05000482/LER-2013-007
| Wolf Creek Generating Station | |
| Event date: | 06-17-2013 |
|---|---|
| Report date: | 11-14-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4822013007R01 - NRC Website | |
There were no structures, components or systems that were inoperable and contributed to the initiation or limited mitigation of the event.
DESCRIPTION OF THE EVENT
On June 17, 2013, the results of an analysis of an oil sample taken from the "A" Class 1E electrical equipment air conditioning (NC) unit [EIIS: VI, ACU], SGKO5A, showed elevated levels of aluminum.
Additionally, the unit was observed to have increased vibration levels and running with elevated Amperes, approximately 20% less than the rated full load amperage of the motor (55 Amperes). This information was discussed with the equipment manufacturer. The compressor's pistons and connecting rods are made of aluminum so the concern was that these parts were damaged. The likely cause of this damage was slugging/hydraulicing of the compressor, which is the introduction of a liquid into a system meant for gas. It was concluded that the equipment could no longer be considered reliable to support the mission time of supported systems. The last maintenance performed on the equipment was on May 28, 2013, when the SGKO5A compressor was replaced.
Since the unit could not be relied on to meet its required function, the Class 1 E electrical equipment air conditioning (NC) unit, SGKO5A, which cools the 'A' train safety related electrical equipment rooms was declared nonfunctional per Technical Requirement (TR) 3.7.23, "Class 1 E Electrical Equipment Air-Conditioning (NC)." TR 3.7.23 requires two Class 1 E electrical equipment NC trains be functional in Modes 1 through 4. Because of the essential support function provided by the Class 1 E electrical equipment NC trains, the correct application of the Technical Specification (TS), upon discovery of a nonfunctional Class 1 E electrical equipment NC train, is to immediately enter the applicable Conditions and Required Actions under TS 3.8.4, "DC Sources — Operating," TS 3.8.7, (Inverters — Operating," TS 3.8.9, "Distribution Systems — Operating," as well as Limiting Condition for Operation (LCO) 3.0.3.
LCO 3.0.3 requires action to be taken within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to place the plant in Mode 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />, in Mode 4 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />, and in Mode 5 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />. The Shift manager entered the applicable TSs including LCO 3.0.3 at 1111 Central Daylight Time (CDT) on June 17, 2013. A plant shutdown was started at 1125 CDT on June 17, 2013.
On June 17, 2013, enforcement discretion was sought to permit noncompliance with TS 3.8.4, TS 3.8.7, TS 3.8.9, as well as LCO 3.0.3, to permit additional time to complete repairs and restoration of SGKO5A before a plant shutdown would be required. An additional '168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> was requested to restore SGKO5A to a functional status such that the Completion Time of LCO 3.0.3 would expire at 1111 CDT on June 24, 2013. The Nuclear Regulatory Commission granted approval of the requested enforcement discretion on June 17, 2013.
When the enforcement discretion was received, the plant shutdown was stopped and stabilized at 150 MWe. The plant returned to full power at approximately 1430 CDT on June 18, 2013.
The replacement of the SGKO5A compressor and testing to restore functionality was completed at 2220 CDT on June 21, 2013.
Wolf Creek Nuclear Operating Corporation (WCNOC) requested and received a notice of enforcement discretion (NOED) from the NRC to not enforce compliance with the actions required in TS 3.8.4, TS 3.8.7, TS 3.8.9, as well as LCO 3.0.3. for a period of 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br />. The SGKO5A unit was nonfunctional for a period longer than allowed by the Completion Time of LCO 3.0.3. As such, the event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) for any operation or condition which was prohibited by the plant's TSs.
LCO 3.0.3 requires action to be taken within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to place the plant in Mode 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />, in Mode 4 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />, and in Mode 5 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />. LCO 3.0.3 was entered in light of the limitations of the Conditions and Required Actions of TS 3.8.7, "Inverters - Operating." Therefore, absent a support system TS LCO to declare inoperable, the control room staff is required to enter LCO 3.0.3 when a Class lE electrical equipment NC train is discovered to be nonfunctional, resulting in an unnecessary plant transient.
Required Actions B.1 and B.2 of TS 3.8.4 would require placing the plant in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> if Required Action A.1 (restore DC electrical power subsystem to Operable status) and it associated Completion Time (2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />) is not met. Required Actions B.1 and B.2 of TS 3.8.7 would require placing the plant in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> if Required Action A.1 (restore one required inverter to Operable status) and it associated Completion Time (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) are not met. Required Actions E.1 and E.2 of TS 3.8.9 would require placing the plant in MODE 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> if Required Actions C.1 and D.1 (restore one AC vital bus subsystem and one DC electrical power distribution subsystem to Operable status) and its associated Completion Time (2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />) is not met.
ROOT CAUSE
The direct cause of the SGKO5A compressor failure occurred from a liquid slug drawn into the compressor resulting in a sudden over pressurization in the cylinder compression volume. One or more thermostatic expansion valves [El IS: VI, TCV] (TXVs) failed open intermittently due to residue on the valve internals. The root cause is that inadequate actions, specifically the chemical flush and evacuation of the system, were taken to restore SGKO5A in May 2013. In May 2013, a chemical flush was performed after the failure of a filter-drier [EllS: VI, FLT] in the system allowed filter element material to enter the refrigerant stream resulting in blockage of the TXVs. This was reported in LER 2013-006-01.
CORRECTIVE ACTIONS
The SGKO5A compressor was replaced. The evaporator TXV internals and hot gas bypass valve internals were replaced. Internal inspections were conducted with a boroscope that verified there was no internal contamination.
Procedure MPE GK-004, "GK Unit Preparation for Work," will be revised to include critical steps that are important for ensuring an effective flush and evacuation of the Class lE electrical equipment NC system.
The Incremental Conditional Core Damage Probability (ICCDP) and Incremental Conditional Large Early Release Probability (ICLERP) was quantified for the requested additional time for replacing the SGKO5A unit. The ICCDP calculated value was 7.038E-8 and the ICLERP calculated value was 6.137E-10. The results of the quantification are within the guidance threshold in Regulatory Issue Summary 2005-01, "Changes to Notice of Enforcement Discretion (NOED) Process and Staff Guidance.
Temporary cooling was put in place for the spaces normally cooled by SGKO5A. This cooling was a temporary A/C unit (elephant trunk type) powered by reliable non-safety power. A second unit was available in case of failure. Monitoring of room temperature was performed every two hours and a fire watch was established to compensate for the fire barriers that were breached by the temporary AC units.
OPERATING EXPERIENCE/PREVIOUS SIMILAR OCCURRENCES
to a calculation that concluded one train of air conditioning was not capable of supporting both trains of Class 1 E equipment. Technical Requirement (TR) 3.7.23 allowed a train to be nonfunctional if compensatory measures were established for the affected unit. During the operability determination and functionality assessment process, it was determined that the operability of the associated train Class 1 E electrical equipment could not be maintained without additional compensatory measures and for a limited period of time.
nonfunctional and one train of control room air conditioning had been inoperable during the previous cycle. This was discovered during refueling outage 19 when the SGKO5A compressor terminal box mounting screws were found over torqued. This resulted in a condition prohibited by Technical Specification and a condition that could have prevented the fulfillment of a safety function.
nonfunctional due to a partial blockage of the thermostatic expansion valves feeding the SGKO5A evaporator coils. This resulted in a plant shutdown required by Technical Specifications.