The ultrasonic examination and monitoring of austenite welds of stainless steel pipelines at Russian Nuclear Power Plants.

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1 The ultrasonic examination and monitoring of austenite welds of stainless steel pipelines at Russian Nuclear Power Plants. V.G. Badalyan, A. K. Vopilkinе Scientific and production center Echo+, Russia, , Kurchatov-place, 1. Tel. (095) , Fax (095) Abstract SPC ECHO+ has developed and during last years has used the complex technology NDT of stainless steel pipeline welds. This technology of quality assessment of first circuit pipe welds during in-service inspection at some Russian NPP was accepted from the beginning The technology consists of three stages. At the first stage the conventional ultrasonic inspection is performed according to the up-date testing rules. At the second stage the expert testing of flaw areas detected at the first stage is performed. The result of the expert testing is the determination of real flaw parameters. For this purpose the Augur system with FT-SAFT treatment is used. At the third stage the strength specialists calculate the danger rate of detected flaws and its affect on the weld strength and residual resource. During the last four years ( ) the 853 welds were inspected using the technology. All welds should to be repaired according to the old testing technology. 532 welds were allowed for further exploitation because they meet strength requirements. It was repaired 93 welds. All these welds after expert testing are monitored periodically. It allows to reduce ungrounded expensive weld repair significantly without decrease of operation safety. On the other hand, the reliability of most dangerous flaw detection such as cracks was increased. The accuracy of flaw height measurements for initial and repeated NDT is discussed. The examples of weld pipeline monitoring at acting NPP are cited. Introduction The ultrasonic testing methods are the wide-spread ones for Nuclear Power Plans equipment testing. The coherent data processing methods were used last time. These methods are allowed to check effectively austenitic welded joints to determine the height and profile of the flaws. SPC ECHO+ together with the experts from ENTEK and Russian scientific center Kurchatov s Institute has developed and during last years used the complex technology NDT of the welds [1]. This technology of quality assessment of first circuit of austenite welds of stainless steel pipelines was accepted in-service inspection at some Russian NPP from the beginning The technology is consisted of the use of the number of the Augur systems, developed in Echo+ center, and of equipment reliability estimation on its base. The coherent signal processing is used in these systems. The algorithm of this technology is shown at Figure 1. The conventional ultrasonic testing is produced firstly by the methods applied in nuclear engineering. The manual or the automatic testing is used. If the flaws were found with the higher then the searching or control levels, the expert testing is provided by the Augur system [2].

2 Figure 1. Technology of quality assessment of first circuit pipeline welds The each flaw diffraction field is registered in details with the step of the order 0.3 mm. These data are treated then and analyzed by the expert in the laboratory in the easy and comfortable conditions. The information about the flaw parameters is transferred to the strength experts. The weld repair advisability decision is taken on the base of the strength calculation. The technology has used in years. Some Features of the Augur Systems The Augur system could be used in two regimes: 1. The search regime along the full length of the weld (the review one) and 2. The detail data registration regime (the expert one). The computer screen images are shown at Figure 2 and Figure 3 after the end of the review and expert testing of the austenite welded joint of the tube with the diameter 325 mm and the wall thickness 16 mm. Horizontal sweeps are performed by using to the transverse wave transducers, located to the left ( negative ) and to right ( positive ) from the tested joint with the angles 75 and 55. The color of background is determined 4the quality of the acoustical contact. The chosen areas are the areas, marked out for the expert testing. The field of the joint scanning and the acoustical block position are shown in the right part of screen. The images were restored then by FT-SAFT for the discontinuity analysis and determination of their real parameters, such as coordinates, length, height, and shape [3,4]. 2

3 Figure 2. The computer screen image at the moment of ending of the review regime of the austenite welds of stainless steel pipelines with the diameter 300 mm. Figure 3. The computer screen image at the moment of ending of the expert regime. Figure 4. The image of flaw in the austenite welded joint of stainless steel pipelines with the diameter 300 mm. 3

4 The one flaw image is shown at Figure 4. The flaw height, its position relatively to joint groove and the flaw type are determined according to the B-image (section across the joint). There is a mask on the screen, showed the joint groove according to the construction documentation. The different amplitudes of the flaw are shown by the different colors (from white to dark- red ones). The white color is corresponded to the maximal amplitude. The Accuracy of Flaw Parameters Determination. The accuracy of flaw length determination. The accuracy of determination of flaw length is corresponded to traditional one, defined by incoherent methods. The results of measurements of flaw length are presented at Figure 5. Figure 5 Results of determination of flaw length according to Augur testing and destructive ones (metallography, method three-point loading). The determination of the flaw length error is ± 5 mm in accordance with Figure 5. The flaw height determination. The relatively high accuracy of the flaw height is one of the important parameters of the Augur system. It is possible to build flaw profile and to follow on its changing during the regular inspection. The corrosion flaw profile peculiarity in the stainless steel pipelines is that the height is changed quickly with the length of flaw. The example of the corrosion crack obtained by three-point loading is shown at Figure 6. Figure 6. Сorrosion crack in austenitic welded joint in stainless steel. The crack contour is shown by yellow line. The scale is in mm. 4

