Resolution of Generic Safety Issues: Item A-10: BWR Feedwater Nozzle Cracking (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–35 )
DESCRIPTION
Inspections of operating BWRs conducted up to April 1978 revealed cracks in the feedwater nozzles of 20 reactor vessels. Most of these BWRs contained 4 nozzles with diameters ranging from 10 in. to 12 in. Although most cracks ranged from 1/2 in. to 3/4 in. in depth (including cladding), one crack penetrated the cladding into the base metal for a total depth of approximately 1.5 in. It was determined that cracking was due to high-cycle fatigue caused by fluctuations in water temperature within the vessel in the nozzle region. These fluctuations occurred during periods of low feedwater temperature when flow is unsteady and intermittent. Once initiated, the cracks enlarge from high pressure and thermal cycling associated with startups and shutdowns. This item was originally identified in NUREG-03712 and was later declared a USI in NUREG-0510.186
CONCLUSION
This issue was RESOLVED with the issuance of NUREG-0619 742 and MPA B-25 was established by DL/NRR for implementation purposes.
REFERENCES
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