ULTRASONIC INSPECTION OF BAFFLE TO FORMER BOLTS IN PRESSURIZED WATER REACTORS

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1 More Info at Open Access Database ULTRASONIC INSPECTION OF BAFFLE TO FORMER BOLTS IN PRESSURIZED WATER REACTORS M. Bolander, L. Alerts, C. Bies, F. Bonitz Westinghouse Electric, Germany R. S. Devlin, P. Minogue, WesDyne International,USA ABSTRACT Industry-wide Aging Management Programs, as well as specific operating experience has resulted in increased interest in the inspection of the baffle to former bolts found in the internals of many Pressurized Water Reactors (PWR s). Age related degradation mechanisms, such as Irradiation Assisted Stress Corrosion Cracking (IASCC) may lead to the failure of these bolts over time. Sufficient baffle bolt failures may lead to the loss of structural integrity of the baffle-former assembly, or damage to the fuel. Ultrasonic testing is used for the inspection of these bolts. The heads of the bolts are accessible from the core side when all the fuel is offloaded. The core barrel may remain in the reactor vessel, or be moved to the storage stand, but it is generally submerged due to the high radiation emanating from the baffle region. Because of the large variation of the configuration of baffle to former bolts in different nuclear power plants, different types of ultrasonic inspection techniques have been developed and qualified. Conventional, phased array, contact, and immersion techniques have been developed to inspect the various bolt types. Qualifications have been undertaken for reactors in Europe, the United States, and Asia. Besides the ultrasonic inspection technique, specific manipulators have been developed to deliver the ultrasonic probe to the bolts to be inspected. In this context the manipulator MIDAS VI used by Westinghouse is introduced. The MIDAS VI is a remotely operated underwater vehicle which positions the ultrasonic probe via camera observation. With MIDAS VI, the time for an on-site inspection, especially the installation time for the manipulator system is substantially reduced. Even the ultrasonic inspection itself can be realized in a shorter time. Longtime experience with the manipulator SUPREEM for European power plants will be also a topic of this contribution. INTRODUCTION In a pressurized water reactor the fuel elements, which possess a quadratic cross section, are assembled in the core during power operation. The core barrel forms an adaptation between these fuel elements and the cylindrical reactor pressure vessel. It consists mainly of the horizontally aligned former plates providing the transition between the reactor pressure vessel to the fuel assembly and the vertically oriented baffle plates constituting the inner shape of the core barrel. The baffle plates are bolted to the former plates by the baffle to former bolts (short: baffle bolt) and the former plates are bolted by the barrel to former bolts (short: barrel bolts) to the core barrel, which is the outer shell of the core assemblies. Figure 1 shows the core barrel and its internals, figure 2 a collection of baffle and barrel bolts used in German nuclear power plants. Depending on several boundary conditions, like material properties, bolt geometry, manufacturing technique of the bolt, chemical water composition, mechanical stress, radiation, etc., the baffle bolts are subject to material damages leading to the fracture of the bolt. The failure of individual baffle bolts distributed on the baffle plate does not affect the proper fixation of the baffle plates at the former plates. If adjacent baffle bolts are broken, the baffle plate may undergo slight displacements yielding to gaps between the baffle plates. 742

Figure 1: Core barrel and its internals inside of the reactor pressure vessel Figure 2: Collection of baffle and barrel bolts used in German nuclear power plants Ultrasonic examinations are carried

2 These gaps can cause irregular flow conditions causing abrasion at the fuel elements. Since demolishing of the bolt head is avoided by means of weld spots, lock washers (locking rings) or lock bars (locking pins), a broken bolt cannot be recognized by visual inspection. Figure 1: Core barrel and its internals inside of the reactor pressure vessel Figure 2: Collection of baffle and barrel bolts used in German nuclear power plants Ultrasonic examinations are carried out to detect cracked baffle bolts. To get access to the bolts the fuel assembly needs to be removed from the core barrel. The high dose rates enforce a mechanized underwater inspection. Different tools, like mast manipulators or complex multiple axis robot systems, were developed, qualified and used to perform these investigations. All these tools required considerable logistic efforts, especially crane support, for their installation. An extended flexibility can be achieved utilizing remotely operated underwater vehicles (ROVs). Due to economic reasons the application of such ROVs is restricted to inspections, where a new qualification of the inspection technique is required or where the expenses of a new qualification are low. Westinghouse Electric Germany developed MIDAS VI, a ROV for the inspection of baffle bolts. This ROV was presented in [1] and [2]. This article reviews the design of MIDAS VI and describes the recent qualifications and inspections, where MIDAS VI was used to apply the ultrasonic probe to the baffle bolt. 743

