Scanning and Testing of Nozzle Welds in the Nuclear Industry

TWI NDT Validation Centre (Wales)

Vasilios Papadimitriou, Ioannis Roditis and Panagiotis Chatzakos
Innora Ltd-Innovation Robotics Automation
59 Ioanni Metaxa Str., 19400, Koropi, Athens, Greece

Paper presented at BINDT 2012 - 51st Annual Conference of the British Institute for Non-Destructive Testing, Daventry, UK. 11-13 September 2012.

Abstract

Regular in-service inspection is important to verify the integrity of welded nozzle sections in the nuclear industry. Nozzle sections can be susceptible to crack growth due to thermal fatigue and stress corrosion. Early detection of cracks is therefore essential to ensure the continued safe operation of the facilities. In order to reduce the time and cost of such inspections there is a need to develop a system capable of performing a full inspection of nozzles without the need to change probes. The aim of this project was to design an inspection system that is able to achieve the following: reduce the inspection times, improve defect detectability and sizing, and reduce human intervention, which will reduce workforce radiation uptake. The developed automated system reduces the requirement for complex robotic manipulation and consequently reduces the size and cost of robotic deployment systems. This paper will present the inspection technique development and ultrasonic simulation using CIVA that was carried out to determine the most appropriate phased array probe and its detection capabilities. The parameters that are assessed are the beam propagation through the material, the beam coverage and the focusing characteristics of the beam as well as the defect response at critical areas of the nozzle. Furthermore, the developed automated inspection system and its functionalities will be presented together with experimental results from nozzle mock up samples with simulated defects at the nozzle to vessel weld.

1. Introduction

NozzleInspect^[1] is a collaboration between EU companies and research organisations supporting the effort of the nuclear industry in achieving better safety standards through improved inspection of plant. The project aimed to develop a novel scanner and probe manipulator combined with a phased array technique for the inspection of Boiling Water Reactor (BWR) nozzles. The project focused on the development of a new automatic NDT phased array ultrasonic system that can be deployed for the inspection of nozzles in nuclear reactors.

Welded nozzles can be found in many sections in nuclear power plants and other critical facilities. Cracking in BWR feedwater nozzless discovered during the 1970s led to a change in both the design and the materials of the nozzle. Guidelines were established for periodic ultrasonic testing with work undertaken since the early 1970s aimed at specialising NDT testing methods for the application of in-service inspection of nozzles in boiled water reactors.^[2] Early investigations serve to illustrate that complicated probe configurations were required to provide the necessary beam angle within a tangent plane of the target area.^[3] The requirement to manage defects in nozzles was addressed through the investigation of the choice of optimal inspection parameters based on three areas of inspection of the geometry of the nozzle. ^[4]

Applications where the external geometry of the test specimen varies across the test area require the control of the degradation of contact between the transducer and specimen.^[5] Experimental techniques were developed to overcome variation through adaptability of different control configurations.^[6-7] Physically flexible phased arrays overcome irregularities in the test surface and improve the coupling between surface.^[8] These arrays have been implemented in the interpretation of irregular shaped components of BWRs including the nozzles and for more challenging inspection configurations in nuclear and conventional power plant applications. Phased array techniques overcome issues associated with complex geometries through adaptability based on a combination of electronic commutation, beam-steering and focusing in complex geometries.^[9-10] In such an approach, each element of the probe is controlled independently and can therefore be delayed in responding to a signal. If all of the elements are activated simultaneously, the probe behaves like a conventional device, but when a delay is introduced, the resulting beam can be controlled and focussed as required.

2. Scanner mechanical design

2.1 Scanner base

The base of the robotic scanner is firmly clamped onto the nozzle feedwater pipe and offers rigid support for accurate positioning of the manipulator that carries the phased array probe. It consists of three main subsystems: the base frame, the clamping/centering mechanism and the rotational degree of freedom.

The base frame is a lightweight and yet rigid structure that supports all components and equipment. It is comprised of two segments, 180° each, that are possible to 'open', one in relation to another, to mount the robotic scanner on the nozzle. Afterwards, the base is easily and effectively secured using a latch mechanism (Figure 1a).

