Abstract
The reliability and performance of a steam generator (SG) is one of the serious concerns in the operation of pressurized water nuclear power plants. Because of high levels of radiation, robotic systems have been used to inspect and repair SG tubes. In this paper, we present a mobile robotic system that positions the inspection and repair tools while hanging down from the tube sheets where the tubes are fixed. All of the driving mechanisms of the mobile robot are actuated by electric motors to start its works, providing that the electric power is prepared without the additional need for an on-site air services. A special tube-holding mechanism with a high holding force has been developed to prevent falling from the tube sheets, even in the case of an electric power failure. We have also developed a quick installation guide device that guides the mobile robot to desired initial positions in the tube sheet exactly and quickly, which helps to reduce the radiation exposure of human workers during the installation work. This paper also provides on-site experimental results and lessons learned.
1. Introduction
The reliability and performance of steam generators are crucial in pressurized water nuclear power plants, because the tubes in the steam generator are the barriers between the highly radioactive primary coolants and the uncontaminated secondary coolants, and should transfer the heat of the primary coolants to the secondary coolants efficiently [1]. Thus, the nuclear regulation authority requires that the tubes are inspected periodically, typically using eddy-current testing methods, to determine if there are any defects such as cracks inside [2]. Several thousand tubes are fixed to a semi-circular plate called a tube sheet, under which is a very highly radioactive water chamber, but human workers cannot endure such high radiation exposure during the inspection and repair tasks.
Thus, for such steam generator inspection and repair tasks, remotely operated robotic systems have been used [3]. There are two main types of robots for such applications. One is a fixed-type manipulator arm [4–6], and the other is a mobile-type robot [7–9]. The base of the manipulator arm robots is usually secured on the entryway, called a man-way, which is provided for human entrance into the water chamber of the steam generator. Inspection and repair tools are mounted on the end-effector side of the manipulator arm and can be positioned by controlling the joint angles between serially connected links of the manipulator arm. In order to access the very large area of the tube sheet from the fixed base, some of the links have to be long. On the other hand, a mobile-type robot can move its base from one tube position to another by hanging down from the tube sheets while holding the internals of several tubes with tube-holding mechanisms and repetitively changing the holding tubes.
Such mobile-type robots have several advantages over the manipulator arm robots. The size and weight of mobile-type robots are relatively so small that one human worker can carry one, compared to the fixed-type manipulator arms, which are about 3 to 4 metres at full stretch and can take above 40kgs in weight. In addition, when the manipulator arm is fully extended while carrying a heavy tool, the end tip can point toward the wrong tube positions owing to the deflection of its links and joints. Thus the kinematic parameters of such a manipulator arm should be statically calibrated within the steam generators to make the position errors as small as possible, in accordance with installation sequences as described in [6]. However, in comparison, the positioning error of a mobile robot can be so small that there are no problems when inserting tools into the tubes, because the links between the tool and the base are relatively short and it can move its base as close to the target positions as possible. Another advantage of the mobile-type robots is that several robots can be used simultaneously to work in one steam generator chamber. Therefore, the time necessary to complete their job can be drastically reduced by increasing the number of robots and enabling them to cooperate without collision. As a result, the mobile robots could be preferred to the inspection and repair works in steam generators.
However, such mobile-type robots [7–9] have only started to be used recently in nuclear plants because they have some possibility of falling from the tube sheets when their tube-gripping functions fail. They all have fault-tolerant pneumatic tube-holding mechanisms to prevent them from falling down, even when the air supply has failed. However, their usage of pneumatic actuators means that air compressors or servicing air should be prepared prior to their operation.
Installing and tearing down mobile-type robots is another problem because they have to be mounted to the tube sheets securely and the mounted tube position should be confirmed as the desired position. Usually, one human worker manually inserts a stick tool, which has a pulley with rope at its end, into the water chamber and finds the desired position to fix the pulley onto the tube sheet. They should then bind the robot with the rope, and pull the rope to raise the robot. Such an installation procedure is relatively time-consuming and it is inevitable that human workers will be exposed to high radiation doses when examining the position of the pulley and the status of the robot in the highly radioactive water chamber through the man-way.
