05000483/LER-2013-001
| Callaway Plant Unit 1 | |
| Event date: | 12-17-2012 |
|---|---|
| Report date: | 02-15-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4832013001R00 - NRC Website | |
1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
At Callaway, the Class 1E electrical equipment air-conditioning (C1EEEAC) system [EIIS system: VI] is part of the control building heating, ventilation, and air-conditioning system [EIIS system: VI]. The CI EEEAC system is safety-related and provides a suitable atmosphere for the two trains of Class lE electrical switchgear [EIIS systems: EB, ED, and EJ, component SWGR] during all modes of plant operation and during movement of irradiated fuel assemblies, including loss of preferred power and post-accident operation.
Each of two C1EEEAC units (internal designation SGKO5A and SGKO5B) [EIIS system: VI, component: ACU] provides safety-related cooling to its respective train of Class 1 E electrical equipment located in the Control Building. Each unit is mounted on a skid and consists of a prefilter, heat exchanger, compressor, condenser [EIIS system: VI, component: COND], four parallel expansion valves, an evaporator unit, associated ductwork, Essential Service Water supply and return lines and valves, capacity control valve, solenoid valves and temperature control instrumentation. Using the thermodynamic principles of the refrigeration cycle, R-22 refrigerant is circulated through the system in order to transfer heat from Control Building air to the Essential Service Water/Service Water (ESW/SW) systems [EIIS system: BI].
During normal operation, the C1EEEAC system is operated in a continuous recirculation mode to maintain the Engineered Safety Feature (ESF) switchgear rooms, battery [EIIS system: EJ, component: BTRY] rooms, and DC switchgear [EIIS system: EJ] rooms at or below a temperature of 90 degrees Fahrenheit (F). The amount of cooling provided by the self-contained refrigeration system is self-regulating. Indication of a loss of preferred AC power, a LOCA, or a Control Room Ventilation Isolation Signal will automatically initiate the Class 1E electrical equipment air- conditioning units if they are not in operation.
While Technical Specifications address the Class 1E electrical systems, they do not directly address the C I EEEAC system. During power operation Technical Specification (TS) 3.8.4 addresses the DC electrical power systems, TS 3.8.7 addresses inverters [EIIS system: EE, component: INVT], and TS 3.8.9 addresses AC, DC, and AC vital bus electrical power distribution subsystems [EIIS system: EF, component: BU]. During shutdown conditions the corresponding Technical Specifications are TS 3.8.5, TS 3.8.8, and TS 3.8.10 respectively. The two ClEEEAC trains (units) constitute a support system for the two Class IE electrical equipment trains. As noted, there is no specific Technical Specification for the Cl EEEAC system.
Background
The C1EEEAC system was designed such that the A ClEEEAC train provides cooling to the A train of the Class IE electrical equipment in the Control Building, and the B CI EEEAC train provides cooling to the B train of Class 1E electrical equipment in the Control Building.
However, each C 1 EEEAC train has also been shown to have sufficient capacity to cool both Class 1E electrical equipment trains if the doors between the electrical equipment rooms are opened (in conjunction with several other compensatory measures). A practice has thus been established at Callaway such that in the event of inoperability of one ClEEEAC unit, operability of the Class lE electrical equipment (both trains) can be supported by the remaining operable ClEEEAC unit once the noted compensatory actions (including opening the electrical equipment room doors) have been completed.
In accordance with the above-described practice, when a ClEEEAC unit/train is declared inoperable during power operation, the applicable Conditions and Required Actions of TS 3.8.4, TS 3.8.7, and TS 3.8.9 are entered due to declaring the supported train of Class lE electrical equipment inoperable. Since there are two inverters in each train of Class lE electrical equipment, but the Conditions and Required Actions under TS 3.8.7 only address inoperability of a single inverter, declaring one train of supported Class lE electrical equipment inoperable also requires entry into TS LCO 3.0.3. After entering LCO 3.0.3 and the Conditions and Required Actions of TS 3.8.4, TS 3.8.7, and TS 3.8.9 upon declaring a Cl EEEAC train/unit inoperable, these Conditions and Required Actions (and LCO 3.0.3) may be exited once supported equipment operability is restored by completion of the noted compensatory actions. Even with the compensatory actions completed, however, the inoperable ClEEEAC unit/train must be restored within seven days per Callaway's requirements.