3 THE REMOTELY OPERATED UNDERWATER VEHICLE MIDAS VI The remotely operated underwater vehicle MIDAS VI, shown in figures 3 and 4, was designed by Westinghouse for visual inspections in the primary system of nuclear power plants and to replace stationary manipulators used for baffle bolt inspections. It consists of the following components: the remotely operated vehicle (ROV), the end-effector allowing the positioning of the ultrasonic probe so sufficient coupling on the bolt head can be achieved, the radiation tolerant camera system including illumination and video recording unit, the operator console, the power and control unit (see figures 7 and 8) and the control and supply lines from the power and control unit to the ROV. An overview underwater camera located on the top of the core barrel allows the surveillance of the MIDAS VI activities during the inspection. The ROV consists mainly of a basic frame carrying a buoyancy vessel, which is driven and maneuvered by four propellers (see figures 3 and 6). By means of these driving and steering elements the ROV can be moved forward, backward, sideward, upward, downward and turned around the vertical axis. A pressure sensor based depth control supports the operator to keep the ROV at constant depth. The radiation tolerant camera possesses a wide angle lens with fixed focal length of 9 mm allowing the visual surveillance of the probe applied on the inspected bolt. Eight 50 W spotlights ensure homogenous illumination of the end-effector with limited shadow casting. The end-effector is mounted under the camera. It can be arranged in a left or in a right position, to reflect the location of the inspected baffle bolts on the baffle plate. That means depending on the accessibility of the baffle bolt the camera and the end-effector are installed on the left or right side of the ROV. A pneumatic cylinder presses the ultrasonic probe onto the bolt head. To turn the probe around its main axis a motor-resolver unit is build into a waterproof housing. The end-effector of MIDAS VI carrying a probe for the inspection of hexagonal bolts with a lock bar is presented in figure 5. During the inspection of the bolt the end-effector is kept in position by suction cups applied to the baffle plate. These suction cups are connected to a vacuum pump connected at the ROV. Figure 6 shows the ROV attached to a mockup. The ROV is brought into the water with the aid of a gripper, which can be connected and disconnected to a bracket located at the top of the ROV. A rack, which is positioned close to the pool, serves as a storage for the ROV between two dives. All ROV operations do not require crane support. 744

Figure 3: Schematic illustration of MIDAS VI Figure 4: Front photgraphy of MIDAS VI All ROV movements are operated utilizing the MIDAS VI control

The movement of the whole ROV and the end-effector is performed from the operator console (see figure 8).

This screen is located on the top of the control and surveillance equipment (see figure 7).

This screen is also shown in the top figure 7 under the overview screen and as a detailed image in figure 9.

4 Figure 3: Schematic illustration of MIDAS VI Figure 4: Front photgraphy of MIDAS VI All ROV movements are operated utilizing the MIDAS VI control and surveillance equipment shown in figures 7 and 8. The movement of the whole ROV and the end-effector is performed from the operator console (see figure 8). A screen showing the camera image of the overview camera helps the operator to position the ROV close to the bolt. This screen is located on the top of the control and surveillance equipment (see figure 7). The camera located over the end-effector delivers an image of the bolt so the operator can move the probe directly in front of the bolt head. This screen is also shown in the top figure 7 under the overview screen and as a detailed image in figure 9. The operator moves the ROV close to the bolt utilizing the surveillance camera image, adjusts it with the aid of the end-effector camera, activates the suction cup mechanism to hold the ROV in position, presses the probe onto the baffle bolt head with the pneumatic cylinder and starts the probe rotation required for the inspection. Figure 5: MIDAS VI end-effector Figure 6: MIDAS VI attached to a mockup 745

Figure 8: MIDAS VI operator console Figure 7: MIDAS VI control and surveillance equipment Figure 9: End-effector camera image ACTUAL QUALIFICATIONS General Steps During the preparation of an

The applicability of an ultrasonic depends mainly on the shape of the baffle bolt head, the material properties and the inspection targets to be found.