To overcome the problems mentioned above, we have developed a fully electrically driven mobile robot and a quick installation guide device.
This paper describes the mobile robot system architecture, including the mechanical architecture of the robot and the control system in Chapter 2, providing experimental results with a mock-up environment in Chapter 3. Chapter 4 concludes with the lessons learned during the pre-service inspection of a real steam generator of a nuclear power plant in South Korea.
2. Description of the Mobile Robot System's Architecture
2.1. Mechanical architecture
Figure 1 shows the overall architecture of the mobile robot. The mobile robot consists of three main parts: a body module, an arm module and a tool guide module. The arm module and the body module are connected by three joints denoted by BLT (Body Lateral Translation), BVT (Body Vertical Translation) and BVR (Body Vertical Rotation). With these joints, the arm module can move in lateral or vertical directions, and rotate along the vertical axis relative to the body module. The tool guide module is connected with the body module by two joints denoted by TVT (Tool Vertical Translation) and TLT (Tool Lateral Translation), and it can also move longitudinally and vertically relative to the body module. At the end of the tool guide module, there is a dovetail joint to attach tools.
Overall mechanical architecture
Each body module and arm module has four finger-like tube-holding mechanisms for holding tubes. In cases when the body module holds some tubes, the arm module can move and hold other tubes by changing its positions and orientating via the sequence of lowering down, lateral movement and rotation, rising up and holding again. When the arm module rises to hold tubes, the touch sensors mounted on the top faces of the arm module can sense and confirm the contact with the tube sheet.
When the arm module holds the tubes, the body module can also then change its holding tubes with similar sequences of movement. In this way, the mobile robot can access all of the tube areas.
Figure 2 shows an example of moving sequences and of how to change the position and orientation of the mobile robot for a steam generator with triangular-type tube arrays. In Figure 2(a), the tube-holding fingers of the arm module are engaged with the tube sheet, but those of the body module are disengaged and lowered down. As shown in Figure 2(b), the body module is moved in the right direction. Figure 2(c) shows that the tube-holding fingers of the body module are engaged again after raising the body module. Then, the tube-holding fingers of the arm module are disengaged, the arm module is lowered down, rotated clockwise and raised up again, then the fingers are engaged as shown in Figure 2(d), 2(e) and 2(f), respectively.
Example of moving sequences
To place the end position of the tool at the desired target tube, the arm module is usually engaged and the body module is lowered down. Because the tool guide module is attached to the body module, the tool can be positioned and oriented in the 2-D space parallel to the tube sheet plane using 3 DOFs which are the rotation and lateral translation of the body module and the longitudinal translation of the tool guide module.
The working area of a single probe guide tool, viewed from below the tube sheet plane, is shown in Figure 3. In this figure, the black solid line is the boundary of the working area that the single probe guide can access. Thus, this mobile robot can inspect 212 tubes in a single position. Because it has 3 DOFs for moving the tool, there is a kinematic redundancy for accessing some of the tubes.
Working area of a single probe guide tool
For ECT inspections, we usually need a dual probe guide tool which can guide two ECT probes simultaneously to increase the inspection speed. In that case, the dual probe guide tool is positioned at the target tubes with exactly 3 DOFs.
As previously mentioned, one of the main differences of our mobile robot with respect to the previously developed robots is that our mobile robot uses electric motors to hold the tubes. Figure 4 shows the finger-like mechanical structures of the tube-holding mechanism. The expandable sleeve has several slits that can be easily expanded radially, and internal tapered surfaces on both ends. The movable shaft and the boss also have outer tapered surfaces that match the internal tapered surfaces of the sleeve. When the movable shaft is pulled down toward the boss, the radius of the expandable sleeve increases, and the outer surface of the sleeve then comes into contact with the internal surface of the tube.
Mechanical structure of tube-holding mechanism
The outer surface of the top head of the movable shaft is also tapered to facilitate the inserting of the sleeve, even if there are some positioning errors with the tube's position, such as gear backlashes.
