05000272/LER-2006-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 03-08-2006 |
|---|---|
| Report date: | 05-03-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2722006001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse - Pressurized Water Reactor Electro-Hydraulic Control System (TG){EHC} Steam Generator Feed Pump (BF/P) {SGFP} Control Rod System (AN-) ndustry Identification System (EllS) codes and component function identifier codes appear in the text s {SS/CCC}.
DENTIFICATION OF OCCURRENCE
Event Date: March 8, 2006 Discovery Date: March 8, 2006
CONDITIONS PRIOR TO OCCURRENCE
Salem Unit 1 was in Operational Mode 1 at 100% reactor power.
No structures, systems or components were inoperable at the time of the discovery that contributed to the event.
DESCRIPTION OF OCCURRENCE
n March 8, 2006 at 1109, with Salem Unit 1 at 100% power, a "first-our Overhead Alarm (F38) Turbine rip & P-9 (Reactor above 49% power) was received in the main control room with an immediate reactor p. The reactor trip actions and plant recovery were performed without complications; however, two quipment issues were noted during the trip. The two issues were: (1) one control rod position ndication for shutdown rod 1SC1 indicated that the rod was at approximately 17 steps, and (2),reports rom field operators (non-licensed personnel) indicated a leak on the condensate line at the suction of he 11 Steam Generator Feedwater Pump. Control room operators (Licensed personnel) initiated a ain Steam Line Isolation to isolate steam flow to the secondary plant and controlled Reactor Coolant ystem average temperature using the atmospheric dump valves. The leak on the condensate line was due to the momentary secondary system pressure perturbation that caused a flange gasket to fail. The failed gasket was replaced.
Later assessments determined that control rod 1SC1 was fully inserted. The erroneous 1SC1 position indication was the result of residual magnetic flux in the individual rod position indicator coil that induced a voltage on the secondary side of the coil. The secondary coil voltage is used to provide the relative rod position and as such any error introduced will directly affect the rod position indication. The rod position indication would have eventually decreased to indicate full insertion through natural decay of the Residual magnetic flux. De-energizing the individual rod position indication system let the residual flux decay almost immediately. The individual position indicator was calibrated during the outage.
DESCRIPTION OF OCCURRENCE (cont'd) Analysis of computer data indicated that the turbine over speed circuit initiated an overspeed signal at 103% and tripped the main turbine as designed. Turbine speed was a constant 1800-rpm as controlled by the electric grid with the generator synchronized to it.
The unit was returned to service the following day, March 9, 2006.
The automatic initiation of the reactor trip and the manual initiation of the Main Steam Line Isolation are reportable in accordance with 10CFR50,73(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).
PREVIOUS OCCURRENCES
A review of reportable events for Salem Generating Station in the last three years identified five licensee ‘rent reports associated with manual or automatic reactor trips.
311/2004-006 "Salem Unit 2 Reactor Trip Due to a Malfunction of a Main Feedwater Regulating Valve (21BF19)," dated September 13, 2004.
311/2004-007 "Salem Unit 2 Reactor Trip Due to a Malfunction of a Main Feedwater Regulating Valve (23BF19)," dated September 13, 2004.
311/2003-001 Salem Unit 2 "Manual Reactor Trip Due to Degradation of Condenser Heat Removal," dated May 22, 2003.
11/2003-003 Salem Unit 2 "Manual Reactor Trip Due to Dropped Control Rod," dated January 20, 2004.
72/2003-002 Salem Unit 1 "Reactor Trip due to Turbine Trip Caused by a 500KV Switchyard Breaker Trip," ated September 24, 2003.
(though these events involved a reactor trip, the root causes were different than the one described in this ER; and therefore they could not have been prevented this occurrence.
KID/. noll/1.1P.AA/1�1 NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (1-2001) Salem Generating Station Unit 1 05000272
CAUSE OF OCCURRENCE
The investigation team explored several possible failure methods which could have resulted in the turbine overspeed trip and determined that the most probable cause was Radio Frequency Interference (RFI) or Electro Magnetic Interference (EMI) by an unknown source; Efforts to pinpoint the source of the interference signals are continuing. Due to the length of cable associated with the turbine speed circuit and the transient nature of the interference, the investigation team has not identified the specific device and location that initiated the EMI/RFI.
his event identified the potential vulnerability of the digital EHC system to RFI/EM1. Tests conducted n the simulator EHC system demonstrated that interference could be induced into the EHC system.
he three turbine overspeed conductors (channels) are routed together in a single cable.
SAFETY CONSEQUENCES AND IMPLICATIONS
There was no actual safety consequences associated with this event.
As stated earlier, later assessments following the reactor trip determined that the control rod indication was erroneous and that the leak on the condensate line was due to the momentary secondary system pressure perturbation caused by the trip. The licensing basis of the Salem plant includes the assumption that the highest worth control rod is stuck completely out of the core; therefore, the current licensing basis accident analyses bound this event.
A review of this event determined that a Safety System Functional Failure (SSFF) as defined in NEI 99-02, Regulatory Assessment Performance Indicator Guidelines, did not occur.
ORRECTIVE ACTIONS
se of electric tools, radios, cellular phones, portable radios, arc flash welders and other equipment that uld result in EMI or RFI in relay rooms has been prohibited.
aming signs have been posted in the effected areas. Adherence to the posting is being emphasized to revent EMI and RFI from interfering with or causing inadvertent actuation or response of sensitive tnstruments in the plant.
Longer term actions such as additional cable shielding or cable separation are being evaluated as well as the extent of the entire digital EHC circuit susceptibility to EMI/RFI.
COMMITMENTS
No commitments are made in this LER.