Unit 3 Steam Generators
Unit 3 operated for approximately 11 months following replacement of the steam generators when, the unit was shut down due to a leak in one of its steam generators (3E088). On January 31, 2012, control room operators received alarms indicating a reactor coolant leak when the main condenser air ejector radiation monitors reached their alarm setpoint. The radiation monitors continuously sample from a vent stack in order to rapidly identify primary-to-secondary leaks. The leak was unexpected, and the licensee responded in accordance with its procedures by performing a rapid shutdown, since the leak, although small, had increased enough in a short period of time to warrant the precautionary shutdown. The estimated leak rate was 75 gallons per day, which is less than .06 gallons per minute. The Plant Operating License allows full- power operation with a steady leak rate of less than 150 gallons per day.
The radiation monitor set point is low enough to ensure early detection such that a direct release of radioactivity to the atmosphere, which is allowed by the plant operating license, up to limits in the regulations is minimized. The licensee reviewed the amount released and estimated that it was much less than is permitted by the plant operating license. NRC inspectors independently reviewed the release data and verified the licensee's findings. The release posed no threat to the public or the workers onsite.
Unit 3 control room operators performed a controlled shutdown and reached cold shutdown conditions on February 2, 2012. The operators then prepared the steam generators for tube inspections, which started on February 12, 2012. The first inspection confirmed the location of the leak in steam generator 3E088. One tube had a small leak, and no other tubes were leaking.
An in-situ pressure test is performed one tube at a time with water, by slowly pressurizing the primary side of the tube (the side that normally sees reactor coolant pressure). The first test pressure point is approximately 3,200 pounds per square inch gauge (psig), which is the differential pressure the tube would see during a main steamline break. The pressure is determined using the expected reactor coolant system (RCS) pressure during a steam line break, and then adding additional pressure due to test conditions (cooler temperature) and gauge uncertainties. This pressure is held for 2 minutes, and then the next test pressure of approximately 5,200 psig is attained and held for 2 minutes. The last pressure is approximately three times the normal tube differential pressure, again adding corrections for temperature and gauge uncertainties.
The licensee determined that there was a high probability of several Unit 3 steam generator tubes failing the in-situ pressure testing. In-situ testing was conducted from March 13 to March 20, 2012. The licensee accurately ranked each tube according to the probability of failure, and the top eight tubes with the highest probability of failure did fail their pressure tests. All other tubes passed, and all of the failures occurred on the 3E088 steam generator. The leaking tube was one of the tubes that failed its pressure test.
The licensee completed extensive plugging and selective staking of 807 tubes in Unit 3 (420 in 3E088 and 387 in 3E089).