Two movable shafts are pulled by one electrical motor through a nut-screw mechanism. Because the nut-screw mechanism prevents backdriving, the contact between the sleeve and tube can be sustained even if the electric supply fails. A total of four DC motors are used to drive eight fingers. To release the holding mechanism manually while the motor is stuck, there is a clutch mechanism between the screw and motor.
Even though the operation of such an electric holding mechanism is somewhat slower than pneumatic methods, it provides benefits for workers, allowing them to resume their jobs quickly without having to wait for the air supplementation. In addition, the frequency of the engaging and disengaging operation is not high because the mobile robot has a relatively large working area of the tool. Therefore, the inspection speed is not slow when compared with the previous mobile system.
Another feature of our system is that it enables quick installation of the mobile robot underneath the tube-sheet plates. Unlike the previously developed mobile systems, a rigid guide device has been developed for installing the mobile robot, as shown in Figure 5. During the installation, at first, the fixture part of the guide device is mounted to the man-way flange of a steam generator. Then, a curved rail is slid in along the fixture part until the support pin is inserted into the desired tube. Because the guide device is sufficiently rigid, the support pin of the curved rail can be inserted securely into the exact tube, as designed, by simply pushing up the curved rail. Then, the basket carrying the mobile robot slides along the curved rail via a basket drive motor. When the mobile robot arrives just underneath the tube-sheet, the arm module holds the tubes.
Guide device for installation
In cases when the robot needs to inspect the area around the support pin of the guide, the guide should be uninstalled to avoid collisions. In addition, to retrieve the robot after all the inspections are finished, the guide should be installed again and the robot moves to the initial mounted position of the tube sheet.
Since such an installation procedure is so simple and does not take much time to finish, and human workers need not examine the water chamber of the steam generator but are able to recognize all circumstances via a monitor displaying the camera view of the tube sheet during the installation operation, the radiation exposure of workers during installation can be reduced significantly, even though the installation guide device is relatively heavier to carry than the installation sticks of the previous mobile robot systems.
2.2. Control system architecture
As previously mentioned, our mobile robot is fully electrically driven to remove the need for air supplementation. The robot is driven by nine electric DC motors in total.
Each DC motor is controlled by one DC motor controller, and the motor controllers communicate with one remote controller through the CANOpen protocol [10]. All controllers are embedded into the mobile robot to simplify the wiring between the mobile robot and the remote controller. Thus we need only one cable, including DC power lines for controllers and CAN communication lines. The remote control system consists of one laptop computer used as a remote controller and one 24V DC power supply, and is easy for one human worker to carry, as shown in Figure 6.
Portable remote control system
In contrast to the single-user interface of the previously developed mobile robot systems, a multiple-user interface can cope with the design of the control software architecture. In short, one human operator controls one mobile robot. In cases when the operator has to conduct small amounts of repairs in a short period of time, it is convenient for the operator to control the robot near the steam generator. However, when the operator has to inspect large amounts of tubes for a longer period of time, it is better for the operator to conduct the task in the control room outside the containment building of a nuclear power plant.
Thus, the control software is divided into two parts, a server and a client, as shown in Figure 7. The two software parts communicate with each other through the TCP/IP protocol. Such a structure enables two parts to run in either one computer or two computers connected by an Ethernet cable. The server software deals with time-critical tasks, such as over-current checks and error message treatment for the motor controller to disable the controller in the event of faults. The client software has a GUI and a 3D graphics module for a human interface, and it also has functions for sending motion commands to the server software and receiving status information from the server software.
Server-client software architecture
The server part is designed to communicate with multiple client software. In multi-user applications, only one user is allowed to operate the corresponding mobile robot, but other users can monitor the status of the mobile robot, just like the operator. For a short operation, the server and client software run on the computer of the remote controller, and one operator controls the mobile robot near the steam generator. If monitoring of the job is necessary outside of the containment building, an additional Ethernet cable can be connected, through a cable penetration hole of the plant, to the monitoring computer, on which another piece of client-software runs. Even in this case, the client-software in the monitoring mode can be switched to the operation mode to control the robot directly, after control privileges have been obtained. This software architecture considering the distributed multi-users is so versatile that it is highly adaptable to various situations.
