05000247/LER-2012-008
| Indian Point 2 | |
| Event date: | 09-07-2012 |
|---|---|
| Report date: | 11-05-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2472012008R00 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets (}.
DESCRIPTION OF EVENT
On September 7, 2012, at approximately 11:52 hours, Control Room operators received a 21/22 Static Inverter (SI) (EF) trouble alarm. A Nuclear Plant Operator (NPO) was dispatched and discovered the 22 SI had swapped to its alternate power source.
Technical Specification (TS) 3.8.7 (Inverters-Operating) Condition A was entered for an inoperable 22 Static Inverter due to the 22 SI being supplied by its alternate power source. Required action A.2 is to restore the affected feature to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
An investigation initially determined all indications were normal and the SI output frequency was within its normal range allowing transfer back to the normal power source. It was noted by inspection personnel that the left-hand side LED light {PEC} in the local SI frequency meter {HZM} was extinguished. Based on a previous Operating Event (OE) recorded in CR-IP2-2012-2557, it was known that a failure of either of the LEDs on the meter face would result in an inverter transfer. The inverter frequency meters are designed with two adjustable set point limits. The alarm set points are set by turning a knob to adjust an arm which sits inside the meter. A photo-coupler is attached to each arm. As the frequency indicator needle passes between the set point arm and the light source, it blocks the light and causes the photo-coupler to change state, which initiates a signal to the Static Switch Control Board to swap the inverter to the alternate power source. Permission from operations management was obtained to attempt a transfer back to the normal power source per System Operating Procedure 2-SOP-27.1.6 (Instrument Bus, DC Distribution and PA system Inverter). At 12:16 hours, an unsuccessful attempt was made to transfer back to the 22 SI normal power source.
Troubleshooting determined the SI frequency meter optical sensor light was out.
Tapping on the meter resulted in the frequency meter light illuminating. The indicated frequency did not change when the meter was tapped. The illumination of the optical sensor light allowed the 22 SI to sense that the correct frequency was present to perform a swap back to the normal power supply. The output frequency signal going to the SI frequency meter was checked and adjusted to 60 Hz. At 12:30 hours, the 22 SI was reset and Operations transferred the 22 SI back to its normal power supply per 2 SOP-27.1.6 and verified the normal TS parameters per 2-PT-W20 (Electrical Verification Inverters and DC distribution in Modes 1-4) were being maintained and exited TS 3.8.7 Condition A on September 7, 2012, at 12:30 hours. The condition of the 22 SI was recorded in the Indian Point Energy Center (IPEC) Corrective Action Program (CAP) as Condition Report CR-IP2-2012-05584. Subsequently, station management representatives, engineering, operations, instrumentation & Control (I&C) and Licensing conducted a conference on the failure to determine a course of action and developed contingency plans for replacing the 22 SI optical sensor frequency meter and installing a temporary modification to bypass the optical sensor out of frequency transfer as was done previously for the 24 SI On September 10, 2012, the seismic qualification of the 22 SI frequency meter was questioned concerning the 22 SI seismic qualification. Because the cause of the failure of frequency meter optical sensor light could not be determined, operations concluded there was no reasonable assurance that the 22 SI would be able to maintain its design function in the event of a seismic event and the 22 SI was declared inoperable at 15:03 hours on September 10, 2012, and TS 3.8.7 Condition A entered. The optical frequency meter for the 22 SI was replaced and satisfactory testing performed per 2-PT-W020 at 16:15 hours. The 22 SI was returned to operable status and TS 3.8.7 Condition A exited at 16:15 hours on September 10, 2012. The question regarding seismic qualification of the 22 SI was recorded in CR-IP2-2012-05620.
NRCFORM366%U.S.NUCLEARREGULATORYCOMMISSION FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) The inverters are the preferred source of power for the 118 VAC instrument buses. Each inverter receives power from a different DC Power Panel. In addition to the normal DC power source (Battery Chargers) for the inverter, each inverter has an associated step down transformer that is used as an alternate input power supply (118 VAC) {ED} to the instrument buses. The alternate power supply is used to synchronize the inverter output to the auxiliary electrical system and to provide continuity of power to the vital 118 VAC loads in the unlikely event of an inverter failure. Power is supplied to the instrument buses from the DC source via the inverter or from the step-down transformer {XFMR}.