2. INITIAL PLANT CONDITIONS:
The plant was in MODE 1 at 100 percent power at the time when a Freon leak was first discovered in the B Class lE AC Unit on December 17, 2012.
3. EVENT DESCRIPTION:
On 12/17/2012, during performance of a quarterly leak check on SGKO5B, a small amount of refrigerant was identified to be leaking from the compressor suction and discharge pressure-sensing lines by use of a halide detector. The leakage was the result of the two 1/8-inch copper sensing lines rubbing together. The identified leakage and inability to quantify leakage rates caused SGKO5B to be declared inoperable at 10:45. LCO 3.0.3 and the Applicable Conditions and Required Actions of TS 3.8.4, TS 3.8.7, and TS 3.8.9 were entered with the intent of completing the necessary compensatory actions for restoring Operability of the affected Class lE electrical equipment. Steps were taken to perform plant procedure, "Control Room and Class lE Air Conditioning Unit Charging," which is used to check the unit for adequate refrigerant charge. Prior to the completion of the refrigerant charge check on SGKO5B, an inspection was performed on SGKO5A. This inspection identified visible leakage on the unit's low oil pressure and compressor discharge sensing lines. As a result of the inability to quantify the identified leakage, SGKO5A was declared inoperable at 14:35. This placed the plant in LCO 3.0.3 (without the ability to implement compensatory measures for the supported Class lE electrical equipment, assuming both ClEEEAC units inoperable). Steps were immediately taken to also perform a refrigerant charge check on the SGKO5A unit.
Upon completion of the refrigerant charge check on SGK05B, it was identified that no refrigerant addition was necessary. It was thus confirmed that SGK05B had in fact remained Operable throughout this time. At 16:00 on 12/17/2012 SGK05B was declared operable. LCO 3.0.3 was exited at 16:12 (as the necessary compensatory actions had been completed to ensure cooling to both Class lE electrical equipment trains via the operable ClEEEAC unit, SGKO5B). The charge check for SGKO5A, however, identified that 9 lbs. 3 oz. of refrigerant needed to be added to the system. Based on the reduced Freon charge, it was apparent that the leak rate would challenge the unit's capability to meet its required mission time.
In order to isolate the leaking sensing lines, a Temporary Modification was installed. This modification disconnected the two 1/8-inch sensing lines and capped their termination ports on the compressor body. (The functions of the sensing lines are not essential to the safety function performed by the ClEEEAC unit, except for being an extension of the Freon pressure boundary).
Following implementation of this modification, SGKO5A was restored to an operable status at 11:08 on 12/18/2012. SGKO5A was inoperable for approximately twenty hours and 30 minutes.
4. ASSESSMENT OF SAFETY CONSEQUENCES:
The Class 1 E Air Conditioning units, SGKO5A and B, are not explicitly modeled in the Callaway Probabilistic Risk Assessment (PRA) model. Analysis supporting the PRA model demonstrates that safety systems supported by the SGKO5 units are capable of meeting a 24-hour PRA mission following loss of a single SGKO5 unit. As such, the Class 1 E electrical equipment air- conditioning units are below the truncation significance level for inclusion in the PRA model.
Further, as previously noted, with one SGKO5 unit inoperable the remaining operable unit can be utilized to support both trains of Class 1 E electrical equipment with the appropriate compensatory actions completed. Although both units had been declared inoperable, it was soon confirmed that the leak condition identified for SGKO5B was not sufficient for actually rendering the unit inoperable. Thus, SGKO5B remained operable, and as such it was capable of providing cooling to both trains of supported Class lE electrical equipment.
5. REPORTING REQUIREMENTS:
This LER is submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) to report a condition or operation prohibited by the plant's Technical Specifications.