The GUI module of the client software consists of the set-up panel, the test panel and the work panel, as depicted in Figure 8. With the set-up panel, the operator can start the joint and position initialization after the selection of the target steam generator, a robot and a tool. The test panel is provided for testing the robot and finding the cause of problems in the velocity control mode or the position control mode. With the work panel, the operator can move the robot to any desired target tube position following the automatically generated path, conducting inspection and repair tasks according to the work plans. This panel also provides the work planning function customized for the preference of operators. The joint compensation control function is included for compensating the tube positioning errors that are induced by large disturbance forces by conduit tubes.
Hierarchy of the GUI module
3. Implementation and Test
Two versions of the mobile robot have been developed based on the mechanical design concepts previously mentioned. The first version is for medium-sized steam generators with a small man-way, such as OPR1000, and the second version is for large steam generators with a large man-way, such as ARP1400 used in South Korea. The first version is relatively small and lightweight, and has a smaller stroke length for the horizontal body motion and no vertical tool motion, as shown in Table 1. Maxon DC motors with magnetic encoders are used for the movements and tube holding. The movements depicted in this table are implemented in position control mode, but the tube-holding function is implemented by the current control mode to compensate for the uncertainty of the tube's internal diameters. The second version is better packaged than the first version, which exposes some cables.
Specifications of mobile robots
During the inspection and repair work, the water chambers of the steam generators are not filled with water. Therefore, such robots do not have to be waterproof, but we may need to feed dry air through the robot's body to reduce humidity in damp atmospheres.
Figure 9 shows a test motion sequence for the body's movement underneath a tube sheet mock-up.
Movement test
We have conducted computational analysis for the contact force between the expandable sleeve and internal surface of the tube in order to examine whether any defects occur by the engagement of the finger, as shown in Figure 10.
FEA Model and Result
The simulation results show that the maximum stress of the tube is about 75MPa, and such a value is sufficiently less than the yielding stress of INCONEL 690 tubes, which is about 461MPa.
We also conducted repetitive engaging and disengaging experiments and visually inspected the internal surface of the tubes. Figure 11 shows the finger used for the experiments and the image of the internal surface of the tube. The blurred circular marks on the surface were found not to be defects.
Experimental finger mechanism and tube
An installation guide device was manufactured and tested in the mock-up; the total installation time was around two to three minutes. Figure 12(a) shows the installation guide after the support pin is secured to the tube sheet mock-up, and Figure 12(b), (c) and (d) show that the mobile robot was carried on the basket, entered into the man-way mock-up and engaged with the tube sheet mock-up.
Installation guide device in the mock-up test
Figure 13 depicts the graphical user interface of the client software when the robot was conducting an inspection task according to a work plan.
Graphical user interface of the client software
4. Conclusion
The new mobile-type robotic systems developed for the inspection and repair of steam generator tubes are fully electrically driven system, without the requirement for service air and including a rigid installation guide device. Such features aim at improving the working time and radiation exposure reduction. Their control system architectures have flexibility for easy operation of the mobile robot in various circumstances.
The second version of mobile robotic system was deployed for the PSI (Pre-Service Inspection) of the steam generator tubes at Shin-kori #3 in 2012.
This nuclear power plant is APR1400 [11], which has two steam generators with water chambers about 2.4 metres in radius, and our system conducted ECT (Eddy Current Test) inspections for all of the tubes of one steam generator. Two mobile robots were used for the hot leg and cold leg tubes, and were operated from a container room at a distance of about 100 metres from the steam generator. During the deployment, several problems were found to have been resolved. The reaction force during the ECT probe shooting was relatively larger than we had expected. Thus, some mechanical parts were redesigned to improve the endurance capability for such high loads. The revised models are undergoing continuous tests at Shin-kori #4, so that they can be used successfully for PSI and ISI at the nuclear power plant.
Footnotes
5. Acknowledgements
This work was supported by the national mid- and long-term nuclear R&D progrm(NRF-2012M2A8A1029350) in South Korea.