The cracking of CANOPY Seal Weld (CSW) of the Control Rod Drive Mechanism (CRDM) ever occurred and led to Boric acid solution leakage in nuclear power plant worldwide. The failure analysis on above failure CSW shows that stress corrosion crack (SCC) is main failure mode of CSW, and the crack usually originate from the weld root and grows along weld or HAZ under PWR service environment.
The CSW structure parameter presented here are as following, the wall thickness is 2–3 mm, radius of curvature of cross-section is 6–12 mm, and the gyration radius is 50–100mm. In the latest year, CSW is required to implement surface examination (i.e. Visual Test or Penetration Test) during the plant outage. The flaw is that the CSW won’t be repaired till its failure is detected; therefore effective preventive maintenance can’t be performed. The specific inspection technology is developed for NDT volume testing method, aiming to detect the root crack occurred in weld and its adjacent base metal.
This paper introduces the CSW structure, in-service inspection requirements, and technical solutions. The NDT technology corresponding UT, ET and CCTV was finally applied as applicable technical solution with automatic scanning devices to detect CSW crack. Ultrasonic inspection technology uses high-frequency water immersion focusing probes with frequency higher than 10MHz which is investigated to detect the defect in the weld root area. Meanwhile, Eddy current inspection technology is applied to detect the surface crack or subsurface crack with orthogonal difference array probes, which helps to distinguish false signals detected by UT in outer surface. Ultrasonic inspection technical solutions based on high-frequency water immersion focusing methods and the key technical parameters are discussed emphatically in this paper. By testing on blocks with characteristic artificial reflectors, characteristic rule of ultrasonic signals in different areas and sizing methods for crack is investigated respectively. It show that UT has higher detecting sensitivity for crack of where the height reaches 0.5mm and the length reaches 4mm, and sizing is accuracy and reliable for crack when the height over than 1 mm and the length over than 4mm.
CSW is not incorporated in-service scope referring to ASME code or Chinese in-service code, and the acceptance criterion of weld flaw is also not found. So, the integrity shall be evaluated as far as CSW with flaw is concerned. Thus, the stress analysis and temperature field are carried out on the CSW mentioned above, and the structural integrity assessment is simultaneously performed based on ASME code. Finally, the acceptance criteria are recommended for both volumetric and planar flaw.