5 The error in determination of flaw parameters is defined by the general resolution of system. It is necessary to discriminate the of flaw parameters determination at their primary analysis and at the repeated measurements. The accuracy is higher at the repeated measurements because the difference between the primary image parameters and repeated ones estimated only. Determination of the flaw profile after primary testing. The height measurement multiple results, received by destructive testing and Augur systems are shown at Figure 7. Figure 7. The height of flaws in separate slices got by Augur systems and at destructive testing. The red lines are limited the error, equaled to ±1 mm. The blue one the area, equaled ±1,5 mm. The comparison of the expert system results and the metallography ones is demonstrated that the accuracy of the crack height measurement is equal to ± 1 millimeter in the 80 percent confidence interval and ± 1.5 mm in the 95 percent confidence interval. Determination of the flaw profile after repeated testing. As it was already mentioned, the error of determination of flaw height is less, at the repeated testing than at primary checking. The results of the flaw height measurements in separate sections for 32 welds are shown at Figure 8. The data are received by Augur systems at the repeated studies in 1999 and It is followed from Figure 8, that the accuracy of the crack height measurement is equal to ± 0,5 millimeter in the 80 percent confidence interval and ± 1 mm in the 95 percent confidence interval. 5

6 Figure 8. The flaw height change distribution in The red line at Figure 8 is corresponded the flaw height change general distribution for the certain slice at the repeated expert testing; the green one to this distribution at the flaw height increase during the exploitation; the blue one is the error of the flaw height determination for the certain slice. The Monitoring of the Weld State. The coherent data processing is allowed to observe the flaw development during exploitation. The results of the corrosion crack regular inspection, performed in years is shown at Figure 9. It is followed from the Figure 9 that the flaw is slightly increased in height and in length. It is seen too, that the flaw is increased in some parts non-uniformly, but no more than 1 mm in a year. It is very difficult to find that without coherent data processing. Figure 9. The profile of the corrosion crack in austenitic welded joint in stainless steel in The height of crack in millimeters is along the Y-axis and its length in millimeters is along the X-axis. The expert testing of fault welds are conducted annually and if parameters of flaw become critical ones for given welded joint, this joint is subjected to repair. 6

7 The Practical Application of the Complex Technology. The mass expert testing of the stainless steel pipelines with the diameter 300 mm was provided for the most crucible welds of the equipment of the first contours of the Smolensk and Kursk Nuclear Power Plants. The welds rejected according to the standard manual testing results were tested. All of the defected welds should be repaired according to the existed technology. Some welds were tested many times, 228 welds were allowed to the exploitation after the testing without the repair and only 93 welds were transferred for the repair after the strength analysis. The generalized testing results are shown in Figure 10. The part of primary testing in the whole testing value is increased as it is seen from Figure 11 firstly and decreased during the last two years. The part of the repeated expert testing in the whole testing volume is increased continuously from 1999 and 90% of the expert testing volume were performed in again. Many welds were checked 2-4 times. Figure 10. The total number of the austenitic welds in stainless steel pipelines of 325x15 mm subjected expert testing. Conclusions 1. The use of the complex technology of quality assessment is allowed to reduce considerably the weld repair volume and raise of the exploitation security of the welds. 2. The analysis of the flaw parameters, measured by the Augur expert system and by the destructive testing, has shown that: The accuracy of the flaw heights determination during the primary expert testing is: ± 1 mm - in the 80 percent confidence interval, ±1,5 мм - in the 95 percent confidence interval. The accuracy of the flaw heights determination during the repeat expert testing is : ± 0,5 мм - in the 80 percent confidence interval, ±1 мм - in the 95 percent confidence interval. 3. The monitoring of the welded joints is possible by Augur system. 7

8 References (1) A.K. Vopilkine, V.G. Badalyan, The application of the system AUGUR at Russian nuclear power station. NDT World, N 6,1999 (in Russian). (2) V.G Badalyan., A.K Vopilkine, The computer systems for the ultrasonic nondestructive testing. Soviet J. of Nondestructive testing, 3, pp (3) V. G. Badalyan, E.G. Basulin, Digital reconstruction of images of reflectors using projection in spectral space technique. Soviet Acoustical Journal, vol. XXXIV, 2, 1988, pp (in Russian). (4) H. Ermert, R. Karg. Multifrequency acoustical holography. IEEE Trans. Sonics a. Ultras.,, SU-26, N 4, 1979, pp (5) V.G. Badalyan, A.K. Vopilkine. The Ultrasonic Testing System Application at the Russian NPP with the Coherent Data Treatment Augur. Proc.15-th WCNDT, Roma, October