Assessment of pressurized water reactor control rod drive mechanism nozzle cracking
This report surveys the field experience related to cracking of pressurized water reactor (PWR) control rod drive mechanism nozzles (Alloy 600 material); evaluates design, fabrication, and operating conditions for the nozzles in US PWR; and evaluates the safety significance of nozzle cracking. Inspection at 78 overseas and one US PWR has revealed mainly axial cracks in 101 nozzles. The cracking is caused by primary water stress corrosion cracking, which requires the simultaneous presence of high tensile stresses, high operating temperatures, and susceptible microstructure. CRDM nozzle cracking is not a short-term safety issue. An axial crack is not likely to grow above the vessel head to a critical length because the stresses are not high enough to support the growth away from the attachment weld. Primary coolant leaking through an axial crack could cause a short circumferential crack on the outside surface. However, this crack is not likely to propagate through the nozzle wall to cause rupture. Leakage of the primary coolant from a through-wall crack could cause boric acid corrosion of the vessel head and challenge the structural integrity of the head, but it is very unlikely that the accumulated deposits of boric acid crystals resulting from such leakage could remain undetected.
- Research Organization:
- Nuclear Regulatory Commission, Washington, DC (United States). Div. of Safety Programs; Idaho National Lab. (INL), Idaho Falls, ID (United States)
- Sponsoring Organization:
- Nuclear Regulatory Commission, Washington, DC (United States)
- DOE Contract Number:
- AC07-76ID01570
- OSTI ID:
- 10196760
- Report Number(s):
- NUREG/CR-6245; EGG-2715; ON: TI95004202
- Resource Relation:
- Other Information: PBD: Oct 1994
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
22 GENERAL STUDIES OF NUCLEAR REACTORS
PWR TYPE REACTORS
CONTROL ROD DRIVES
NOZZLES
CRACKS
EVALUATION
STRESS CORROSION
STRESSES
LEAK TESTING
CRACK PROPAGATION
RUPTURES
ALLOYS
REACTOR SAFETY
ASSESSMENTS
EQUIPMENT
PRESSURIZED WATER REACTORS
210200
220900
POWER REACTORS
NONBREEDING
LIGHT-WATER MODERATED
NONBOILING WATER COOLED