Table of Contents ECNDT '98
Session: Nuclear Industry
Innovative Robotics and Ultrasonic Technology at the Examination of Reactor Pressure Vessels in BWR and PWR Nuclear Power Stations.F. Dirauf, B. Gohlke, E. Fischer
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Inservice inspections on the reactor pressure vessels of nuclear power plants not only demand high standards of technology and quality but increasingly require that more emphasis be placed on economics. The aim is to cut down on the time required for inspections thereby shortening nuclear power plant outages and to cut the cost of performing such inspections. This demands continual innovation both in the field of ultrasonic examination technology as well as regarding manipulators and robotics. The development of the SAPHIR ultrasonic examination equipment has made a considerable contribution towards achieving these goals. The equipment allows for application of phased-array technology and incorporates comprehensive data acquisition, storage and evaluation capabilities. Advanced inspection robots are being developed for use both in BWR and PWR plants which offer more flexibility of application, are faster and offer economic advantages in comparison with standard manipulators.
Non-destructive inspections on reactor pressure vessels are characterized by the following criteria: :
Fig. 1: The SAPHIR system with workstation for data acquisition and evaluation
SAPHIR requires fewer scanning passes and fewer changes to the search unit systems and has allowed the time for which the reactor well is blocked for other activities (which influences the critical path of the outage) to be reduced by more than half in German PWR plants. For example, the time recorded in Unit 2 of the Neckarwestheim nuclear power plant in 1997 was reduced to 3.75 days as compared to 8.33 days four years previously for a 100% inspection.
SAPHIR is characterized by an extremely high degree of reliability due in part to the use of advanced technologies and to the automatic internal self-testing capability, which is only confirmed by the fact that a total of twelve complete systems have been produced to date which are now being used successfully for example in Hungary, Slovakia, Ukraine, and Russia.
The equipment was introduced onto the German market in 1996 in the course of a test run in Unit 1 of the Neckarwestheim plant. In 1997 SAPHIR took over from the systems used previously and has since been used successfully for numerous examinations in Germany and abroad. This was made possible through qualification and approval by the German authorized inspection agency, TÜV, which functions as the authorized inspector for in-service inspections. A license has also been issued by SVTI in Switzerland. License applications for the USA and Sweden are also presently being processed by the competent authorities.
The SAPHIR ultrasonic device is linked to the controls for the inspection manipulator as well as to the workstation for data acquisition and evaluation by an Ethernet LAN (Local Area Network). This allows setting of parameters for system subsections, operation of the equipment and transmission of inspection data and manipulator position data during scanning. These functions are also controlled by a superordinate inspection data management system which has access not only to data from the inspection in progress but also to historical data from previous inspections allowing rapid direct comparison of results.
Fig. 2: Overview of complete SAPHIR inspection equipment system
The ultrasonic device comprises a front-end electronics package and a main electronics package as well as a control unit. The front-end electronics can be operated close to the search units at a distance of up to 60 m from the main electronics. The device allows simultaneous operation of up to 6 phased-array search units each with 16 elements, or 96 conventional search units. Any combination of the two types of search units is also possible.
In order to allow the measured data, which are supplied at a high-frequency following digitization, to be stored on standard data storage devices, the data are compressed using one of the approved and conventional methods, the pixel mode or the 'i/k algorithm' (ALOK) mode. The possibility of parameter adjustment with these data compression methods in conjunction with adjustable gates (FEB) and amplitude thresholds makes it possible for the complex signals from the inspection to be stored as a simple sequence of compressed data in a form which simplifies data analysis and the presentation of results. The use of distributed processors with real-time operating systems enables SAPHIR to offer high data processing rates even up to the acquisition of on-line A scans..
The modular design of the ultrasonic device using standard VME bus modules together with the variable configuration and the use of new advanced modules mean that system capabilities can be continually expanded. SAPHIR is designed for use as mobile equipment in problematic environments. The electronics are housed in a rugged portable casing which protect them from impacts, high ambient temperatures, dust, contamination and electromagnetic interference. Trouble-free operation is further ensured by rugged plug connectors and data transfer technology with standardized interfaces which is resistant to interference. Automatic self-monitoring capabilities ensure that relevant functions and parameters are continuously monitored and provide the operator with timely diagnostic messages. The system is run down and switched off automatically in the event that critical limits are reached.
Fig. 3: Block diagram of SAPHIR ultrasonic device
Proper functioning of all channels, as well as of the processing of all analog and digital signals can be checked within a very short time before and during an inspection using an automated procedure. A burst signal generator with highly stable amplitudes and times-of-flight is integrated in the front-end electronics for this purpose so that no external generators or calibration blocks are required. During this automatic self-test procedure, current equipment parameters are compared with those of the last calibration performed by an authorized laboratory. This allows long-term drifts to be detected. The result is output and recorded as a good/bad statement, if desired together with all the measured values and dedicated tolerances, and in the course of quality assurance is stored together with the measured data on the original data storage device.
The specification for this equipment self-testing routine was prepared together with an authorized calibration agency and qualified by the German authorized inspection agency, TÜV. It is therefore recognized as providing verification of reproducibility in line with the applicable German industrial standard DIN 25450.
