05000416/LER-2007-001
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 04-11-2007 |
|---|---|
| Report date: | 06-05-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4162007001R00 - NRC Website | |
On March 24, 2007 at 1221, it was discovered during Technical Specification surveillance testing that the Division 2 Emergency Bus [EB,BU] (4.16 kV) 16AB feeder breaker [BKR]152-1611 from ESF (Engineered Safety Feature) Transformer [EK] 12 would open at a 0.35 second time delay upon receipt of a loss of voltage signal. This was contrary to its required Technical Specification 3.3.8.1 Function 1.b allowable value of >1=0.4 and Investigation revealed that the 4.16 kV Emergency Bus 15AA (Division 1) and 16AB (Division 2) loss of voltage protective time delay relays [27] (15AA-162-1, 15AA-162-2, 16AB-162-1, and 16AB-162-2) were set such that their time delay (0.3 seconds) did not allow adequate surveillance testing of the Technical Specification 3.3.8.1 Function 1.b. Loss of Voltage - Time Delay Load Shedding and Sequencing System (LSSS) credited time delay devices (XA22-TD1 and XA22-TD2). Subsequent bench testing of the 15AA 162-1, 15AA-162-2, 16AB-162-1, and 16AB-162-2 time delay relays indicated as-found time delays between 0.28 and 0.32 seconds. Technical Specification credited time delay devices (XA22-TD1 and XA22-TD2) have not been properly surveillance tested in accordance with Technical Specification Surveillance 3.3.8.1.2 for both Division 1 (15AA) and Division 2 (16AB) 4.16 kV Emergency Buses.
This condition is considered a violation of Technical Specification 3.3.8.1 Loss of Power (LOP) Instrumentation allowable value for Function 1.b and is reportable as required by 10CFR50.73(a)(2)(i)(B).
This condition was considered an Operation or Condition Prohibited by Technical Specifications since there was a failure to comply with Technical Specification surveillance testing criteria (Function 1.b allowable value). The reportability event discovery date was April 11, 2007 at 0957 due to the time needed to evaluate the condition and affected Technical Specifications.
B. INITIAL CONDITIONS
At the time of the event, the reactor was in OPERATIONAL MODE 5 with reactor power at 0 percent.
Moderator temperature, reactor pressure vessel (RPV) pressure, and RPV water level were at approximately 93 degrees F, 0 PSIG, and at refueling floor high water level with the reactor vessel head off and reactor steam dryer and moisture separator removed. There were no additional inoperable structures, systems, or components at the start of the event that contributed to the event.
C. DESCRIPTION OF OCCURRENCE
On March 24, 2007 at 1221, it was discovered during Technical Specification surveillance testing that the Division 2 4.16 kV Emergency Bus 16AB feeder breaker 152-1611 from ESF (Engineered Safety Feature) Transformer 12 would open at a 0.35 second time delay upon receipt of a loss of voltage signal. This was contrary to its required Technical Specification 3.3.8.1 Function 1.b allowable value of >1=0.4 and seconds for a loss of voltage time delay. As discussed in Technical Specification Bases B 3.3.8.1 Loss of Voltage on a 4.16 kV emergency bus indicates that offsite power may be completely lost to the respective emergency bus and is unable to supply sufficient power for proper operation of the applicable equipment.
The bus undervoltage allowable values are low enough to prevent inadvertent power supply transfer, but high enough to ensure power is available to the required equipment. The time delay allowable value for 1.b (>1=0.4 and normal voltages, but short enough to ensure that power is available to the required equipment.
Investigation revealed that the 4.16 kV Emergency Bus 15AA (Division 1) and 16AB (Division 2) loss of voltage protective time delay relays (15AA-162-1, 15AA-162-2, 16AB-162-1, and 16AB-162-2) were set such that their time delay (0.3 seconds) did not allow adequate surveillance testing of the Technical Specification 3.3.8.1 Function 1.b. Loss of Voltage - Time Delay Load Shedding and Sequencing System credited time delay devices (XA22-TD1 and XA22-TD2). Subsequent bench testing of the 15AA-162-1, 15AA-162-2, 16AB-162-1, and 16AB-162-2 time delay relays indicated as-found time delays between 0.28 and 0.32 seconds. The 15AA-162-1, 15AA-162-2, 16AB-162-1, and 16AB-162-2 relays were sending trip signals to ESF offsite power feeder breakers 152-1601, 1611, 1614, 1501, 1511, and 1514 prior to the XA22-TD1 and XA22-TD2 time delay devices sending their LSSS trip signals to the same breakers. The offsite power feeder breakers 152-1501, 1511, and 1514 supply power to the Division 1 4.16 kV Emergency Bus 15AA from ESF Transformers 11, 12, and 21. The offsite power feeder breakers 152 1601, 1611, and 1614 supply power the Division 2 4.16 kV Emergency Bus 16AB from ESF Transformers 11, 12, and 21. Figure 1 provides detail on breaker arrangement and affected breakers.
