Adavanced Technology for Root Cause Analysis

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Background

One of the 17 tubes (stainless steel 304L) from a feeder cabinet instrument line in a unit of a large Nuclear Generating Station (NGS) was found leaking at a location below a carbon steel support clamp. Failure analysis of this tube indicated that the leak was attributed to circumferential transgranular stress corrosion cracking (TGSCC). The cracks appeared to be OD initiated and located at the tension side of the tube (from bending) and chloride was detected in the deposits on the tube and fracture surfaces. It was suspected that chlorides originated from the heat-resistant paint that was applied to the tube.

During the pressurization of the unit in preparation for restart, a second leaking tube was found within the same 17-tube bundle along with some white deposits on another suspected tube. A third leaking tube was then discovered during inspection close to this bundle, at a 90° bend area (elbow). In addition, dripping water was found from another 4-pack tube bundle that was about 2-3 m away from the 17-tube bundle and at a higher elevation. These observations resulted in additional failure analyses. All tubes were removed from the instrument line and submitted to Kinectrics Inc. for metallurgical analysis.

Project objective

The objective of the examination was to establish the failure mode(s), and to investigate the root cause of these failures.

Scope of work

Metallographic examinations of cross-sectional components and fracture surface morphologies were carried out. Sample preparation, micrography, and elemental composition chemical analysis were performed at Kinectrics Inc. in accordance with standard ISO 9001 procedures. The following steps were carried out to effect a detailed inspection and analysis of the tubes.

Visual and Stereomicroscopic Examination indicated that there were no visible cracks in the tubes examined, with one exception.

 Fluorescent Dye Penetration (FDP) Examination. Fluorescent dye penetrant (FDP) was applied to the affected area in order to locate the cracks in the specified area. No extra cracks were found.

Scanning Electron Microscopy (SEM) Examination. In order to acquire a better resolution, the two suspected leaking tubes were examined again by SEM for possible OD and ID surface cracks.

Metallographic and SEM Cross-Sectional Examination. The OD surface examination revealed that two tubes contained cracks. The cross-sectional examination will help to identify the cracking was ID and/or OD initiated.

Energy Dispersive Spectroscopy (EDS) Analysis. The corrosive products and the deposits on both OD and ID surfaces were analyzed by EDS in the area close to the cracking, as well as on the fracture surface in order to understand the cracking mechanism.

Results

Based on the metallographic and fractographic evidence it was concluded the mechanism of cracking was the result of both ID and OD initiated transgranular stress corrosion cracking (TGSCC). It has been reported that the Degree of Sensitization (DOS) determines the fracture mode in Type 304 stainless steel. At low DOS, TGSCC occurs, while at high DOS intergranular stress corrosion cracking (IGSCC) is the predominant mode. Since the stainless steel employed for the instrument tube lines is the low carbon type and will therefore have low DOS, TGSCC is expected. A relatively high concentration of chloride from the paint/insulation together with moisture provided a potential environment for OD initiated cracking. The combination of chloride and sulphur impurities in PHT system, as well as the presence of copper and magnetite deposits in a thermally gradient area may have facilitated the ID initiated cracking.

Conclusions

The leaks in feeder cabinet instrument lines were due to transgranular stress corrosion cracking (TGSCC) and an incomplete weld. Four more tubes and one elbow tube were found to have TGSCC. The source of OD cracking environment was likely due to leaks from other tubes and leached chloride/fluorides contained in the heat resistant paint and/or insulation materials.

Benefit

It was ascertained that locations of potential high residual stress that might be caused by the installation of tubing and the clamping stresses at supports, are at risk from both OD and ID initiated TGSCC mechanisms.

For more information, contact

Allan Jarvine, 416.207.6000 x5785, allan.jarvine@kinectrics.com

Malcolm Clark 416.207.6000 x6846, malcolm.clark@kinectrics.com