Abstract
Weld cracks in safety class systems are a serious concern, because these systems are part of the primary barrier providing containment of radioactive coolant. Loss of weld integrity yields leaks, or, under catastrophic failure, can be the basis for a severe loss of coolant accident. A circumferential indication was found by ultrasonic examination (UT) in the N4A-2 inlet feedwater nozzle to safe-end weld during the second refueling outage of River Bend Station Unit 1 in March 1989. The indication, approximately 15cm (6in) long with a reported maximum depth of 0.5cm (0.1in), was located in the Alloy 182 weld butter on the safe-end side of the weld. (The safe-end base metal was ASME SA 508 Class 1 carbon steel.) The reported characteristics of the UT indication were indicative of intergranular stress corrosion cracking. This indication was reexamined during the second and third fuel cycles in March 1990 and September 1991, respectively, and during the third refuel outage in November 1990. Crack growth was reported during each examination. The safe-end was replaced during the fourth refueling outage in the summer of 1992. The US Nuclear Regulatory Commission (NRC) subsequently contracted with Brookhaven National laboratory (BNL) to conduct a confirmatory investigation to establishmore » the failure mode and determine the root causes of cracking in the safe-end weld.« less
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