Jung Ho Han

Sliding Wear Behavior of Steam Generator Tube Materials in Nuclear Power Plants

Wear damage of steam generator tubes of nuclear power plants can cause leakage of radioactive substances. So the evaluation of tubes’ integrity is very important from the viewpoint of nuclear ecocide. In the present study, sliding wear behaviors of Inconel 600 and 690 steam generator tube materials mated with 409 stainless steel commonly used as the support plate were investigated at room temperature in an air environment. For more precise prediction of wear behaviors of steam generator tubes, Archard equation was modified, and the modified wear coefficients were estimated as a function of sliding distance. When using the modified Archard equation, the reliabilities for prediction of wear behavior of Inconel 600 and 690 mated with 409 stainless steel increased from 71.8% to 83.8% and from 60.2% to 85.2%, respectively.

An Automated System for Detection of Through-Wall Cracks in Racks in Steam Generator Tubes

Through-wall axial cracks occurred by primary water stress corrosion are one of the serious defects in steam generator (SG) tubes (made of alloy 600) in pressurized water reactors. Therefore, it is necessary to detect and size them by eddy current testing (ECT) conducted during in-service inspection of SG tubes. To address this issue, it has been recently proposed an effective method, namely „M-shape profile“ approach, which relies on the difference in the amplitude between the pancake and plus point coils in a MRPC probe. Even though the M-shape curve approach is straightforward in principle, it requires time-consuming data processing if performed by human operators. In order to get rid of this tedious task, an automated system is developed in the present work. This paper addresses the principle of the M-shape approach together with the automated system and its performances for the detection of natural axial cracks in SG tubes. The results observed in the present work demonstrate the high potential of the developed system as a very promising tool for detecting through-wall cracks in many practical field applications.

The Geometry of Steam Generator Tube and Its Relevance to the Occurrence of Stress Corrosion Cracking in Operating Nuclear Power Plants

Occurrences of a stress corrosion cracking in the steam generator tubes of operating nuclear power plants are closely related to the residual stress existing in the local region of a geometric change, that is, expansion transition, u-bend, ding, dent, bulge, etc. Therefore, information on the location, type and quantitative size of a geometric anomaly existing in a tube is a prerequisite to the activity of a non destructive inspection for an alert detection of an earlier crack and the prediction of a further crack evolution [1].Copyright

Sliding Wear Behaviors of Steam Generator Tubes in Various Environments

Wear damage of steam generator tubes for nuclear power plants can cause the leakage of radioactive substances. Therefore, the evaluation of integrity and safety for tubes is very important from the viewpoint of nuclear ecocide. In the present study, to investigate the wear properties of Inconel 600 and 690 steam generator tube materials mated with 409 stainless steel commonly used as support plate, sliding wear tests were performed with increasing sliding distance in air and in elevated temperature water environment, respectively. The wear volume of tube materials was less than those of supports under all conditions. There were no significant differences in the wear behavior for the Inconel 600 and 690 tubes, independently of the testing environment.

A New Eddy Current 3-D Profilometry for a Quantitative Measurement of Geometric Anomaly in Steam Generator Tubes of Nuclear Power Plants

Most of the corrosive degradations in steam generator tubes of nuclear power plants are closely related to the residual stress existing in the local region of a geometric change, that is, an expansion transition, u-bend, ding, dent, bulge, etc. Therefore, accurate information on a geometric anomaly in a tube is a prerequisite to the activity of a non destructive inspection for a precise and earlier detection of a defect in order to prevent a failure during an operation, and also for a root cause analysis of a failure. In this paper, a newly developed eddy current technique of a three-dimensional profilometry is introduced and the proof for the applicability of the technique to a plant inspection is provided. The quantitative profile measurement using a new eddy current probe was performed on steam generator expansion mock-up tubes with various geometric anomalies typically observed in the operating power plants, and the accuracy of the measured data was compared with those from the laser profilometry.

Development of an ECT Technique for Discriminating between a Through-Wall/Non Through-Wall Crack in a Steam Generator Tube

In a lot of pressurized water reactors, PWSCC (Primary Water Stress Corrosion Cracking) has been observed in the expansion or u-bend transitions of the alloy 600 steam generator tubes. Particularly, the development of a through-wall crack may cause leakage of the primary coolant during operation and the resultant forced outage, and the in-situ pressure test has often been used to evaluate the integrity and the leakage of the cracked tube during an in-service inspection. However, this process requires additional equipment and hours, and the tested tubes are plugged due to the plastic deformation induced by the internal pressurization. This paper describes a new evaluation technique of the eddy current test, by which it can be determined whether the crack is through-wall or non through-wall. The technique is based upon the analysis of the characteristics in the eddy current signals from the crack and also includes the method of measuring exact through-wall length of the crack. The proof and applicability of the technique is discussed with the results of the destructive tests on cracked tubes extracted from a commercial power plant. Also, the effect of crack opening, which reflects the driving force for a crack propagation of non through-wall crack and the leak rate of a through-wall crack, upon the characteristics of the eddy current signals from the coils of the motorized rotating probe is investigated and discussed using steam generator tube samples with manufactured axial cracks of a through-wall and non through-wall. Introduction Many steam generators of alloy 600 tubes in pressurized water reactors (PWR) have been affected by primary water stress corrosion cracking (PWSCC). The cracks are observed at the tube locations of the high residual tensile stress such as the expansion transition at the top of the tube sheet, u-bends with small radius and dents at the tube support plate [1]. Even though the cracking at the u-bends and the tube support plate could be mitigated through the stress relief heat treatment of the u-bends, and changes of the tube support plate material and hole design, respectively, the cracking at the expansion transition is an unresolved issue. In the two Korean PWR power plants, which have been operated since 1988 and 1989, axial cracks due to PWSCC at the roll expansion transition of the alloy 600TT steam generator tubes were first detected during the 4 th in-service inspection(ISI) by the eddy current test(ECT) using motorized rotating pancake coil (MRPC) probes and a large number of cracked tubes were observed during the 5 th ISI. Even though shot peening was performed on the inner surface of the tubes around the hot leg transition to mitigate further cracking, PWSCC continued and the leakage of primary coolant occurred during the 8 th fuel cycle in both the plants. For the axial cracks due to PWSCC at the expansion transition regions, an alternative maintenance strategy based upon the appropriate limitations of the crack length, the number of cracks over the tube circumference and the leak rate has been implemented to increase the steam generator life. In this regard, besides the accurate detection and sizing of the cracks including the length and depth, it is very important to discriminate the through-wall crack from the non through-wall crack in order to prevent Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 600-605 doi:10.4028/www.scientific.net/KEM.270-273.600