05000341/LER-2009-002
| Telephone Number (Include Area Code) | |
| Event date: | 09-30-2009 |
|---|---|
| Report date: | 11-17-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 45394 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3412009002R00 - NRC Website | |
Initial Plant Conditions:
Mode � 1 Reactor Power� 100% percent Description of the Event .
At 11:09 EDT on September 30, 2009, the reactor mode switch [HS] was taken to shutdown and the main turbine generator [TA] was manually tripped in response to hydrogen gas leakage into the stator water cooling system [TJ] from the main generator [TB]. The Operations Department had been performing frequent additions of water to the stator water cooling head tank which is indicative of increased in-leakage of hydrogen into the system.
This had been identified as an Operational Decision Making Issue and an assessment of the condition had been performed. The rate of water addition and hydrogen in-leakage was being closely monitored, and trigger points and actions were developed to address this condition. Preparations were also being made for an orderly shutdown to address the problem. As part of this process, limits were set for a leakage rate past which operation should not be continued. Operating Experience indicated that these types of leaks can progress very slowly or more rapidly.
When a rapid increase in the hydrogen leakage rate was observed in excess of the predetermined trigger point, the reactor was promptly shutdown.
The scram was uncomplicated. The reactor protection system (RPS) [JD] performed as expected, and all control rods fully inserted into the core. Reactor water level reached a low of approximately 122 inches above top of active fuel and recovered to normal level automatically without operator intervention. Reactor water level was maintained above Level 2, and as expected, none of the primary containment isolations or safety injection systems initiations associated with Level 2 occurred. The main steam isolation valves (MSIVs) remained open, and the reactor feedwater [SJ] system automatically restored reactor water level and maintained it in the normal band of 173 to 214 inches. The main steam bypass valves [JI] opened and automatically controlled reactor pressure. There was no transient increase in reactor dome pressure and no safety relief valves (SRVs) actuated.
All primary containment isolations [JM] and actuations for reactor water Level 3 occurred as expected. At the time of the manual scram all Emergency Core Cooling Systems (ECCS) and Emergency Diesel Generators (EDGs) [DG] were operable, and safety related plant equipment responded as expected to the scram. There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
This report is being made in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in an unplanned manual actuation of the reactor protection system (RPS) when the reactor was critical.
Significant Safety Consequences and Implications This event posed no significant safety implications because the reactor protection and safety related systems functioned as designed following the manual reactor trip. Important safety-related and non-safety related equipment performed as discussed in the description of the event, and plant response was as expected. There was no increase in reactor pressure, and the reactor core was adequately covered and cooled throughout the event.
This event is being reported under 10 CFR 50.73(a)(2)(iv)(A), as an event or condition that resulted in manual actuation of the reactor protection system and a reactor scram. A 4-hour non-emergency notification was made to the NRC Operations Center at 14:11 EDT on September 30, 2009 (EN 45394) in accordance with 10 CFR 50.72(b)(2)(iv)(B) for an actuation of the reactor protection system.
Cause of the Event
The manual scram was inserted due to hydrogen leakage from the generator hydrogen cooling system into the stator water cooling system that exceeded a predetermined maximum allowable operational leakage rate. The cause of the leak was attributed to a particle of carbon steel in the generator end winding area. The particle was magnetized and oscillated with the four- pole rotor field. The oscillation, in conjunction with a magnetic attraction towards the stator bar, damaged end winding insulation laminates over time, eventually reaching the copper stator water bar. The oscillation of the particle continued until a hole was worn into the generator stator water bar, allowing hydrogen gas to leak into the generator stator water.
Corrective Actions
The leak in the generator stator water cooling system was located and repaired in accordance with the generator manufacturer's recommendations. Other preventive and corrective maintenance tasks, and a thorough cleaning and magnetic sweeping of the generator internals were also performed during the outage. This event is documented and evaluated in the Fermi 2 corrective action program. Other actions are being considered to address this event. These actions will be tracked and implemented by the corrective action program.
Additional Information
A. Failed Components: Main Turbine Generator Component: Stator Water Cooling System Function: Generator Stator Cooling Manufacturer: English Electric (Alstom) Model Number: N/A Failure Cause: Foreign Material B. Previous LERs on Similar Problems: None