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 view more