MRP-146 Thermal Fatigue Examinations

History

In the late 1980’s, there was thermal fatigue cracking and leakage in several PWR plants. This led to the issuance of NRC Bulletin 88-08. The cracking was attributed to thermal cycling mechanisms and was found in normally stagnant lines attached to RCS piping. In the late 1990’s to early 2000’s, there were several other instances of thermal fatigue cracking in other components such as drain lines and a high pressure injection/makeup line. In January 2001, MRP-24 was made available as an interim guideline. After the issuance of MRP-24, additional testing and evaluations were done to better characterize thermal fatigue damage. In 2009, supplemental guidance to MRP-146 was issued providing revised evaluation guidance for screened in branch piping.

Exams

Structural Integrity (SI) recently completed three exams looking for thermal fatigue and craze cracking at a client’s site in April 2015. The exams were successfully performed on reactor coolant pump drain lines with an elbow to pipe configuration. The exams were completed using the guidelines of MRP 146 and supplemental guidance report MRP-146S. We used the OmniScan MX ultrasonic instrument with the OMNI-MPA32 (128 PR) phased array module running a 20°-70° shear wave azimuthal scan with an angle resolution of 1°. This allowed for a thorough inspection of the exam area. Figure 1 shows an example of coverage for an exam. Figure 2 shows craze cracking in a mockup. Figure 3 shows an example of a thermal fatigue crack in a mockup.

Our involvement in the development and implementation of the governing documents, along with the ability to develop customized examination solutions, positions us to provide our clients with fully integrated, best in value solutions.

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