Each alternate power supply can be used to support the 118 VAC loads via the inverter internal static transfer switch or via an external manual bypass switch. The static switch is composed of two sets of Silicon Controlled Rectifiers (SCRs) which are turned on or off to determine the source of supply to the instrument bus (either battery charger or the 480 volt plant supplies). In the event of a loss of DC power to the inverter, the inverter's internal static transfer switch will automatically transfer the 118 VAC loads to the alternate power supply. The alternate power supply to the instrument busses will be interrupted during accident conditions involving a safety injection actuation (with or without loss of offsite power) and during a loss of offsite power.
The SIs provide assurance in modes 1, 2, 3 and 4 of the availability of AC electrical power for the system instrumentation required to shut down the reactor and maintain it in a safe shutdown condition after an anticipated operational occurrence or a postulated DBA. The four SIs ensure an uninterruptible supply of AC electrical power to the 118 VAC instrument buses even if the 480 volt safety buses are de-energized.
The SIs are manufactured by SCI {S095}.
An extent of condition (EOC) review determined the condition applies to the SCI Static Inverters (unit 2 SI 21 - 23). A temporary modification to bypass the 24 SI transfer on out of frequency has previously been implemented. For unit 3 SI 31, 32 and 33 are manufactured by Elgar and do not have the out of frequency transfer function. The 34 SI is manufactured by SCI but is a newer unit which does not have the out of frequency transfer function. A corrective action (CA) for CR-IP2-2012-02557 was previously issued to initiate a modification to change the out of frequency transfer to an alarm only function based on the redundancy due to the installation of the new Sync and Oscillator boards in SIs 21 through 24. Installation of the modification would prevent similar events as the SI would not swap based solely on loss of the optical sensor.
During spring 2012 refueling outage, the Sync and Oscillator boards were replaced in SI 21 -24 with a new design which does not drift and would only transfer upon a board failure fulfilling the same function as the out of frequency transfer rendering the out of frequency transfer unnecessary. As an interim measure, a Temporary Modification was issued to allow the installation of a jumper on the frequency detector board to prevent transfer on SI out of frequency. Included were Work Orders on the remaining SIs that were held open as contingencies for installation in the event of a SI issue due to a degraded frequency meter.
Cause of Event
The apparent cause of the event was an inadequate use of human performance tools.
Shift management did not effectively use questioning attitude before declaring the 22 SI operable. The Shift Manager (SM) incorrectly assumed that as long as the SI met TS Surveillance Requirement 3.8.7.1 that the SI was operable. Proper consideration was not given to the fact that the inverter was able to be placed on its normal power supply by tapping on the frequency meter to reenergize the optical sensor and the possibility that the 22 SI could transfer to its alternate power source during a seismic event.
The Operation team's level of knowledge on the operation of the optical sensor was insufficient to assess the seismic effect on its operation. The SM believed the frequency meter left side optical sensor was not lit because the needle indicating frequency reached the low frequency alarm set point. It was believed that the optical sensor would stay off until the frequency returned above the alarm set point. When the meter was tapped, the SM believed that the frequency meter cleared the low frequency alarm set point causing the low frequency optical sensor to illuminate.
Corrective Actions
The following corrective actions have been or will be performed under the Corrective Action Program (CAP) to address the cause of this event.
- The 22 SI frequency meter was replaced and tested satisfactorily.
- An OE of this event was included in operations training.
- A clock reset was performed and shared with all operations teams to ensure heightened awareness of seismic considerations when determining operability.
- A Temp Mod had been implemented for the 24 SI and Temp Mods were issued to bypass the SI transfer capability for out of frequency on the 21 thru 23 SIs.
- The SI vendor manual was updated to include information on meter function.
- The System Description for electrical systems (27.1) will be revised to include a discussion of the SI frequency meter optical sensor.
