Development of On-Line Monitoring System for Cracking in Nuclear Piping based on Equipotential Switching Array Probe Direct Current Potential Drop

Abstract

Unacceptable cracks exceeding the limits of ASME Boiler Pressure Vessel Code Sec XI at dissimilar metal weld of piping during non-destructive testing using ultrasonic testing (UT), under the conventional periodic inspection approach. As many nuclear power plants (NPP) renter into long-term operation beyond their design life more sensitive on-line cracking monitoring approach became necessary. In this thesis, a direct current potential drop (DCPD) method utilizing array probes with measurement ends maintaining an equalized potential designated as equi-potential switching array probe direct current potential drop (ESAP-DCPD) technique has been developed for on-line crcak monitoring. The developed ESAP-DCPD method has been validated by showing consistency among experimental measurements, analytical solution and numerical predictions using finite element analysis (FEA) with crack growth in weld metals. The FEA-based numerical prediction was verified by a good agreement with an analytic solution for center-cracked tension (CCT) specimen in accordance with ASTM Method E647. The optimal configuration of array probes was obtained from sensitivity study of verified FEA prediction method.

In order to examine the viability of ESAP-DCPD for an in-service inspection (ISI), artificial inner surface cracks were introduced in a full-scale dissimilar metal weld (DMW) mockup pipe with circumferential weldment between low alloy steel and stainless steel. A round-robin measurement has been made by participation of three independent laboratories. It has been found that the developed ESAP-DCPD can detect circumferential cracks with a depth of 40% or greater of wall thickness in stainless steel. In contrast, the ESAP-DCPD system is shown to have very good viability and sensitivity under on-line monitoring condition.

In order to examine its sensitivity under on-line monitoring of crack growth at the DMW, fatigue tests were performed by using modified CCT specimens manufactured from a full-scale mock-up welded pipe. The depth of initial crack was 20% of pipe wall thickness (a/t) and crack growth under fatigue testing was measured by both ESAP-DCPD and direct optical inspection. ESAP-DCPD detected the crack growth from the beginning of the test with high signal to noise ratio, demonstrating its suitability for on-line monitoring.

Using the experimental results, the probability of detection (POD) curve was obtained for use in In-Service Inspection (ISI) mode and on-line monitoring mode of ESAP-DCPD through the signal sensitivity and noise analysis. Results were compared with POD of conventional ISI using UT. A threshold crack size of detection was defined by measured ESAP-DCPD value exceeded four times standard deviation of noise distribution measured at the initial state prior to the crack growth. The POD of ESAP-DCPD under on-line monitoring mode reaches almost 100 percent when size of crack is only about 10% of wall in contrast to the conventional ISI using UT where excessive cracks including even through-wall cracks are often undetected in DMW region. However ESAP-DCPD used under ISI mode is shown to suffer from POD decrease due to increased noise, like ISI using UT. ESAP-DCPD is, therefore, found to be one of the best method for on-line monitoring of piping weld crack growth during long-term operation of nuclear power plants.