Hydrogen and water leakage in the main generator stator cooling water (SCW) equipment forced two separate shutdowns of a nuclear plant in a three-month period. Manufacturer weld defects on the SCW exciter end ring header are the likely cause for the hydrogen leakage.

The first nuclear plant shutdown was initiated in mid-May when an SCW leak internal to the main generator was confirmed. Events that led up to the shutdown decision started three days earlier and included an upward trend in stator coil temperatures. After two days of an elevated temperature trend, an SCW tank high-pressure alarm indicated hydrogen leakage. Per alarm response procedure, the operators vented the tank. Hydrogen leakage was determined to have increased from about 300 to 1400 ft3/day with stator water view more