The 15AA-162-1, 15AA-162-2, 16AB-162-1 and 16AB-162-2 time delay relays being set at a 0.3 second (as-found 0.28 and 0.32 seconds) time delay did not allow adequate surveillance testing of the Technical Specification 3.3.8.1 Loss of Voltage Load Shedding and Sequencing System time delay devices XA22- TD1 and XA22-TD2. The XA22-TD1 and XA22-TD2 are required to be surveillance tested in accordance with Technical Specification Surveillance 3.3.8.1.2. This condition affected both Division 1 (15AA) and Division 2 (16AB).
D. APPARENT CAUSE
The cause has been determined to be a failure to recognize in the original design documents for the ESF Division 1 (15AA) and Division 2 (16AB) Emergency Bus switchgear [SWGR] that the standard protective bus under-voltage device time delay (set at 0.3 second) relays could react faster than the Technical Specification 3.3.8.1 credited LSSS time delay devices (XA22-TD1 and XA22-TD2) which were set at 0.5 seconds. Review of the E009.1 Technical Specification that was issued for purchase of 15AA and 16AB Emergency Bus switchgear indicates that the under-voltage relays are listed as generically supplied equipment that were supplied with the switchgear. The switchgear supplied time delay relays 15AA-162-1, 15AA-162-2, 16AB-162-1 and 16AB-162-2 set at 0.3 seconds were not recognized to react faster than the LSSS time delay devices during installation and design review nor during Technical Specification Surveillance testing of the 15AA and 16AB switchgear. There were no component failures that resulted from the identified condition.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions — The Loss of Voltage — Time Delay relay timers 15AA-162-1, 15AA-162-2, 16AB-162-1 and 16AB-162-2 setpoints were changed from as found 0.28 — 0.32 seconds to 0.9 seconds to ensure there would be no interference with the Technical Specification 3.3.8.1 Function 1.b time delay devices (XA22-TD1 and XA22-TD2). After changing the 15AA-162-1, 15AA-162-2, 16AB-162-1 and 16AB 162-2 setpoints, Technical Specification Surviellance 3.3.8.1.2 was successfully performed for Emergency Buses 15AA and 16AB to verify the Technical Specification 3.3.8.1 Function 1.b allowable value of >1=0.4 and found to be set correctly and provided the required time delays of >1=0.4 and Long Term Corrective Actions - Condition Report GGN-2007-1405 was written to address any additional actions.
Prior to the discovery of this Technical Specification non-compliance there have been no known spurious power supply transfers due to conditions identified in this LER. This condition did not prohibit 15AA and 16AB Emergency Buses from performing their intended safety function. There was a loss of powerloss of voltage event as documented in LER-2005-001-00 when a ground fault on the 34.5 kV bus work of Service Transformer ST11 resulted in the loss of power to 12HE, 13AD and 15AA buses. Emergency Diesel Generator (EDG) 11 started on a loss of power and connected to the 15AA Emergency bus. This event demonstrated that the loss of voltage feature for the 15AA bus was functional and did trip the required breakers. The Loss of Voltage — Time Delay relay timers 15AA-162-1, 15AA-162-2, 16AB-162-1 and 16AB 162-2 are designated and installed safety related equipment. This condition did not prevent the fulfillment of a safety function therefore; it was not a safety system functional failure.
G. ADDITIONAL INFORMATION
Previous Similar Events - Pursuant to 10CFR50.73(b)(5) this issue is to be an infrequent event. There has not been any occurrence of the same underlying concern in the past two years at Grand Gulf Nuclear Station.
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