Toshiba has been developing the eddy current techniques for nuclear components. The application for the inner surface of the RV nozzle in PWR plants has been developed as the surface examination pre- and post-underwater laser beam welding. As the advanced application of the eddy current technique, the array coil probe makes the duration of inspection shorter. Furthermore, flexible array probe fit curved surface, such as the inner surface of the bottom of the reactor pressure vessel in BWR plants. Although visual testing, penetrant testing, ultrasonic testing and eddy current testing are considered as the surface testing, eddy current testing has the advantages of the surface testing for in-vessel components; applicable in water, higher sensitivity of detection, crack length measurable and captured the test results as digital. While inner coil probes have been applied for many components such as the inspection of small bore tubes, surface coil probes have been less applied for the nuclear components. The new application of the eddy current technique with surface coil is expected to assure the higher quality of the inspection for nuclear components.

1. (1)Components:
   Reactor pressure vessel, reactor vessel and in-vessel components of BWRs and PWRs

1. (2) Material:
   Non-magnetic material（Austenitic stainless steel, Nickel-based alloy）

1. (3) Condition:
   Underwater of reactor vessel

1. (1)Applicable as the inspection underwater remotely
   Eddy current technique is applicable for in-vessel components underwater condition during outage in nuclear plants. Although eddy current testing is also applicable in dry condition, it is possible to reduce irradiation exposure by using it underwater condition.

1. (2)Higher sensitivity of detection
   Eddy current technique is applicable for the inspection to detect 0.5 mm or deeper open end SCC crack according to JEAG 4217-2010 “Eddy Current Examination for Nuclear Power Plant Components” which is the guideline for eddy current examination using the surface coil. [1] 100% detection rate was verified for the inspection to detect 0.5 mm or deeper open end SCC crack according to “Report on the Project of Nondestructive Inspection Technologies on the Ni Alloy Welded Joint (2008)”. [2] Although the sensitivity of detection is affected by surface condition of components and the aspect of crack, about 0.1 mm deep open end SCC cracks were detected by eddy current testing as shown in Fig.1. Herein the detective sensitivity was calibrated as 100% by using 1mm deep and 0.3 mm wide EDM slit. [3, 4]

1. (3)Quantitative performance and storage of test data as digital
   The crack length is measurable quantitatively using eddy current technique, and the test data is recordable as digital. The digital data is applicable to analyze after testing, because the digital date is recorded as signal waveform.

1. (4)Duration saved by array probe
   Array probe, multiple coils are arranged in a probe, makes the duration of the inspection shorter. And flexible array probe, which are mainly consisted of coils and flexible materials, is applicable for the inspection of the curved surface.



Fig. 1 Example of inspection results to detect artificial SCC crack

1. (1)Surface inspection pre- and post-underwater laser beam welding
   Weld repair and high SCC resistant weld material hard facing utilizing the underwater laser beam welding are technologies for repair and preventive maintenance that are applied to reactor pressure vessel and in-vessel components. Although liquid penetrant technique is commonly used for surface inspection before and after welding, but liquid penetrant technique is difficult to apply as surface inspection due to the water which is filled in reactor for in-service term. Underwater laser beam welding process flow is shown in Fig. 2. Surface inspection is required before and after welding. By applying eddy current technique as the surface inspection instead of liquid penetrant testing, all processes are possible to work remotely underwater in order to lower the exposure.
   Application of the underwater laser beam welding is discussed as repair and/or preventive maintenance domestically and abroad. [5-8] In this preventive maintenance, existing alloy 600 weld material inside the reactor vessel nozzle in PWR plants would be removed by excavation machine, and high SCC resistant alloy 690 weld material is welded by underwater laser beam welding. Fig.3 shows the schematic images to inspect the surface of the weld area before welding and after welded surface machining. [3, 4, 9, 10]



Fig. 2 Process flow for underwater laser beam welding



Fig. 3 Schematic images of reactor vessel nozzle in PWR plants to inspect before and after underwater laser beam welding

