05000482/LER-2013-004
| Wolf Creek Generating Station | |
| Event date: | 03-08-2013 |
|---|---|
| Report date: | 05-03-2013 |
| 4822013004R00 - NRC Website | |
There were no structures, components or systems (SSC) that were inoperable at the start of the event and that contributed to the event.
BACKGROUND
On 03/08/2013, Wolf Creek Generating Station was in Refueling Outage 19 with the core defueled and no movement of irradiated fuel assemblies. The A train essential service water system [EIIS: BI] was out of service for maintenance. The B train Class lE electrical equipment air conditioning unit (SGKO5B) [EIIS: VI] and the B train control room air conditioning unit (SGKO4B) [EllS: VI] were in service. With the reactor defueled, Technical Specifications (TS) and the Technical Requirements Manual (TRM) do not require these units to be in service.
DESCRIPTION OF THE EVENT
On 03/08/2013, during replacement of the SGKO5A Class lE electrical equipment air conditioning compressor [EllS: VI, ACU], technicians were re-installing the compressor terminal box and torquing the terminal box mounting screws to the compressor at 50 inch pounds per the maintenance work instructions. The first mounting screw snapped off at 40 inch pounds, leaving the threaded body of the screw inside the compressor. The mounting screw was successfully extracted.
Engineering recommended that the terminal box mounting screws for both SGKO5A and SGKO5B be removed and replaced with safety related mounting screws and tightened to a torque value of 20 inch pounds. Corrective work was completed for SGKO5A on 03/10/2013.
An extent of condition review determined that the mounting screws on SGKO4B and SGKO5B compressors [EIIS: VI, ACU] were also over torqued. SGKO4A [EllS: VI, ACU] was unaffected as no maintenance had been performed on the compressor terminal box that specified torque requirements for the mounting screws.
SGKO5B had maintenance performed on 05/30/2012 and the mounting screws were over torqued to 50 inch pounds. On 03/09/2013 at 1715 Central Standard Time (CST), SGKO5B was declared nonfunctional.
50 inch pounds. Since the mounting screws had been over torqued, there was no way to ensure the mounting screws would keep the terminal box in place during a seismic event. Though not in the TS mode of applicability, the plant was defueled and there was no movement of irradiated fuel assemblies, on 03/10/2013 at 1208 CST, SGKO4B was declared inoperable.
As the plant was in a refueling outage, the A train could not be restored because the A train essential service water was out of service for maintenance. Due to the protected status of the B train that did not allow the de-energization of SGKO4B and SGKO5B units to allow replacement of the mounting screws, a temporary modification to restrain the terminal boxes was installed on 03/14/2013. The temporary modification was removed after the A train was restored to operation.
The mounting screws on SGKO5B were replaced and torqued to 20 inch pounds. SGKO5B was returned to service on 03/22/2013 at 1816 Central Daylight Time (CDT). The mounting screws on SGKO4B were replaced and torqued to 20 inch pounds on 03/23/2013. SGKO4B was restored to operable status on 3/23/2013 at 1021 CDT.
A review of the Instruction Manual for Packaged Air Conditioning Units, Bolt Torque on Copeland Compressors, did not reveal any torque value or the need to torque the compressor termination box machine screws. Furthermore, the Instruction Manual did not specify the size of the machine screw used for the termination box. Interviews with the Maintenance Planner did not determine how or why a torque requirement was placed in the work instructions for the compressor terminal box mounting screws.
BASIS FOR REPORTABILITY
Technical Requirement (TR) 3.7.23, "Class 1 E Electrical Equipment Air-Conditioning (A/C)," requires two Class 1 E electrical equipment A/C trains be functional in Modes 1 through 4. SGKO5B had maintenance performed on 05/30/2012 and the mounting screws were over torqued to 50 inch pounds.
As a result, SGKO5B was nonfunctional from 05/30/2012 through 2/5/2013. Because of the essential support function provided by the Class 1 E electrical equipment A/C trains, the correct application of the TS, upon discovery of an 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. This resulted in a condition prohibited by TS.
Further review determined that SGKO5A was nonfunctional from 6/4/2012 through 6/6/2012. As result, two Class 1 E electrical equipment A/C trains were nonfunctional for a period of time. Because of the essential support function provided by the Class 1 E electrical equipment A/C trains, both trains of Class 1 E electrical equipment were considered to be inoperable during this time period. This resulted in a condition that would have prevented fulfillment of the safety function required of the Class 1 E electrical equipment.
The LCO for TS 3.7.11, "Control Room Air Conditioning System (CRACS)," requires two CRACS trains to be operable in Modes 1 through 6 and during movement of irradiated fuel assemblies. SGKO4B had maintenance performed on 08/12/2011 and the mounting screws were over torqued to 50 inch pounds.
As a result, one CRACS train was inoperable since 08/12/2011. Condition A of TS 3.7.11 requires restoring the inoperable CRACS train to operable status in 30 days. Since Condition A and Condition B were not met, this resulted in a condition prohibited by TS.
A review since 08/12/2011 revealed multiple instances when SGKO4A was not operable. As a result, two CRACS trains were inoperable in multiple instances since 08/12/2011. This resulted in a condition that would have prevented fulfillment of the safety function required of the CRACS.
This event reports a single cause or condition, which caused independent trains to become inoperable.
This resulted in a common-cause inoperability of independent trains or channels.
ROOT CAUSE
The apparent cause of this event was less than adequate usage of self-checking, planner peer review and questioning attitude. Maintenance planners did not validate the torque requirements for the terminal box mounting screws by using self-checks and/or planner peer review. In addition, maintenance failed to question why terminal box mounting screws were being torqued.
CORRECTIVE ACTIONS
The mounting screws were replaced and torqued to 20 inch pounds for SGKO4A/B and SGKO5A/B.
Procedure Al 16C-007, "Work Order Planning," was revised to require a reference source document for any work instruction that requires torquing data for an installed SSC.
SAFETY SIGNIFICANCE
The CRACS provides temperature control for the control room and consist of two independent and redundant trains, SGKO4A/B, that provide cooling of recirculated control room air. A single unit will maintain a temperature of less than or equal to 84 degrees Fahrenheit for the control room during normal or emergency operations. The CRACS are designed to remove sensible and latent heat loads from the control room, which include consideration of equipment heat loads and personnel occupancy requirements, to ensure equipment operability. Control room temperatures remained below the area temperature limits.
The Class 1 E electrical equipment air conditioning system operates in a continuous recirculation mode to maintain the engineered safety features (ESF) switchgear room [EIIS: EB, SWGR], battery rooms [EIIS: EJ, BTRY] and the DC switchgear rooms [EIIS: EJ, SWGR] at or below the design temperature of 90 degrees Fahrenheit during all modes of plant operation, including loss of preferred offsite power and post-accident operation. Room temperatures remained below the area temperature limits for qualification of electrical equipment.
The safety significance of this event is low since the units with the over torqued terminal box mounting screws were capable of operating. The units may not have functioned if a seismic event had occurred.
OPERATING EXPERIENCE/PREVIOUS EVENTS
Licensee Event Report (LER) 2012-005-00 reported a Class 1 E electrical equipment NC train was declared nonfunctional. Technical Requirement (TR) 3.7.23 allowed a train to be nonfunctional if compensatory measures are 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.
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