APPLICATION OF ADVANCED ULTRASONIC TEST TECHNIQUES COUPLED WITH FATIGUE AND LEAKAGE MONITORING TO ASSURE INTEGRITY OF BWR FEEDWATER NOZZLES

As a result of feedwater nozzle cracking observed in Boiling Water Rea ctor (BWR) plants, several design modifications were implemented to el iminate the thermal cycling that led to crack initiation. BWR plants w ith these design changes have successfully operated for over ten years without any recurrence of cracking. To provide further assurance of t his, the U.S. Nuclear Regulatory Commission (NRC) issued NUREG-0619, w hich established periodic ultrasonic testing (UT) and liquid penetrati on testing (PT) requirements. While these inspections are useful in co nfirming structural integrity, they are time consuming and can lead to significant radiation exposure to plant personnel. In particular, the PT requirement poses problems since it is difficult to perform the in spections with the feedwater sparger in place and also leads to additi onal personnel exposure. Clearly, an inspection and monitoring program that eliminates the PT examination and still verifies the absence of surface cracking would be extremely valuable in limiting costs as well as radiation exposure. This paper describes a program involving the a pplication of advanced UT techniques coupled with fatigue and leakage monitoring to assure integrity of BWR feedwater nozzles. The inspectio n methods include: (1) scanning with optimized transducers and techniq ues from the outside vessel wall surface to inspect the nozzle inner r adius region, and (2) scanning from the nozzle forging outside-diamete r to inspect the nozzle bore region. Methods of analyzing the data usi ng 3-D graphics displays have been developed that show crack location, size, and maximum depth of penetration into the nozzle inner surface. These techniques have been developed to the point where they are now considered a reliable alternative to the liquid penetrant requirements of NUREG-0619. An important supplement to the UT program is the use o f automated fatigue, leakage and crack growth monitoring to verify the absence of cracking. This approach provides for a continuous assessme nt of the integrity of the nozzle structure by tracking the actual fat igue duty, measuring thermal sleeve bypass leakage and performing crac k growth predictions based on actual thermal duty. Collectively, the m onitoring and inspection program provides technically sound assurance of nozzle integrity and a firm basis for plant operational planning.