The clamping mechanism is located at the base frame and consists of four linear actuators, two manually and two pneumatically actuated. The purpose of the clamping mechanism is to stabilize the base on the nozzle offering as well, the required traction during operation. The two manually adjustable actuators are coupled and this ensures that the robotic scanner base remains concentric with the nozzle. They are mechanically coupled through a system of two timing belts stages and two low backlash worm gears. As a result the manual rotation, achieved by means of a hand knob, is transformed to an accurately coordinated linear motion of each linear actuator, adjusting the 'clamping' diameter of the base on the specific nozzle. The adjustment of the manual actuators is made prior to taking the robotic scanner on-site to avoid the calibration procedure near the vessel. After the robotic scanner is placed on the nozzle and the base is 'closed' and secured using the latch, the base is forced to become concentric with the nozzle due to the geometrical restriction of the manual actuators. The two other linear actuators are pneumatically actuated and are activated to firmly clamp the robotic scanner on the nozzle, through high friction yet low elasticity pads. To implement the peripheral movement of the probe around the nozzle, the manipulator is mounted on a carriage that rolls on a double V profiled precision stainless steel circular slide. The carriage incorporates 8 rolling wheels that allow it to slide with precision and low friction. The movement is actuated using a 60 Watt brushed DC motor and a timing belt stage mounted on the carriage (Figure 1b).

Figure 1. Design details of
(a) base of the robotic scanner; (b) peripheral degree of freedom

2.2 Manipulator

The robotic manipulator (Figure 2) is mounted on a carriage that can rotate 360° around the nozzle to fully cover the nozzle to vessel weld. Structurally, it is comprised of two links and is able to accurately place the end-effector on the vessel in a workspace up to 820mm axially from the axis of the nozzle (400mm maximum axial distance from nozzle-vessel weld). The power to both links is given by two 60 Watt brushed DC motors accompanied with low backlash planetary gearheads. The payload of the manipulator is estimated to 5Kg and it is able to apply 40N of force to the probe holder to keep it against the surface of the vessel.

Figure 2. The two degrees of freedom manipulator

2.3 End-effector

The end-effector of the robotic scanner carries and deploys the probe holder that encloses the 2D phased array UT probe. During inspection, in order for the probe holder to comply with the varying surface of the vessel, two degrees of freedom are required. A gimbal joint design was conceived for the purpose which allows two axes of rotation perpendicular to each other (Figure 3).

Figure 3. The end-effector

In addition, during the inspection it is required that a vertical force of 30N is applied to the probe holder to maintain sufficient ultrasonic coupling. Two springs mounted on linear shafts provide the required force. The deflection of the springs is measured using and absolute encoder (4096 ppr) and a two link mechanism, i.e., a 'knee' mechanism, for converting linear movement to rotational. The linear resolution achieved with this mechanism is 0.03mm.

2.4 Control software

For the control of the robotic scanner, a Galil controller was programmed to execute the low level functionalities using Galiltools software. The low level software includes the sequence of functions for the motion and the synchronization of the axes movement so that the scanner can move the probe smoothly on the curved surface of the vessel. The software is stored in the memory of the controller and it is executed on a chip in real time. A dedicated custom graphical user interface (GUI), shown in Figure 4, was developed using the InspectionWare® (product of UTEX^[11]) to combine the motion control with the acquisition of the ultrasonic data.

Figure 4. The InspectionWare® (product of UTEX) GUI

3. Modelling approach

This paper presents a series of modelling studies undertaken using CIVA 10^[12] ultrasonic modelling software developed by CEA. These models enable the simulation of the wave propagation and interaction of ultrasound with simulated flaws in models of the nozzle to vessel weld. The analysis of the beam propagation and interaction with the defects was simulated in order to optimize and predict the performance of the phased array technique in terms of critical area coverage and defect response. These simulations included several different configurations of probes, defect parameters and locations.

2.1 Defect parameters

The position, size and orientation of the defects were defined from the end user requirements and ASME Code Case 235. The defects to be detected in the main weld are planar (cracks) and parallel to the axis of the nozzle to vessel weld. The defects are likely have an allowable tilt angle in the range 5-15° with respect to the normal of the vessel wall. In addition, defects may be skewed with respect to the weld axis within the range ±10°. The length of defects varies in the range of 5.8mm to 23.6mm while the height is in the range of 2.9mm-11.8mm.

2.2 Ultrasonic modelling

2.2.1 Feedwater nozzle parameters

The nozzle under investigation is at the junction of the 300mm diameter feedwater pipe with the cylindrical wall of the reactor vessel. The diameter of the nozzle to vessel weld is 840mm. The wall thickness of the reactor vessel is 139mm and the material is low alloy ferritic steel. 3D modelling and simulation of the inspection of the nozzle were performed using CIVA 10 software. It can be observed that the weld volume can only be inspected from the side of the vessel. Inspection from the side of the pipe is problematic due to the curved surface of the nozzle and insufficient distance. The inner surface of the vessel wall outside the weld is normally covered by cladding and in such a case the use of the techniques based on the reflected from inner surface of the wall signals (full skip) is not recommended.

