05000254/LER-2012-001
| Docket Number Sequential Revmonth Day Year Year Month Day Year Quad Cities Nuclear Power Station Unit 2 05000265Number No. | |
| Event date: | 02-03-2012 |
|---|---|
| Report date: | 03-23-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 47634 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2542012001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EllS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Control Room Emergency Ventilation Air Conditioning System Inoperable
A. CONDITION PRIOR TO EVENT
�Unit: 1 Event Date: February 3, 2012� Event Time: 1035 hours0.012 days <br />0.288 hours <br />0.00171 weeks <br />3.938175e-4 months <br /> �Reactor Mode: 1 Mode Name: Power Operation� Power Level: 100%
B. DESCRIPTION OF EVENT
On February 3, 2012, at 1035 hours0.012 days <br />0.288 hours <br />0.00171 weeks <br />3.938175e-4 months <br />, Operations discovered that the indicating light was off for the safety-related Train "B" Control Room Emergency Ventilation (CREV) [VI] system HVAC Refrigeration [RFU] Condensing Unit (RCU) [CDU] 0-9400-102 on the Main Control Room (MCR) [NA] Panel 912-5. An attempt was made to change the light bulbs [IL] in MCR but no light indication returned. Operators were dispatched to Motor Control Center (MCC) 18-4 Cubicle 1D to investigate the light indication issue and found the breaker [52] in the tripped condition. This resulted in declaring the "B" CREV and RCU inoperable and subsequently securing the system. Operators performed required actions safely, in accordance with procedures and training, and without complications.
As a result, Technical Specification 3.7.5, Condition A (30 day Action) was entered for an inoperable Control Room Emergency Ventilation Air Conditioning (AC) System for Units 1 and 2. ENS Notification (# 47634) was made in accordance with 10 CFR 50.72 (b)(3)(v)(D) since the CREV system is a single train safety system, and loss of the CREV AC could impact the plant's ability to mitigate the consequences of an accident. This is a Maintenance Rule Functional Failure for the 480 Volt System.
Train B Control Room HVAC had been successfully run from February 2, 2012, at 0845 hours0.00978 days <br />0.235 hours <br />0.0014 weeks <br />3.215225e-4 months <br /> until February 2, 2012, at 2207 while troubleshooting and maintenance was being performed on the non-safety related Train A Control Room HVAC System. The Train B Control Room HVAC RCU runs a pumpdown cycle every four hours when idle to ensure that the compressor [CMP] does not start in a flooded condition. A flooded start is caused by the return of a large quantity of liquid refrigerant to the suction side of the compressor. Refrigerant condenses during idle times in any part of the system where the temperature becomes cold enough, generally in the suction line between the evaporator [EVP] and the compressor. When a flooded start occurs, the liquid refrigerant has a tendency to wipe away any oil on the compressor internals and therefore, any damage that may occur is mostly caused by lack of lubrication. The automatic pumpdown feature allows the compressor to run periodically during idle periods to pump out Freon in the suction side of the compressor. Therefore, the RCU would have run a pumpdown cycle on February 3, 2012, at approximately 0207 hours0.0024 days <br />0.0575 hours <br />3.422619e-4 weeks <br />7.87635e-5 months <br />, 0607 hours0.00703 days <br />0.169 hours <br />0.001 weeks <br />2.309635e-4 months <br /> and 1007 hours0.0117 days <br />0.28 hours <br />0.00167 weeks <br />3.831635e-4 months <br />. The breaker trip likely occurred during one of those pumpdown cycles.
An Engineering review of the February 3, 2012, breaker trip indicated that the most likely cause of the breaker trip was insufficient trip margin. A new Engineering Change (EC) evaluation for breaker trip settings was completed on February 8, 2012. This new EC utilized locked-rotor currents which were calculated using the provided motor [MO] nameplate data. When the locked-rotor currents were calculated, it was determined that this motor load had higher locked-rotor currents, and as such, the breaker setting needed more margin and was required to be set up to 17 times the Full Load Current (FLC). This resulting new breaker setting is significantly higher than the original breaker setting of 12 times the FLC, and will eliminate nuisance trips of the breaker and reduce equipment unavailability.
The breaker setting for MCC 18-4 cubicle 1D was changed in accordance with the new EC evaluation, and on February 9, 2012, at 1628 hours0.0188 days <br />0.452 hours <br />0.00269 weeks <br />6.19454e-4 months <br />, Train "B" Control Room HVAC system successfully completed its 10 hour1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> operability run, was declared operable, and Technical Specification 3.7.5, Condition A, was exited.
Given the impact on the CREV AC system, this report is submitted in accordance with the requirements of 10 CFR 50.73(a)(2)(v)(D), which requires the reporting of any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
C. CAUSE OF EVENT
The CREV RCU chiller compressor breaker trip was caused by insufficient margin in the breaker setting for MCC 18-4 Cubicle 1D. The original breaker setting did not take into account that the compressor motor load had the potential for a high locked-rotor current, and the breaker setting for instantaneous (magnetic) setting needed to be set higher to avoid nuisance trips.
The initial EC evaluation for breaker settings in April 2009 and May 2009 used the assumption that the motor for the RCU was a continuous duty motor without higher locked-rotor currents, and therefore the instantaneous (magnetic) setting of the breaker should be set between 10 to 12 times the FLC. The motor manufacturer had provided motor nameplate data, and it was used to calculate the breaker setting under the initial EC evaluation, however specific locked-rotor amps were not provided on the motor nameplate.