These conditions include the evaluation of obstacles like fuel alignment pins at the bottom of the core barrel or niches formed by the baffle plate.

5 Figure 8: MIDAS VI operator console Figure 7: MIDAS VI control and surveillance equipment Figure 9: End-effector camera image ACTUAL QUALIFICATIONS General Steps During the preparation of an ultrasonic technique for the inspection of baffle bolts it must be verified if an existing ultrasonic probe can be applied. The applicability of an ultrasonic depends mainly on the shape of the baffle bolt head, the material properties and the inspection targets to be found. After the ultrasonic probe characteristics are known the question of the accessibility must be answered. That means the geometric boundary conditions need to be examined. These conditions include the evaluation of obstacles like fuel alignment pins at the bottom of the core barrel or niches formed by the baffle plate. If the environment of the baffle bolts to be inspected is known, the next task is to determine whether the baffle bolts can be reached utilizing the end-effector mounted on MIDAS VI or not. This task can be solved by performing experiments or by CAD modeling. If the probes are available and the accessibility of the bolts is proven, the qualification process can be initiated. This process is finalized by an integral performance demonstration. Actual Qualifications in Europe Since major components of the inspection equipment used for an existing qualified inspection of baffle bolts in Belgium power plants were substituted, the inspection technique for these examination was qualified utilizing the new inspection system. The existing technique was widely kept due to the reliable results achieved in the past. In 2005 baffle bolts exhibiting indications had been removed after ultrasonic inspection. The indications found were confirmed by penetrant testing. The equipment was modified as follows. 1. The ultrasonic device PARAGON was replaced by the DYNARAY / Ultravision inspection system. 2. The SUPREEM manipulator was replaced by the MIDAS VI remotely operated underwater vehicle. In addition the technique was qualified for baffle bolts possessing lock bars (see figure 10) and replacement bolts with a safety flange. 746

Figure 10: Baffle bolts possessing lock bars The ultrasonic probes used for that

Figure 11: Schematic of the probe and configured channels In figure 12 a sample of an

6 Figure 10: Baffle bolts possessing lock bars The ultrasonic probes used for that qualification were not modified, since these have shown excellent detection capabilities in the past. Figure 11 shows schematics of the probe used in pulse-echo and pitch-and-catch mode. Figure 11: Schematic of the probe and configured channels In figure 12 a sample of an indication is given. The test reflector located under the bold head is clearly detected in pulse-echo mode. Figure 12: Reflector underneath the bold head detected with channel 1 (see arrow) 747

Actual Qualifications in the US In order to inspect baffle bolts, which could not be examined with ultrasound in the past, two ultrasonic techniques were developed and qualified.

The inspection of inner socked bolts (allen screws) secured by a lock bar with a technique utilizing the few areas at the edge of the bolt head for the coupling of the a multi-element ultrasonic

The inspection of inner socked bolts (allen screws) secured by a lock washer with a technique utilizing the conical bottom of the hexagonal socked for the coupling of a phased array probe.

7 Actual Qualifications in the US In order to inspect baffle bolts, which could not be examined with ultrasound in the past, two ultrasonic techniques were developed and qualified. For both qualifications MIDAS VI was used to attach the probe at the baffle bold. 1. The inspection of inner socked bolts (allen screws) secured by a lock bar with a technique utilizing the few areas at the edge of the bolt head for the coupling of the a multi-element ultrasonic probe. This bolt type is shown in figures 13 and The inspection of inner socked bolts (allen screws) secured by a lock washer with a technique utilizing the conical bottom of the hexagonal socked for the coupling of a phased array probe. This baffle bolt type is displayed in figures 15 and 16. Figure 13: Installed inner socket bold with lock bar Figure 14: Bold with lock bar Figure 15: Drawing of an installed inner socket bold with lock washer and weld pins Figure 16: Bold with lock washer and weld pins During the examination the shape of the cone is determined utilizing a sector scan. As an example for the results, two sector scans are given in figure 17. The left image displays an indication underneath the bold head, the right image the back-wall echo from the bolt bottom. 748

