IE Information No. 86-99: Degradation of steel containments

The Oyster Creek Nuclear Generating Station first discovered water in the gap between the boiling-water-reactor drywell and the concrete shield in 1980 and began investigation of the cause in 1983. It appeared that the collection of water varied from a few drops to 2 gallons per minute, depending on whether the unit was in operation or an outage for refueling. During the spring and summer of 1986, the licensee planned work to identify and eliminate this water problem. The bellows at the drywell to cavity seal was repaired and a gasket was replaced, thus stopping the leakage. Since the bellows is located at the top of the drywell and region above the bellows is flooded during refueling, it would explain why leakage was high during refueling and low during operation. To determine if the water in the gap had caused damaged to the steel containment, the licensee measured the wall thickness, using an ultrasonic testing (UT) technique at two elevations. The 51-ft level near the drywell seal was sound, but there appeared to be loss of metal on the gap side at the 11-ft 3-in. level immediately above the concrete floor. A total of 143 measurements were made at this level and 60 indicated a reduction in thickness of more than 1/4 in.from the drawing thickness of 1.154 in. These readings were found throughout seven of the ten downcomer bays. The licensee plans to cut the steel containment and remove about 12 samples to confirm and evaluate the corrosion damage