Simulation of eddy current testing of steam generator tubes with wear and secondary side magnetite deposits

Abstract

The steam generator (SG) tubes of pressurised water reactors are prone to degradation and are periodically inspected using eddy current testing (ECT) techniques. SGs provide a safety barrier between the radioactive primary side and the non-radioactive secondary side. Historically, a major degradation issue was stress corrosion cracking of tubes made of alloy 600. In modern SGs, better resistance to stress corrosion cracking is observed with the replacement of alloy 600 by alloy 690. However, this alloy does not prevent tubes from wear at support locations or wear resulting from fretting with loose parts. Long-term operation of SGs also leads to non-volatile species accumulation in the secondary side of the SGs, mainly magnetite. Magnetite deposits may build up as a dense and solid material on top of the tubesheet (hard sludge) and also in the flow slots of tube support plates, therefore modifying the general flow pattern and velocity of the secondary fluid. This has detrimental effects on the safe operation of SGs. The clogging-up ratio of the flow slots therefore needs to be assessed. The development of an ECT simulation model to address tube wear and magnetite deposits in SGs is presented. This model is based on the CIVA NDT platform. A bobbin coil signal is simulated and its performance for tube wear and magnetite deposit evaluation is discussed.