2.2.2 Probe and probe holder design

Several configurations were evaluated to assess the most suitable, taking into account the inspection requirements and the manufacturability of the probe. Among these configurations were (see Figure 5):

• 1D array (Figure 5a): standard phased array configuration; only for comparison purposes as this configuration does not allow the deflection of the ultrasound beam in all directions
• 2D array (Figure 5b): this configuration allows the deflection of the beam in all directions
• 2D annular segmented array (Figure 5c): this is the most complex configuration that leads to differences in elementary areas.

Figure 5. Examples of phased array probe configurations evaluated during the design phase
(a) 1D array; (b) 2D array; (c) 2D segmented annular

The modeling was performed on the YZ plane as defined in Figure 6(a) for the sectorial scanning. The modeling was also performed in the plane that is perpendicular to YZ plane and at an angle of 40° with respect to the Y axis (this plane is called XS as described in the Figure 6(b) to assess the skewing abilities of the different probe designs.

Figure 6.
(a) position of the transducer when used for sectorial scanning; (b) beam steered at 40° and skewed at 10°

The modeling demonstrated that a 2D array configuration at 2MHz centre frequency with 128 rectangular elements (3mm x 2mm) in water is the most suitable configuration for this application. The resulting ultrasound beam presents some grating lobes but these are low compared to the main beam and are not detrimental to the inspection (Figure 7).

Figure 7. Modelling results of the beam profiles for the 2D array with the beam steering at
(a) 40° in the YZ plane, (b) 65° in the YZ plane, (c) 85° in the YZ plane, (d) 40° in the YZ plane and the profile shown in the XS plane (e) 40° in the YZ plane and 10° in the XS plane and the profile shown in the XS plane

For the different configurations, the beam amplitude, focal area and focal depth were also modelled and confirmed the suitability of this 2D array configuration. It was demonstrated that the selected configuration gives the highest amplitude of ultrasonic beam when steering the beam at 40°, that the focal depths of -3dB and -6dB are the largest and that the focal zone is able to reach the bottom of the inspected component. The capability of this array on electronic beam skewing was also assessed and a maximum skewing angle of ±20° can be achieved.

The developed 2D matrix annular phased probe was fixed into a flexible probe holder (designed by Phoenix ISL) filled with water as shown in Figure 8. A flexible membrane was used to compensate for variations in the surface profile around the nozzle circumference. The ultrasonic beam was transmitted from the probe into the water and then passed through the flexible membrane.

Figure 8. Schematic drawing showing the designed probe holder and an actual photo of the flexible membrane

4. Experimental set up and results

The probe holder assembly was placed on the nozzle reference samples as shown in Figure 9. Gel coupling was used between the membrane and the sample in the laboratory development work and water coupling will be applied to the inspection of the real sample through the use of miniature spray nozzles. The profile of the nozzle to vessel weld varies circumferentially and so three reference samples were designed and manufactured to represent 0°, 45° and 90° parts of the nozzle weld profile and surface curvature profile. Figure 9 shows the 2D matrix annular phased array placed on the sample representing 0° part of the vessel. The sizes, orientation and position of the introduced defects into the reference samples were selected in accordance to ASME Code Case 235.

Figure 9. Experimental set-up of the position of the probe holder on the reference samples

Defects were introduced into the samples using Electro Discharge Machining (EDM). The skew angle of each defect ranges from 0° to 10°, and the tilt angles are -5° and 5°. The defect sizes vary from 3.8mm x 1.9mm (L x H) for surface breaking defects to 14.6mm 7.3mm for postulated defects.

For the experimental work, a Micropulse 5PA (manufactured by PeakNDT) phased array pulser-receiver was used to acquire the data. For the generation of the focal laws and display of the data the ArrayGen software was used. The focal laws can also be incorporated into the InspectionWare software which was used to drive the scanner/manipulator, and to acquire and analyse the data.

