revised design basis accident loading value.

In May 2011, in an attempt to mitigate low amplitude, high frequency oscillations present in the fuel rack control system for the emergency diesel generators, the licensee made adjustments to the actuator compensating potentiometer. The actuator compensation setting was reduced from an original value of 2.0 to about 1.0.

Based on continued questioning by the resident inspectors, the licensee decided to test the emergency diesel generators at the new, higher design basis accident loading during a surveillance test. On September 1, 2011, operators started Emergency Diesel Generator A for a monthly surveillance test in accordance with Procedure STS KJ-005A. When load on the unit was increased to approximately 5800 kW, the control room operator observed excessive load swings. Specifically while paralleled to the power grid in droop mode, Emergency Diesel Generator A experienced load swings of +/- 400 to 500 kW when operating above approximately 5800 kW; below 5800 kW the unit exhibited load variations of +/- 75 kW.

The licensee later determined that the actuator compensation setting on the Woodward 2301A governor control system was set too low. The actuator compensation setting had been decreased in May 2011, from an original value of 2.0 to about 1.0 to address excessive fuel rack movements during engine operation. The gain and reset were also tuned slightly at this same time. Returning the actuator compensation to a value of 2.0 in September 2011 corrected the load swings.

Evaluation of Industry Operating Experience

The inspector reviewed internal operating experience by obtaining a list of plant modifications related to the emergency diesel generators and selecting those modifications that would have affected the electronic governor. The inspector requested key word searches of the corrective action program related to load swings and unstable loads/frequency control on emergency diesel generators. Additionally, the inspector selected corrective maintenance activities that had affected the emergency diesel generator governors.

For external operating experience, the inspector selected operating experience information that was applicable to this inspection.

Observations 

The licensee was able to provide only three examples of external operating experience related to emergency diesel generators being reviewed in their operating experience program. The inspector identified multiple applicable operating experience items and NRC Information Notices which were not in their program, for example:

-        Information Notice No. 83-58, TransAmerica DeLaval Diesel Generator Crankshaft Failure, dated August 30, 1983

-        Brunswick, Emergency Diesel Generators (EDG) Inoperable Due to Fuel Rack Limiter Problems, dated December 30, 2008

-        San Onofre, Emergency Diesel Generator Unrealized Inoperability Due to Load Swings, dated December 2007

-        Seabrook, B Emergency Diesel Generator Experienced a High KVAR Fluctuation, dated July 24, 2002

-        V.C. Summer, Failure of Electronic Governor, dated January 1998

Industry experience with emergency diesel generators and their governors is quite extensive, and although not specific to Fairbanks Morse equipment, it reflected similar challenges to operation for other diesel manufacturers. The licensee did not make use of operating experience until after performance problems occurred. The inspector performed a search of operating experience and identified reports that pertained to the malfunction of the Digital Reference Unit, setting of governor controls, frequency oscillation issues, and procedure adequacy and clarity. The licensee did not effectively utilize these reports.

Review of Preliminary Cause Determination 

Observations 

September 1, 2011 Emergency Diesel Generator A Load Swings The Wolf Creek Generating Station is equipped with two emergency diesel generators, which consist of a Pielstick 2.5 14-cylinder engine and a Beloit Power Systems TGZDO  synchronous generator manufactured by Fairbanks-Morse Engine. The emergency diesel generators are controlled by a Woodward Governor Company 2301A speed governing system and a Westinghouse static excitation system.

The Woodward 2301A governing system can work in either of two modes of operation. When the emergency diesel generator is the only source of power to the emergency bus, the unit runs in the isochronous mode, and maintains a stable frequency regardless of the load it is supplying. When both the emergency diesel generator and offsite power are supplying the emergency bus (the two sources are “paralleled”), as is done during monthly surveillance testing, the unit operates in the “droop” mode to allow proper load sharing between the two power sources. In the droop mode, the governor allows a linear decrease in frequency as load increases. At Wolf Creek, the emergency diesel generators were designed to have 3.5 percent droop, or a drop of 2.1 hertz from no load to full load (6201 kW).

On September 1, 2011, operators started Emergency Diesel Generator A for a monthly surveillance test in accordance with Procedure STS KJ-005A, “Manual/Auto Start, Sync & Loading of EDG NE01.” When load on the unit was increased to approximately 5800 kW, the control room operator observed excessive load swings. Specifically while paralleled to the power grid in droop mode, Emergency Diesel Generator A experienced load swings of +/- 75 kW below 5800 kW. This equates to a 0.05 hertz frequency change. Above 5800 kW, the kW swing was observed as approximately +/- 500 kW, which equates to 0.339 hertz. This change in frequency is less than the technical specification allowed limit of +/- 1.2 hertz. Current design calculations show that safetyrelated equipment is capable of performing its design function within these frequency fluctuations.

 

The licensee later determined that the actuator compensation setting on the Woodward 2301A governor control system was set too low. The actuator compensation setting had been decreased in May 2011, from an original value of 2.0 to about 1.0 to address excessive fuel rack movements during engine operation. The gain and reset were also tuned slightly at this same time. Returning the actuator compensation to a value of 2.0 in September 2011 corrected the load swings.

