Improved Criteria for the Repair of Fabrication Flaws
Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code for nuclear power plant components requires radiographic examinations (RT) of welds and requires repairs for RT indications that exceed code acceptable sizes. This paper describes research that has generated data on welding flaws, which indicated that the largest flaws occur in repaired welds. The fabrication flaws were detected in material removed from cancelled nuclear power plants using high sensitivity Nondestructive Examination (NDE) and validated by complementary NDE and destructive testing. Evidence suggests that repairs are often for small and benign RT indications at locations buried within the vessel or pipe wall. Probabilistic fracture mechanics calculations are described in this paper to predict the increases in vessel and piping failure probabilities caused by the repair-induced flaws. Calculations address failures of embrittled vessel welds for pressurized thermal shock (PTS) transients and piping failures caused by fatigue crack growth. For vessels the small flaws, which are relatively common, can cause brittle fracture, such that the rarely encountered repair flaws of large sizes gave only modestly increased failure probabilities. Calculations for piping show that only relatively large fabrication flaws can cause failures because of the ductile nature of the piping material. The large repair flaws therefore significantly increased the failure probabilities. The paper recommends the use of more discriminating ultrasonic examinations in place of RT examinations along with repair criteria based on a fitness-for-purpose approach that minimize the number of unjustified repairs.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 993676
- Report Number(s):
- PNNL-SA-41947; 401001060; TRN: US1008106
- Resource Relation:
- Conference: Transactions of the 17th International Conference on Structural Mechanics in Reactor Technology (SMiRT 17), Prague, Czech Republic, August 17 - 22, 2003, Paper No. O01-3
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BOILERS
CRACK PROPAGATION
DEFECTS
DESTRUCTIVE TESTING
ENGINEERS
FABRICATION
FRACTURE MECHANICS
NUCLEAR POWER PLANTS
PRESSURE VESSELS
REACTOR TECHNOLOGY
REPAIR
SENSITIVITY
THERMAL SHOCK
TRANSIENTS
ULTRASONIC WAVES
WELDING
Nondestructive testing
Nondestructive examination
Nondestructive evaluation
Fabrication flaws
Preservice examination
Construction codes and standards
Fracture mechanics