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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.

Reactor Pressure Vessel (hereinafter RPV) is the core component of PWR nuclear power plants. The nuclear power plants operation experience shows that the dangerous defects could produce in the material of Bottom Mounted Instrument and J type weld (hereinafter BMI). In early nuclear power plants, the BMI is made of corrosion-sensitive materials (such as Alloy 600). The stress corrosion crack is easy to grow up and destroy the integrity of RPV in operation. Therefore, inspection for BMI during the service stage is necessary to ensure the integrity of the primary circuit pressure boundary. The BMI is general made of Alloy 600 or Alloy 690 and penetrate the reactor pressure vessel lower head vertically to form J type weld on the inner wall of the RPV lower head. The inner diameter is generally 15 to 16 mm and the wall thickness is about 15 mm. In order to improve the welding performance, the dissimilar metal J-type weld groove is usually overlaid with nickel-based pre-heap edge. The component structural and material characteristics have brought great difficulties to the test technology research as well as the location. In this paper, the main failure modes of BMI are introduced and the technical requirements of in-service inspections are presented. The technical scheme and inspection system are designed according to the above requirements and weld structural characteristics. Both UT and ET methods are used for the inspection. UT is based on the time of flight diffraction (hereinafter TOFD) technique. According to the structural and material characteristics of the BMI, many researches have been done on block with simulated defect, and characteristic rule of defect indication signals in different areas are summarized. Then, the optimal design parameters of the TOFD probe are gained and the disadvantages of ultrasonic inspection are overcome. The eddy current inspection system adopts the inspection technology with the Bobbin probe and the rotary point probe combined together. The use of its surface and near the surface of the detection ability has become a powerful complement to ultrasonic testing. From the test result, TOFD probe has high defect height measurement accuracy. Measurement error isn’t more than 1mm and mostly is positive deviation. The surface defects with a height of not less than 2 mm can be measured. Considering the complex geometry of BMI, the multi-mode multi-axis scanning program and remote automatic docking device are used in the system. The multi-mode multi-axis scanning program can be achieved axial and circumferential rotation of the probe with the axial positioning accuracy within 2mm and circumferential positioning accuracy within 3°. The remote automatic docking device can be used to connect the bottom of the mechanical device and the top of the BMI in fast and seamless security to avoid the mechanical device bumping of the component. The probe connecting shaft is made of flexible material, which avoids the violent bumping of the tested parts and greatly enhances the safety of the equipment operation. The system simulates the actual scanning environment by establishing three-dimensional model and testing it on the simulation body.