05000482/LER-2013-008
| Wolf Creek Generating Station | |
| Event date: | 09-11-2013 |
|---|---|
| Report date: | 11-12-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4822013008R00 - NRC Website | |
There were no structures, components or systems (SSC) that were inoperable at the start of the event and contributed to the event.
DESCRIPTION OF THE EVENT
During Operator rounds on September 11, 2013, low oil level on the 'A' train Class 1 E electrical equipment air conditioning (A/C) unit [EllS: VI, ACU] (SGKO5A) compressor, elevated vibration and an increase in motor current was observed. The compressor refrigeration parameters (suction pressure, discharge pressure, oil pressure, thermostatic expansion valve [EllS: VI, TCV] (TXV) superheat) were running normal and the room temperatures were being maintained. It was determined that the symptoms were similar to a June 2013 compressor failure reported in LER 2013-007-00. SGKO5A was declared nonfunctional per Technical Requirement (TR) 3.7.23, "Class 1 E Electrical Equipment Air- Conditioning (A/C)." TR 3.7.23 requires two Class 1 E electrical equipment A/C trains be functional in Modes 1 through 4. Because of the essential support function provided by the Class 1 E electrical equipment A/C trains, the correct application of the Technical Specification (TS), upon discovery of a nonfunctional Class 1 E electrical equipment A/C train, is to immediately enter the applicable Conditions and Required Actions under TS 3.8.4, TS 3.8.7, TS 3.8.9, as well as Limiting Condition for Operation (LCO) 3.0.3. On September 11, 2013 at 1645 Central Daylight Time (CDT), the plant entered LCO 3.0.3.
A plant shutdown was commenced on September 11, 2013 at 1645 CDT. Wolf Creek Generating Station (WCGS) entered Mode 3 on September 11, 2013 at 2312 CDT.
Following the plant shutdown, while in Mode 3, the 'A' steam generator (SG) level [EllS: SB, LI] was observed approaching the Auxiliary Feedwater Actuation Signal (AFAS) setpoint of 23.5 % level. The Control Room operators initiated a manual reactor trip. As a result of the trip, a feedwater isolation signal (FWIS) and a motor-driven AFAS was generated.
After the actuation, SG levels were controlled by manually adjusting the,auxiliary feedwater pump discharge valves [EllS: BA, V].
Work on 'A' train Class 1 E electrical equipment A/C unit was completed and the unit returned to a functional status on September 26, 2013 at 2316 CDT. WCGS returned to Mode 1 on September 29, 2013 at 1632 CDT.
10 CFR 50.73(a)(2)(i)(A) requires reporting "the completion of any nuclear plant shutdown required by the plant's Technical Specifications." A shutdown of the plant was completed per LCO 3.0.3.
Additionally, a 4-hour notification was made per 10 CFR 50.72(b)(2)(i) when the plant shutdown was initiated.
10 CFR 50.73(a)(2)(iv)(A) requires reporting "any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B)." A manual reactor trip was initiated that actuated the Reactor Protection System and the PWR auxiliary feedwater system actuated.
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 [EllS: 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.
The apparent cause of the manual reactor trip and auxiliary feedwater actuation was the lack of crew proficiency to maintain SG levels in Mode 3, immediately following a rapid shutdown.
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 1 E electrical equipment A/C system.
The operator training cycle that commenced October 28, 2013, includes a simulator training scenario addressing the control of SG levels following a rapid plant shutdown.
The Class 1 E electrical equipment NC system operates in a continuous recirculation mode to maintain the engineered safety features (ESF) switchgear rooms [EllS: EB, SWGR], battery rooms [EllS: EJ, BTRY] and the DC switchgear rooms [EllS: EJ, SWGR] at or below the design temperature of 90 degrees F during all modes of plant operation, including loss of preferred offsite power and post- accident operation. The safety significance of this event is low since only one train of Class 1 E electrical equipment was potentially affected. The 'B' train Class 1 E electrical equipment NC unit, SGKO5B, was not affected and remained functional, and its associated train of Class 1 E electrical equipment was operable. Additionally, the 'A' train Class 1 E electrical equipment room temperatures, cooled by SGKO5A, remained below 90 degrees F.
OPERATING EXPERIENCE/PREVIOUS SIMILAR OCCURRENCES
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 TXVs feeding the SGKO5A evaporator coils. Failure of a filter-drier in the system created the contamination that led to the blockage. This resulted in a plant shutdown required by Technical Specifications.
nonfunctional due to an analysis of an oil sample that showed elevated levels of aluminum. The NRC granted enforcement discretion that allowed the plant to remain at power while the train was restored to functional status.