1. (2)Application of array probe to inspect faster and/or to enlarge the inspection area
   Array probe, the example of the arranged coils shown in Fig.4 and Fig.5, makes the duration of the inspection shorter. And the deterioration of detection sensitivity caused by flaw orientation is reduced, because the eddy current is exited diagonally by a pair of coils and the direction of eddy current is invertible. [11] In addition to these, flexible array probe, which mainly consists of coils and flexible materials, is applicable for the inspection of the curved surface such as the bottom of in-vessel components in BWR plants. The flexible array probe reduces the lift off signal which affects the accuracy of the detection. Fig.6 shows the calculated result of the eddy current excited by the array probe. Fig. 7 shows the application of the flexible array probe for the mock-up with curved surface.



Fig. 4 Schematic view of eddy current flow exited by the multiple coils in an array probe



Fig. 5 Schematic view of eddy current flow exited by the multiple coils in an array probe



Fig. 6 Analytical result of eddy current flow exited by the multiple coils in an array probe



Fig. 7 Example of the application of flexible probe for curved surface

1. [1] The Japan Electric Association, Eddy Current Examination for Nuclear Power Plant Components, JEAG 4217-2010
2. [2] Japan Nuclear Energy Safety Organization, Report on the Project of Nondestructive Inspection Technologies on the Ni Alloy Welded Joint (2008), pp. 870-872, December 2009
3. [3] S. Ueno, N. Kobayashi, T. Kasuya, M. Ochiai, Y. Yuguchi, Defect Detectability of Eddy Current Testing for Underwater Laser Beam Welding, ICONE 19, Osaka Univ., October 24-25, 2011
4. [4] N. Kobayashi, T. Kasuya, S. Ueno, M. Ochiai, H. Ichikawa, Feasibility Assessment of Eddy Current Testing in Underwater Laser Beam Welding, Journal of the Japanese Society for Non-Destructive Inspection, 61(9), pp. 475-479, 2012
5. [5] T. Fukuda, M. Tamura, Y. Tongu, W. Kono, M. Obata, Y. Morishima, Development of Temper-Bead Welding by Under-water Laser Welding, 15th National Symposium on Power and Energy System (SPES 2010), The Japan Society of Mechanical Engineers, Waseda Univ., June 21-22, 2010
6. [6] M. Yoda, M. Tamura, Underwater Laser Beam Welding Technology for Reactor Vessel Nozzles of PWRs, Toshiba Review, 65(9), pp. 36-39, 2010
7. [7] Y. Tokunaga, I. Chida, K. Shiihara, M. Tamura, T. Fukuda, T. Maehara, M. Yoda, Development of Underwater Laser Beam Welding Equipment, 8th Japan Society of Maintenology Annual Conference, Tokyo, October 21, 2011
8. [8] I. Chida, K. Shiihara, T. Fukuda, W. Kono, M. Obata, Y. Morishima, Study on Laser Beam Welding Technology for Nuclear Power Plants, Transactions of the Japan Society of Mechanical Engineers (B), 78(787), pp. 73-77, 2012
9. [9] N. Kobayashi, T. Kasuya, S. Ueno, M. Ochiai, Y. Yuguchi, C. S. Wyffels, Z. Kuljis, D. Kurek and T. Nenno, Utility Evaluation of Eddy Current Testing for Underwater Laser Beam Temperbead Welding, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Berlin, September 29- October 1, 2010
10. [10] N. Kobayashi, T. Kasuya, S. Ueno, M. Ochiai, Y. Yuguchi, Applicability Evaluation of Eddy Current Testing for Underwater Laser Beam Welding, 7th Japan Society of Maintenology Annual Conference, Shizuoka, July 13-15, 2010
11. [11] S. Ueno, N. Kobayashi, K. Nomura, M. Ochiai, Y. Kitajima, S. Maruyama, Suppression of Reduced Sensitivity of Eddy Current Testing Depending on Defect Orientation, 9th Japan Society of Maintenology Annual Conference, Hitotsubashi Univ., July 25-27, 2012