05000416/LER-2008-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | 01-12-2008 |
|---|---|
| Report date: | 03-10-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| LER closed by | |
| IR 05000416/2008004 (17 October 2008) | |
| 4162008001R00 - NRC Website | |
On January 12, 2008, at 1626 hours0.0188 days <br />0.452 hours <br />0.00269 weeks <br />6.18693e-4 months <br /> Grand Gulf Nuclear Station initiated a manual reactor scram from 99 percent power due loss of cooling to the Main Transformers [EL XFMR]. After the scram reactor water level decreased to Level 2 (-41.6 inches) and Reactor Core Isolation Cooling (RCIC) initiated. The following occurrences were considered reportable:
> Manual Actuation of Reactor Protection System (RPS) [JC] Manual Scram (Reference:
10CFR50.73(a)(2)(iv) (A) & (B)(1)) > Reactor Core Isolation Cooling (RCIC) [BN] automatic initiation and injection on Reactor Vessel Water Level — Low Low, Level 2 into the reactor. (Reference: 10CFR50.73(a)(2)(iv) (A) & (B)(5)) Notification was made to the NRC's Emergency Notification System (ENS) reporting this condition pursuant to 10CFR50.72(b)(2)(iv)(B) and 10CFR50.72(b)(3)(iv)(A) and this event is reported under 50.73(a)(2)(iv)(A) — "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section....
B. INITIAL CONDITIONS
At the time of the event, the reactor was in OPERATIONAL MODE 1 with reactor power at approximately 99 percent. 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 January 12, 2008, at 1558, Main Transformer trouble alarms were received for all three in-service transformers. Investigation revealed a malfunction had occurred on the "B" phase Auxiliary Power Supply (panel 1SMT1) to the Main Transformer Auxiliaries. Operations and Electrical personnel were dispatched to the transformers. The "A" Transformer was found with no cooler groups running. The "B" and "C" Transformers were found with only partial cooler groups running. Subsequently, all cooling groups stopped for the "B" and "C" Transformers. Further investigation in the field indicated that power to the transformer cooling systems could not be restored.
Because there was a loss of cooling to the Main Transformers, a manual SCRAM was initiated at 1626 followed by a Main Turbine trip at 1627. After the scram reactor level decreased due to shrinkage to Level 2 (-41.6 inches) and Reactor Core Isolation Cooling initiated. Operators manually tripped Reactor Feed Pump Turbine (RFPT) "B". Reactor Pressure Vessel (RPV) water level increased to a maximum of +60 inches resulting in a RCIC shutdown on Level 8 (+53.5 inches) and a RFPT "A" trip on Level 9 (+56 inches).
Operations returned RFPT "A" to startup level control and placed RCIC in the normal standby lineup. The scram was reset at 1636. The normal heat sink (main condenser) remained available and no Main Steam Safety Relief Valves (MSRV) actuations occurred during the event. All control rods fully inserted and all safety systems functioned as designed and responded properly. This was NOT a SCRAM with complications.
The cause of this event was the loss of cooling to the Main Transformers caused by loss of Auxiliary power to the Main Transformer cooling system. Inspection of the complete circuit for Main Transformer Auxiliary Power showed that the "B" phase power cables were found to be burned in two in panel 1SMT1 (transfer switch panel). The cause of the "B" phase power cable failure is attributed to failure of an electrical compression connection on a lug connection; the connection was manufactured by BURNDY®, Model No.
D1281. The root cause of the power cable failure could not be conclusively determined by the initial investigation. This particular cable and connection has been inservice since plant start-up. One potential root cause was determined to be use of a compression connection for high current applications versus the preferred crimp type connection. Another potential root cause was there was an inadequate preventive maintenance strategy to provide early detection. Figure 1 provides a picture of the failed connection.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions — Completed inspection of the 1SMT1 Transfer Switch Panel. All damaged cabling has been replaced. Inspection results indicate that no damage occurred to the affected transformers and all equipment responses were as designed and expected.
Long Term Corrective Actions - Condition Report GGN-2008-00174 was written and will address any additional actions.
F. SAFETY ASSESSMENT
Immediate actions performed by the Operations staff were adequate and appropriate in placing and maintaining the reactor in safe shutdown condition. No margin of safety was affected or encroached. All safety systems responded as designed in this event.
Reactor Core Isolation Cooling initiated at Level 2 (-41.6 inches). No Emergency Core Cooling System (ECCS) initiations occurred. The Group 2 and 3 Primary Containment Isolation initiated at Level 3 (+11.4 inches), however no valves changed position because these valves were already in the normal isolated position. The Group 2 and 3 isolations were for valves in the same system.
This condition did not prevent the fulfillment of a safety function therefore there were no safety system functional failures. Based on the discussion provided, the health and safety of the public was not compromised by this event.
G. ADDITIONAL INFORMATION
Previous Similar Events - Pursuant to 10CFR50.73(b)(5) this issue is considered an infrequent event. There has not been any occurrence of the same underlying concern in the past two years at Grand Gulf Nuclear Station.