A previously identified generator hydrogen gas leakage into the stator cooling water system exceeded the predetermined maximum operational allowance and the nuclear plant was shut down from 100 percent power in accordance with plant operating procedures. The leak was identified by monitoring the stator water cooling system detraining tank. Following reactor shut down and generator rotor removal, a small hole was located in the collector end winding area of a slot on the top stator bar. A small particle of carbon steel (estimated to be 2 mm/0.078 inch by 0.6 mm/0.023 inch) is believed to have caused the damage. The source of the carbon steel particle was either foreign material introduced during previous generator internal work or from a phenomenon called "back-of-core burning", where electrical arcing caused the release of internal metal particles. See Attachment 1 for additional details.
The magnetized carbon steel particle vibrated with the changing magnetic field of the generator (60 Hz). This motion, along with the magnetic attraction of the stator bar, acted as a driver to bore through bar insulating tape and eventually into and through the surface of the hollow copper strand. The penetration of the generator strand caused the steep increase in monitored hydrogen in-leakage that necessitated the nuclear plant shutdown. This event did not affect nuclear safety.
Shutdown/outage activities to repair the hydrogen leakage in the stator cooling water system included identification and repair (plugging) of the damaged generator strand as well as thoroughly cleaning and magnetic sweeping of generator internals.
Incorporate external operating experience lessons learned into site program controls. Other nuclear plants had similar strand failures and back-of-core issues that were not evaluated for impact on procedures or system/component health plans.
Be more sensitive to precursor indications of declining system/component health; in this case the main generator. Insensitivity resulted in material condition deficiencies and elevated risks to generation that are undesirable given the economic importance of this high-value asset.