- An equipment failure analysis will be performed to determine the cause of the failure of the SI frequency meter photocell LED.
- A modification will be prepared to replace the installed SI frequency meters with an updated model that does not include an out of frequency transfer.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(i)(B). The licensee shall report any operation or condition which was prohibited by the plants Technical Specifications.
Technical Specification (TS) 3.8.7 (Inverters - Operating), Limiting Condition for Operation (LCO) requires four inverters to be operable to support onsite 118 VAC instrument bus electrical power distribution subsystems required by TS 3.8.9 (Distribution systems - Operating). TS 3.8.7 Condition A required action for one inverter inoperable is: A.1, Declare affected required feature supported by associated inverter when the required redundant feature is inoperable AND A.2 Restore inverter to operable in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Condition B, Required action and associated completion time not met, B.1 be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and B.2, be in mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. There was no redundant feature associated with the 22 SI inoperable therefore TS 3.8.7 condition A.1 was not applicable. The actions required by TS 3.8.7 Condition A.2 were not performed resulting in a TS prohibited condition. The condition of the 22 SI resulting in inoperability was identified on September 7, 2012, at 11:52 hours, and subsequently returned to service. On September 10, 2012, after questioning the seismic qualification of the 22 SI transfer on September 7, 2012, the 22 SI was declared inoperable, repaired and returned to operable, at 16;15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br />, on September 10, 2012. This period of time when the 22 SI was inoperable exceeds the TS AOT time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and the required TS actions not taken. During the time the 22 SI was inoperable the 21, 23 and 24 SIs were operable and in service. There was no safety system functional failure as the minimum required 118 VAC instrument bus power was available.
Past Similar Events
A review was performed of the past two years of Licensee Event Reports (LERs) for events that involved inoperable SIs due to defective SI frequency meters. No LERs were identified. Further review was performed for LERs whose causes reported ineffective or weak use of Human Performance (HP) tools or HP tools not applied.
Three LERs were identified (LER-2012-002, LER-2012-001, LER-2010-005). LER-2012-002 reported fuel assembles stored in a configuration prohibited by the TS caused by weak/ineffective use of HP tools as a result of poor self check and peer check/review.
LER-2012-001 reported a TS prohibited condition caused by an inoperable 23 Emergency Diesel Generator Fuel Oil (FO) Storage Tank due to FO below the TS limit. The apparent cause was lack of a questioning attitude (HP tool) and understanding by operations watch personnel of out of service equipment. LER-2010-005 reported a TS prohibited condition due to an inoperable Control Room Ventilation System caused by a closed normally open damper. The apparent cause was HP tools were not applied to ensure work activities did not affect other equipment in the work area. Corrective actions for these causes included coaching of applicable personnel on lessons learned and management expectations on use of HP tools. Corrective action for LER-2012-002 included training to reinforce expectations of TS compliance and independent verification and self checking. The corrective actions for LER-2012-002 and LER-2010 005 would not have prevented this LER as the personnel were different (Reactor Engineering and Maintenance). LER-2012-001 involved Operations but only the applicable field operators.
Safety Significance
This event had no significant effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents or transients requiring the inverters nor was there a seismic event during the time the SI was declared inoperable. In addition, the assumption of a single failure and concurrent loss of all offsite or all onsite power is not required. During the time the 22 SI was inoperable the 21, 23 and 24 SIs were in service. Also, the instrument bus for the 22 SI can be directly powered by an alternate power supply via an associated step-down transformer.
The likelihood of a seismic event occurring during the time the 22 SI was inoperable with the failed frequency meter photocell LED was assessed. The probability of a 0.15g seismic event, as described in the Indian Point IPEEE, is approximately 2.1E-4 per year. The probability of such a seismic event during the exposure time of approximately 76 hours8.796296e-4 days <br />0.0211 hours <br />1.256614e-4 weeks <br />2.8918e-5 months <br /> was approximately 1.82E-6. Since the remaining mitigation capability is not impacted by the condition, the conditional probability of a core damage event given the seismic failure of the 22 SI is expected to be far below the 1E-6 screening criteria.