As a result of this February 2012 Train B CR HVAC RCU motor breaker trip event, reviews were performed on the breaker settings for other similar air conditioning compressors. These reviews concluded that air conditioning compressor loads on breakers, in some cases, were determined to be continuous duty motors with higher locked rotor currents (i.e., greater than 6 times FLC). As such, a new EC evaluation for breaker trip settings was completed on February 8, 2012, utilizing locked-rotor currents which were calculated from the provided motor nameplate data.
As a result, it was determined that this motor load had higher locked-rotor currents, and the breaker setting needed more margin, which required it to be set up to 17 times the FLC.
The extent of condition of this event is limited to equipment that utilizes a motor load with higher locked-rotor currents.
A higher locked-rotor current is more likely to occur on equipment which does not run continuously, but rather cycles on and off, as determined by the device logic. This equipment may include other station compressors and various sump pumps [P].
D. SAFETY ANALYSIS
The purpose of the CREV system is to maintain the proper air environment for instrumentation and personnel in the Control Room [NA]. The Train B CREV AC system is a single train safety-related system that is designed to operate in a post-accident condition to maintain design temperature in the Control Room Envelope (CRE). The Train B CREV RCU 0-9400-102 is a Carrier reciprocating compressor; Model 5H120, with a capacity of 90 tons. The backup to the Train B CREV system is the Train A Control Room HVAC system, which is a non-safety-related system. This is a Maintenance Rule Functional Failure for the 480 Volt System.
Insufficient breaker setting margin caused MCC 18-4 Cubicle 1D breaker to trip which resulted in declaring the "B" CREV and RCU inoperable and subsequently securing the system. Operators performed required actions safely, in accordance with procedures and training, and without complications.
�This resulted in entering Technical Specification 3.7.5, Condition A (30 day Action). The loss of the CREV AC could impact the plant's ability to mitigate the consequences of an accident.
The safety significance of this event was minimal. The CREV system filtration capability was not lost, and control room temperature was maintained during this event. In addition, the non-safety related Train A Control Room HVAC system was available and operating throughout this event.
The CREV AC system is not explicitly modeled in the Plant Probabilistic Risk Assessment (PRA). Certain Operator Actions (OAs) are modeled in the PRA, however, the effect of this event on OA human error probabilities (HEPs) would have a negligible quantitative impact on the calculated Core Damage Frequency (CDF) and Large Early Release Frequency (LERF) since loss of CREV AC events are slow progressing events (being reduced temperature control of the Control Room). In the loss of CREV system event, Station procedures direct the opening of doors and use of temporary fans, as necessary. Applicable Operator training is performed on these actions and the equipment is pre-staged as a part of the response to station blackout requirements.
Although opening Control Room doors to support room cooling would introduce the increased potential for radiological or chemical events impacting the MCR, the probability of a loss of CR HVAC event coincident with a radiological or chemical event is small and therefore, would also have a negligible quantitative impact on CDF and LERF.
In conclusion, the insufficient breaker setting margin which caused MCC 18-4 Cubicle 1D breaker to trip and resulted in securing the "B" CREV and RCU, was not risk significant.
E. CORRECTIVE ACTIONS
Immediate:
1. The breaker setting was modified for MCC 18-4 Cubicle 1D that feeds the Train B Control Room HVAC RCU.
Follow-up:
1. Review breaker settings for all loads identified in the extent of condition to determine if they have higher locked-rotor current settings. Revise the breaker settings as necessary for those loads.
2. Review the need for additional training on breaker sizing for non-continuous loads which may have high locked rotor currents.
F. PREVIOUS OCCURRENCES
The station events database, LERs, EPIX, and NPRDS were reviewed for similar events at Quad Cities Nuclear Power Station. This event was a CREV RCU chiller compressor breaker trip, which was caused by insufficient breaker setting margin.
- Station Events Database — IR 895604 (03/20/09), Unit-1A Instrument Air [LD] Compressor Trips (IAC) (Equipment Apparent Cause Evaluation). The apparent cause was the 1A IAC trips on feed breaker TOL rather than its installed TOL relays. A breaker setting change was made for the 1A IAC and no further breaker trips have occurred. The IAC compressors have a very similar load to the B CR HVAC RCU in that it cycles on and off frequently during operation and this load has caused high currents under certain conditions. This event is similar in that breaker trips and settings were near the operating limit and the trips were spurious, however, at the time of this event the causes and required corrective actions were limited to the IACs, and therefore this event is not directly applicable and is not considered a significant station experience that would have directly contributed to preventing the event of this current LER.
- Station Events Database — IR 914984 (05/02/09), Train B Control Room HVAC RCU Multiple Breaker Trips at MCC 18-4 Cubicle 1D (Equipment Apparent Cause Evaluation). The apparent cause of the breaker trips was a double start condition (not due to locked-rotor current conditions) on the compressor motor start logic. At the time of this event the causes and required corrective actions were limited to the double start condition, and therefore this event is not directly applicable and is not considered a significant station experience that would have directly contributed to preventing the event of this current LER.
G. COMPONENT FAILURE DATA
The inoperable component was the Train B CREV RCU 0-9400-102, which is a Carrier Model 5H120 reciprocating air conditioning compressor with a capacity of 90 tons.
There was no specific component damage in this event, however, the breaker trip setting for MCC 18-4 Cubicle 1D was originally set with insufficient margin and led to nuisance trips of the breaker. Breaker MCC 18-4 Cubicle 1D is a Cutler-Hammer/Eaton Molded Case Circuit Breaker with Trip Unit, Model # KD3250/KT3250T.
This event has been reported to EPIX as Failure Report No. 1411.