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 conductivity increasing from 0.1 to 0.25 μmho/cm. Shutdown was completed the next day and efforts began to locate and repair the leak. A pinhole leak (calculated to be 0.017") was subsequently located at the corner of three welds on the lower west side of the drain box where two plates are welded to the SCW ring header on the exciter end of the generator. The defective weld was repaired and post-maintenance pressure decay testing confirmed that the repair was satisfactory and no other leaks existed. A preventive maintenance (PM) task was planned for the next main generator outage to remove paint and inspect other welds on the SCW for defects.
The second nuclear plant shutdown was initiated at the end of July with the same symptoms as above. Monitoring of SCW temperatures indicated that a pinhole leak had developed somewhere internal to the generator, thereby allowing hydrogen to leak into the SCW system. Power was maintained at levels to sustain SCW temperatures near normal as preparations for a forced outage were completed. Seven days later, the plant was shutdown. The leak was determined to be in the same stator water collector box as the leak above, but at a different weld. Except for the weld repaired during the first shutdown above, all remaining manufacturer's welds on the exciter end SCW ring header drain box were surfaced prepared, PT examined, and repaired as needed. Weld overlays were performed on all welds. A total of four welds required repair. Three of the four welds requiring repair were similar geometry to the weld repaired during the first shutdown. At reassembly of SCW equipment, various checks and tests were satisfactorily performed, including natural frequency and pressure drop testing.
- Defects in equipment, such as welds, may not be evident during initial proof testing and operation.
- When an equipment defect is found, check equipment for possible similar defects in other areas of the equipment.