A 2D matrix annular phased array was used to inspect three reference samples (0°, 45° and 90° parts of the nozzle vessel). The inspection set up has been designed to generate shear waves in the material. This paper presents some of experimental results obtained from the 0° part of the nozzle vessel sample. Because of the presence of stainless steel cladding in the inner surface of the vessel, full skip ultrasonic techniques cannot be used. The direct response from the defects located in the middle of the vessel was measured. Figure 10 shows that defect 3 positioned in the middle of the weld, size 5.8mm x 2.9mm, can be detected when the beam was electronically skewed at 5°. Also the vessel inner surface breaking defect 4, with a size of 3.8mm x 1.9mm, is shown in the same phased array data display. The phased array data is displayed in corrected and uncorrected sectorial scan and A-scan. Defects 3 and 5 are both tilted in the same direction. Suitable electronic skewing was applied to improve the detectability and increase the reflected amplitude response from the induced defects. There are some other strong reflections that can be seen in the phased array data in Figure 10. The first reflection that can be seen at approximately 70mm ultrasonic range is the reflection from the interface between the flexible membrane and the top surface of the steel sample. There are some other reflections noticeable that are due to internal ultrasonic reflections and reverberations occurring inside the probe holder immersion bath. These reflections are present in most of the acquired phased array data, but did not affect the detection capabilities of the ultrasonic technique because they occur outside the region of interest in which the defects are located. Furthermore, in the data set, a reflection from the corner of the reference sample can be seen because of the small size of the reference sample.

Figure 10. Experimental results of the inspection of defect 3 (middle, 5.8 x 2.9mm, skew 5° and tilt 5°) and defect 5 ((bottom, 3.8 x 1.9mm, skew 10° and tilt -5°) in 0° sample

The use of the designed 2D array phased array probe on the reference samples manufactured for the phased array technique development has shown that the introduction of electronic skewing significantly improves the detection capabilities. The electronic beam skewing is particularly important when the defect is mis-orientated with respect to the incident ultrasonic beam. The technique developed on the reference samples was used during the final integration trials on a full nozzle mock up with induced cracks.

5. Testing and field trials

A mockup of a N5 feed water nozzle was manufactured (shown in Figure 11), in order to test the functionalities and validate the performance of the prototype. This mockup did not have the thickness of the actual nozzle, but the geometry of the nozzle where the robotic scanner is mounted and the vessel curvature where the end effector is placed were accurate.

Figure 11. The robotic scanner on the geometrical mockup

The robotic scanner was able to be managed by two persons and was successfully clamped onto the nozzle in less than a minute. The base was very rigid and stable when the pneumatic actuators clamped it onto the nozzle. No movement or wobbling occurred during testing. The manipulator moved smoothly and the probe followed the curvature of the vessel. The prototype of the robotic scanner and phased array probe was tested on a full nozzle mock up with induced defects in the nozzle to vessel weld. The mockup that was used is an exact replica of an actual N5 nozzle that is used for qualification of scanning systems, ultrasonic techniques and inspection personnel. The system fitted in the constrained environment that exists in nuclear facilities (Figure 12). The robotic scanner was placed and clamped onto the nozzle without calibration or fine tuning. The manipulator was able to move the ultrasonic phased array probe smoothly on the surface of the vessel and all the defects in the nozzle to vessel weld of the mockup were detected at the expected positions according to the defect plan. Furthermore, the detected defects were accurately sized using the acquired phased array data.

Figure 12. The robotic scanner on the nozzle mockup

5. Conclusions

The developed automated robotic scanner/manipulator and phased array ultrasonic testing technique is a working prototype that satisfies the requirements for nozzle inspection in the harsh and challenging environment of the nuclear plant. It can be mounted on a variety of nozzle diameters and nozzle-vessel configurations. The design and control of the robotic scanner allows the operator to place and fasten the system on the nozzle faster than any available equipment without the need of calibration and fine tuning.

Furthermore, the developed scanner/manipulator provides the operator with the flexibility of carrying out different scanning patterns, depending on the inspection needs, by using a user-friendly interface. Additionally the manipulator maintains sufficient force to the end effector to ensure constant ultrasonic coupling. As the developed scanner/manipulator system is an integrated part, it is only required to be transferred to the inspection area and clamped onto the nozzle pipe, using a quick latch mechanism. All the initial system calibration and scanning set up can be completed remotely away from the the high radiation area.

It was shown that the developed system is capable to detect all the defects with very good sizing capabilities. The flexible probe holder was shown to run smoothly on the mock up surface during scanning, and the water irrigation system provided good ultrasonic coupling. The application of 2D steering capabilities enhanced the detection capabilities of the phased array ultrasonic inspection technique when the defects were misorientated with respect to the incident ultrasonic beam.

Acknowledgements

NozzleInspect is collaboration between the following organisations: Iberdrola Generacion, Phoenix Inspection Systems Limited, PeakNDT Limited, Vermon, Innora, Cereteth and Kaunas University of Technology. Furthermore, the authors would like to thank the consortium collaborator for the access given to the full nozzle mock up that allowed the final integration and testing of the complete automated inspection system. The project was co-ordinated and managed by TWI Ltd and is partly funded by the EC.

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