Figure 17: Sector Scan of a phased array baffle bolt inspection The phased array technique for inner socked baffle bolts was qualified in December 2011 and successfully used in spring 2013 at the

8 Figure 17: Sector Scan of a phased array baffle bolt inspection The phased array technique for inner socked baffle bolts was qualified in December 2011 and successfully used in spring 2013 at the Point Beach 1 nuclear power plant. By means of the qualification of these ultrasonic examination techniques, the power plant owners avoided the replacement of baffle bolts, which were regarded as not inspectable by the authorities. Experiences gained during inspections in French nuclear power plants In 2009 inspection techniques for the original and the replacement baffle bolt built into French power plants (900 MW / 1300 MW) were qualified. In contrast to the examinations described above, the SUPREEM inspection robot, which originally was developed by WesDyne for the inspection of the reactor pressure vessel, was used (see figure 18). The examination of the original hexagonal baffle bolts was performed utilizing a four element ultrasonic probe; for the inspection of the replacement bolts a two element ultrasonic probe was applied. The inspection techniques have proven their practical applicability during the following interventions: Bugey 4 September loop plant Bugey 5 December loop plant Belleville 1 May loop plant Bugey 2 August loop plant Bugey 4 February loop plant Fessenheim 2 May loop plant Bugey 5 June loop plant Dampierre 2 March loop plant 749

Figure 18: SUPREEM inspection robot General comments on the analysis The analysis of the collected signals is generally based on pattern comparison, evaluation of the indications signal to noise

9 Figure 18: SUPREEM inspection robot General comments on the analysis The analysis of the collected signals is generally based on pattern comparison, evaluation of the indications signal to noise ratio and the surveillance of the back-wall echo generated at the bolt bottom. Further actual activities The present development efforts concentrate on the inspection of barrel bolts, which can only be accessed through the 25 mm wide gap between the thermo shield and the core barrel. Such configurations are found in pressurized water reactors exhibiting a Westinghouse design. Figure 19 illustrates how the probe is attached to the barrel bolt. The recent qualification in that area focuses on the development of an inspection specification of barrel bolts with a lock bar [3]. The probe used for that purpose was developed with the aid of a simulation of the sound field described in [4]. Figure 19: Ultrasonic probe attached to a barrel bolt 750

10 Summary This article was meant to provide insight in some existing inspection technique of baffle and barrel bolts. These challenging inspections underwent permanent improvements during the last 15 years. The improvements comprise both, the manipulator technique and the inspection technique. The manipulator used became more and more flexible and complicated by means of adding axis providing a higher degree of freedom and finally by changing the basic design towards the application of remotely operated underwater vehicles. The inspection techniques evolved from simple transducers, over multi-element probes to phased array probes used in the recent designs. Due to the presence of a qualification, techniques established over that development period are still in use and serve as a base of future improvements. References 1) Debnar A., Spies M., Bonitz F., "Prüfung der Kernumfassungsschrauben in Druckwasserreaktoren", DGZfP Jahrestagung ) Debnar A., Alaerts L., Spies M., Minogue P., Epineau C., "Recent Experiences with Ultrasonic Inspection of Baffle Former Bolts", Proceedings of the 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Berlin, ) Bonitz F., D Annucci F., Knierriem L., Franke H., Kappes W., "Auslegung, Herstellung und Qualifikation einer Prüfeinrichtung für Kernbehälter von Druckwasserreaktoren", DGZfP Jahrestagung ) Spies M., "Semi-analytical elastic wave-field modeling applied to arbitrarily oriented orthotropic media", Journal of the Acoustical Society of America 110 (1), July

ULTRASOUND & SUBMARINE ADVANCEMENTS FOR RPV INTERNALS BAFFLE BOLT INSPECTION. S.W. Glass, B. Thigpen, AREVA, USA A. Bleuze, E. Brau, AREVA, France

More Info at Open Access Database www.ndt.net/?id=18551 ULTRASOUND & SUBMARINE ADVANCEMENTS FOR RPV INTERNALS BAFFLE BOLT INSPECTION S.W. Glass, B. Thigpen, AREVA, USA A. Bleuze, E. Brau, AREVA, France