 

The inspector observed that Nuclear Plant Information System computer setpoints NEP002 and NEU001 were unreliable and the condition was previously documented. Condition Report 2011-34654 was initiated on 3/11/11 due to NEU0001 and NEP0002 either failing low or to unreasonable values during the performance of STS KJ-011A. This system is used to provide accurate and precise indications and recordings of various plant parameters to allow monitoring of plant performance. Work Request 11-085841 (Work Order 11-339130-000) was generated as a result. The operators were unable to use the more precise Nuclear Plant Information System data points to perform trending of emergency diesel generator kW and amperage after initial resetting of actuator compensation adjustment. During interviews with the plant operators, they stated that because these indications were not available, they used wide range indications, which made it more difficult to detect the load swings. If these computer points had been available, it is likely the emergency diesel generator load oscillation would have been identified in a more timely manner.

 

Observations

The licensee determined that the direct cause of this event was that the actuator compensator for the governor that had been improperly adjusted in May 2011. The asfound condition was that the potentiometer was set at 1.0, which was less than the procedural minimum value of 1.5 given in Section 7.2.8 of Procedure MPE NE-003, “Governor Adjustments For Emergency Diesel Generator NE01”.

 

The inspector agreed with this determination.

 

The licensee determined that the apparent cause for the event was that the organization

did not use conservative actions in adjusting the emergency diesel generator governor.

Section 7.2.8 of Procedure MPE NE-003 contained a note that warned to not set actuator compensation adjustment below a value of 1.5 (it was set to a nonconservative 1.0 setting). The vendor manuals contained amplifying information that indicated settings below 1.5 for the actuator compensating potentiometer could result in an unstable feedback circuit at high load conditions. The inspector considered that the licensee’s nonconservative actions included the lack of sufficient monitoring by use of plant computer to assess the engine performance after making adjustment.

 

While this meets the procedural requirements for an apparent cause evaluation, the

inspector determined that a more appropriate evaluation of the cause was inadequate

work orders/procedures. Additionally, the inspector reviewed previous inspection reports

to assess the extent of problems the licensee had experienced with inadequate maintenance procedures and practices. The inspector identified that during the previous 4 years, the NRC had issued 19 noncited violations related to inadequate procedures/work instructions or inadequate post maintenance testing.

  

Observations

The inspector determined that the monthly surveillance run data was not reviewed with sufficient thoroughness to identify the issue. The system engineer relied on the operators to evaluate the emergency diesel generator performance during the surveillance and did not normally review the computer data in detail. The system engineer felt the computer data had limited resolution and only a few parameters were logged, so it was not of value for analyzing system performance. Despite this, the inspector was able to discern critical data in the history plots of the emergency diesel surveillance runs and identify the increased load swings from May 2011 through September 2011.

 

The inspector noted that Wolf Creek issued six licensee event reports from October 2007 to November 2011, that involved maintenance or procedural issues related to the emergency diesel generators. Many of these failures resulted from maintenance or data review issues. The inspector concluded that the licensee had a history of events that challenged operation of their emergency diesel generators. The inspector verified that the licensee had taken appropriate corrective actions to address each of the failures.

 

Findings and Observations

 

The licensee limited the extent of condition review to the A and B emergency diesel generators and the monitoring of performance during test/surveillance for safety related systems. This was based on the governor performance associated with emergency diesel generator A, which exhibited excessive load swings and abnormal fuel rack movement. The licensee’s basis for not considering the other governors onsite was:

 

-        The other diesels on site are not as complex as the Fairbanks Morse emergency diesel generators and do not employ a Woodward 2301A control system.

-        Although the Turbine Driven Auxiliary Feed Pump uses a Woodward governor system, it is an older system with a limited gain and stability control.

-        The remaining diesels that could have a plant impact included the Security diesel generator, Technical Support Center diesel generator, and the Emergency Operations Facility diesel generator, none of which use a 2301A governor system.

 

The inspector determined that this limited scope evaluation met the procedural requirements for the apparent cause evaluation. During a recent 95002 inspection, the inspection team had noted that extent of condition evaluations were narrowly focused even for root cause evaluations. Because of this narrow focus, the licensee missed several opportunities to improve overall plant performance and material reliability.

 

The inspector found 2006 operating experience from Vermont Yankee related to improper tuning of the Terry turbine flow/speed controllers for both the high pressure core injection and reactor core isolation cooling systems that resulted in failure of the systems to operate in the automatic mode. This operating experience indicates that potential adverse consequences can result from improper adjusted governors that are less complex than the Woodard 2301A system, which the licensee’s review did not consider.

 

Potential for Generic Issues Related to the Discovery of the Load Swings

 

Inspection Scope

 

The inspector evaluated the Emergency Diesel Generator A load swing event and associated deficiencies to determine whether any potential generic issues should be communicated to the industry (e.g., Information Notices, Generic Letters, and Bulletins).

 

Findings and Observations

The inspector determined that although there were a number of different issues related to the emergency diesel generator maintenance practices at this facility they were similar in nature to operating experience already available to the industry and may not warrant a generic communication to inform other licensees of the types of problems encountered. The inspector determined that the problems with the emergency diesel generator load swings resulted from poor work practices, improper post maintenance testing, and less than adequate review of available data.

 

Procedure Guidance Not Commensurate with Work Complexity or Significance

 

Licensee Personnel

M. Sunseri, President and CEO

S. Hedges, Site Vice President

L. Ratzlaff, Manager Maintenance

G. Sen, Manager Regulatory Affairs

D. Hooper, Supervisor, Licensing

L. Rockers, Licensing

W. Muilenburg, Licensing

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