Investigation of Abnormal TOFD Signals from Coarse Grain in Alloy 690 Tube Materials

Since PWSCC(Primary Water Stress Corrosion Crack) was in the Reactor Vessel Head Penetration(RVHP) weld of Oconee and Davis-Besse Nuclear Power Plant (NPP) in 2001 and 2002, Korean regulatory body, KINS(Korea Institute of Nuclear Safety) recommends Korean Utility inspects the Korean RVHP J-welds and tube. These J-welds and tube of RVHP are periodically inspected by related Code requirements according to ASME Code Case N-729-1. Also, these J-welds and tube of RVHP are inspected during fabrication process of RVHP. During fabrication process, several abnormal signals from tube of RVHP were found. These abnormal signals could be confused with defect signal when analysts evaluate TOFD signal. Therefore, this article try to reveal what this abnormal signals is through simulation modeling and mock-up of the alloy 690 tube material. In the computer simulation, we have simulated ultrasonic scattering signals with variation of grain size using FEM simulation. To find equivalent flaw size to grain signals, we have performed simulation of ultrasonic signals with various sizes of SDH. Thus, from the simulation results, ultrasonic signals from the coarse grain could be classified as flaw signals in the TOFD inspection. In the experiment, we have acquired TOFD signal of SB-166 N06690 tube specimen to investigate abnormal signal from a coarse grain. The results of tube test, one abnormal signal was detected in specimen and compared to typically TOFD signal of PWSCC then, the abnormal signal have some different patterns. Thus, we could be confirmed that it clearly discriminate between the coarse grain signal and the PWSCC signal. By comparing computer simulation and experimental result. It is needed to investigate the effects by coarse grain on the signal characteristic and the causes of the effects in the TOFD signal by more studies.

References
  1. ASME B&PV Code Committee, Code Case N-729-1, Alternative Examination Requirements for PWR Reactor Vessel Upper Heads with Nozzles Having Pressure-Retaining Partial-Penetration Welds, 2006.
  2. Fei Zeng, Sean R. Agnew, Babak Raeisinia, Ganapati R. Myneni, “Ultrasonic Attenuation Due to Grain Boundary Scattering in Pure Niobium,” J. Nondestruct Eval, Vol. 29, 2010, p. 93–103.
  3. M Nygards, P Gudmundson, “Three-dimensional periodic Voronoi grain models and micromechanical FEsimulations of a two-phase steel, Computational Materials Science, 24 (2002) 513-519.
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