A condition or operation prohibited by the plant Technical Specifications was determined to have occurred due to the leakage condition identified for SGKO5A. As explained previously, when a ClEEEAC unit/train is declared inoperable, LCO 3.0.3 is required to be entered due to the impact on the supported Class lE inverters. Upon declaring a ClEEEAC unit inoperable (i.e., when it is known to be inoperable), compensatory actions can be taken to restore operability of the inverters (and the other supported Class 1 E equipment) via the remaining operable ClEEEAC unit, thus allowing LCO 3.0.3 to be exited. If the compensatory actions are not (or cannot be) taken, LCO 3.0.3 remains in effect such that per the requirements of that TS, action must 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 />. Thus, with a ClEEEAC unit inoperable and no compensatory actions taken, the plant must be in Mode 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> (and so on) to comply with the plant's Technical Specifications (i.e., LCO 3.0.3).
When the tubing leak was identified for SGKO5A on 12/17/2012, it was declared inoperable on the basis that the leak condition would not have enabled the unit to fulfill its 30-day post-accident mission time from that point in time. Although it is not certain when the leak began, it was concluded that the leakage had existed for a period of time prior to the discovery of the leak such that the period of SGKO5A inoperability exceeded the time allowed by the Technical Specifications (i.e., longer than the 7-hour period of plant operation allowed by LCO 3.0.3 with no compensatory measures in place).
6. CAUSE OF THE EVENT:
The Direct Cause of the leak condition identified for each of the SGKO5 units was determined to be the seismically qualified design of the skid-mounted unit which exposes some sub-components to excessive vibration during unit operation. The leakage was the result of two sensing lines rubbing together. The Root Cause was determined to be an inadequate scope of previously conducted equipment reliability evaluations on the Control Building HVAC System. These evaluations should have identified the long-term trends in equipment failure that led to the event in this LER.
7. CORRECTIVE ACTIONS:
As noted in the Event Description section of this LER, a temporary modification was installed to disconnect two 1/8-inch sensing lines and cap their termination ports on the compressor body.
The direct cause is addressed by an increase in preventative maintenance and monitoring of vibration susceptible sub-components and piping on the Class 1 E electrical equipment air- conditioning units.
In order to identify other latent, long-term trending issues, Engineering will document a review of the history of the safety-related Control Building HVAC system Air Conditioning units. As part of the review, an analysis of historical data, including subcomponent failure data, will be performed.
Engineering will then revise or implement new Preventative Maintenance Tasks to address any failure mechanisms that are identified.
8. PREVIOUS SIMILAR EVENTS:
On March 31, 2011, Callaway submitted LER 2010-010-00 about a violation of Technical Specification 3.0.3 due to inoperability of the B Class lE electrical equipment air conditioning unit.
The air conditioning unit was out-of-service for a timeframe longer than permitted by the Technical Specifications; however, it was for a cause other than a Freon leak.
An internal operating experience search for vibration-induced leakage from copper tubing during the last three years was completed using Callaway's corrective action program and the Equipment Out of Service Logs (EOSLs). This review identified multiple examples of Freon leakage from similar air conditioning units.
On 12/17/2012: Corrective Action Program document, SGKO5B Freon Leaks on sensing line, was written on the same day as the SGKO5A leak was identified. This documents nearly identical leakage due to vibration induced wear on sensing lines.
On 9/18/2011: Corrective Action Program document, Hazardous Environment developed in A CRAG Room, documents Freon leakage on hot gas piping due to mechanical fatigue leading to a though wall crack.
On 5/15/2011: Corrective Action Program document, Small Freon Leak on SGKO5A, documents small refrigerant leak due to an incomplete solenoid joint.
On 3/8/2011: Corrective Action Program document, Oil Leak on SGKO4A, documents oil leaking from an oil pressure sensing line. The leak occurred where a sensing line was bent at a sharp angle. This was determined to be the result of work in area contributing to the failure.
On 2/27/2011: Corrective Action Program document, SGKO5A 'A' Train Class 1 E A/C Unit declared inoperable, identifies leakage discovered coming from the high side of the piping about a foot from the compressors high side isolation valve. The failure was attributed to wear from the edge of the installed fabric insulation on the compressor discharge piping which was vibrated by the compressor.
From the internal examples, only the 3/8/2011 Oil Leak on SGKO4A event, deals with leakage on small diameter copper sensing lines. Corrective actions from this event only repaired the leak.
This event was determined to be a maintenance preventable functional failure with the basic cause being that work in the area contributed to failure.