05000482/LER-2013-006
| Wolf Creek Generating Station | |
| Event date: | 05-07-2013 |
|---|---|
| Report date: | 08-14-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 4822013006R01 - 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
On May 5, 2013, Operators identified an increasing temperature trend for the 'A' train safety related electrical equipment room. The trend began one month earlier with a gradual increase of approximately 11 degrees Fahrenheit (F). On May 6, 2013 at 1733 Central Daylight Time (CDT), the Class lE electrical equipment air conditioning (A/C) unit [EDS: VI, ACU], SGK05A, 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 (A/C)." TR 3.7.23 requires two Class lE electrical equipment A/C trains be functional in Modes 1 through 4. Because of the essential support function provided by the Class lE 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.
A plant shutdown was commenced on 5/6/2013 at 1801 CDT. Wolf Creek Generating Station (WCGS) entered Mode 3 on 5/7/2013 at 0009 CDT.
Troubleshooting determined that the SGK05A evaporator coils [EllS: VI, EVP, CL] were being starved of refrigerant, which was consistent with blockage of the thermostatic expansion valves [EllS: VI, TCV].
Further examination identified a foreign material on the inlet thermostatic expansion valve screens. The foreign material was determined to be filter drier assembly [EllS: VI, FLT] core material that had broken down and entered the system. The thermostatic expansion valves are Sporlan Model Number SVE-8- CP100. The filter drier assembly is a Sporlan Model C-1449. Examination of the liquid line filter drier assembly internals identified damage to one of the three filter cores and the outlet plate and filter tube assembly. The filter tube and outlet plate are designed to prevent any carryover of the filter drier core material into the refrigerant stream. The damage to the core material appears consistent with erosion caused by liquid refrigerant flowing across the edge of the core element created by the failure of the connection between the filter tube and the outlet plate.
The system was flushed and the thermostatic expansion valves and filter drier assembly were replaced. It was confirmed that this issue did not exist with the 'B' train Class lE electrical equipment A/C unit, SGKO5B. The operating history of similar A/C units was reviewed and no similar problems were found at WCGS.
Work on 'A' train Class lE electrical equipment A/C unit was completed and the unit returned to a functional status on May 11, 2013 at 0451 CDT. WCGS returned to Mode 1 on May 13, 2013 at 0832 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.
ROOT CAUSE
Over tightening of the wing screw, that holds the filter drier assembly together, resulted in damage of the outlet plate assembly. As a result, the joint between the filter tube and the outlet plate in the liquid line filter for SGKO5A failed. This failure allowed filter element material to enter the liquid refrigerant stream leading to blockages of the thermostatic expansion valves, gradually reducing the cooling capacity of SGKO5A.
CORRECTIVE ACTIONS
The system was flushed and the thermostatic expansion valves and filter drier assembly were replaced. It was confirmed that this issue did not exist with the 'B' train Class 1 E electrical equipment A/C unit, SGKO5B. The operating history of similar A/C units was reviewed and no similar problems were found at WCGS.
Procedure MPE GK-004, "GK Unit Preparation for Work," will incorporate the vendor guidance on replacement of the filter driers, including the direction to hand tighten the filter driers wing screw. It will require an inspection of the outlet plate assembly when a filter drier is removed and an inspection as the final assembly step.
SAFETY SIGNIFICANCE
The Class 1 E electrical equipment A/C 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 A/C 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.
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.