The software package for data acquisition and evaluation runs on high-performance HP workstations under UNIX. Both rugged industrial computers for mobile applications and more economic desktop variations for office applications are available. The computers are equipped with two 2.6 GB magneto-optical disk drives for archiving the data. Hardcopies can be output on a fast color printer. A modem is integrated in the system to allow remote diagnosis and software maintenance.
Fig. 4: Example of SPAHIR Evaluation with Projections in Top-, Side- and End- Views
The workstation initially saves onto a magneto-optical disk all the parameter settings for the inspection equipment system in a header together with the ultrasonic measured data. The measured data are simultaneously sorted and pre-processed ready for evaluation. This includes depth-amplitude correction unless this has already been performed in the ultrasonic device. During data acquisition coupling images are computed for selected inspection functions, which can be seen immediately as a screen image. Processing and evaluation of the test results is performed directly upon completion of data acquisition.
The UNIX multiprocessing operating system allows the workstation to perform several tasks simultaneously using additional terminals (X terminals), such as data acquisition during evaluation of a previous scan or during the setting of parameters. The SAPHIR software package is based on the graphic user interface OSF/Motif. It is of a strictly modular design to allow additions tailored to customer requirements. The standard version offers the following possibilities for data evaluation. All outputs can also be documented on color hardcopies.:
The software package also contains numerous tools for preparing scans, for securing original data as well as a database for parameter settings and measured data.
In German BWR plants inservice inspections on the reactor pressure vessels are performed from the outside using manipulators inserted in the annular gap between the RPV and the biological shield and traveling on vertical tracks. To date this required a special manipulator for each reactor type onto which various pivot arms and numerous search unit systems often had to be mounted in the course of the inspection. An inspection in the cylindrical area of the RPV, including the nozzles has therefore usually taken more than 20 days up to now, which meant that the entire scope of the inspection could not be performed within a single outage. Therefore it had to be distributed over a period of several years to complete an inspection cycle.
Fig. 5: Computer Simulation of OD inspection of a BWR pressure vessel
Siemens' newly-developed inspection robot for BWR pressure vessels has a replaceable carrier so that it can be tailored to the various profiles of the vertical guide tracks in the individual power plants. Through its variable kinematics it is therefore suitable for universal applications and can replace all the standard manipulators. The robot has a pivot arm with five degrees of freedom, its shoulder joint being fixed to the carrier, and provides all-round access to all areas for examination. This precludes having to interrupt the inspection to allow different arms to be mounted on the robot. Changing of search unit systems is also no longer necessary. Using the SAPHIR equipment, a new compact search unit system with five phased-array search units has been developed which allows simultaneous inspection for longitudinal and transverse defects in the welds and in the near-surface areas as well as for radial defects at the nozzle inner corners. When used in the wave conversion mode the equipment even allows the detection of defects normal to the surface in several depth zones corresponding to the tandem inspection technique required in Germany. It then replaces the total of eight to ten different conventional search unit systems required to date for the same tasks.
As a result of these improvements the inspection can now be performed in 10 to 12 days, around half the time previously required, and can be fitted in within a single annual inspection outage. This leads not only to cost savings but also simplifies planning and logistics within the nuclear power plant. Plant operators benefit from further cuts in costs as they are no longer required to have a special manipulator on hand for each power plant.
Fig. 6: Inspection robot for OD inspection of reactor pressure vessel in BWR plants
The inspection robot for BWR pressure vessels, which has six position-controlled axes, was built up using a novel modular robot system. Each axis of the robot forms an autonomous unit with integral servomotor, gearing, position sensors and brake, and miniaturized control electronics with an intelligent microcontroller. This allows the robot modules to be connected in varying mechanical and electrical configurations with a minimum of effort. Each robot module possesses a digital address and communicates with the central robot control equipment via a "Profibus DP" serial field bus. As a consequence a single cable, as thick as one finger, is sufficient to provide the serial bus connection for the entire robot and all the drive axes as well as to accommodate the DC power supply for the robot modules.
A high-performance robot continuous-path control system using a portable PC with real-time operating system guides the robot arm with the search unit system over the surface of the RPV. Only very few operator inputs are required to define and implement the indexed scanning motions necessary for inspection of vessel welds and nozzles. Anti-collision monitoring is performed automatically by the robot. Previously-defined obstructions are automatically avoided in the scans. Actual robot movements can be monitored on video cameras and via graphic displays on the screen of the robot control system.
As described earlier, a more than 100% increase in productivity has been achieved in the ID inspection of PWR pressure vessels by optimizing the inspection technology using SAPHIR and by streamlining work procedures. The extremely short inspection times achieved are to a great extent due to the proven central mast manipulator (CMM), which has been used successfully on numerous occasions, not only in all PWR plants supplied by Siemens/KWU, but also in nuclear power plants supplied by Framatome, Westinghouse and Babcock&Wilcox as well as in Russian WWER 440 and WWER 1000 plants.
Inspection using the central mast manipulator has always been very fast and effective owing to the symmetrical arrangement in the RPV allowing direct access to all areas for inspections and straight runs through inspections without requiring repositioning and realignment of the manipulator. The concept for the new mobile underwater robot 'SISTAR' for PWR RPV inspection therefore incorporated the specific advantages offered by the CMM. However in comparison to the CMM the new robot is much smaller and easier to handle. It is immediately ready for use once unpacked from its shipping container, therefore cutting down on setup times at the beginning and end of an inspection. The reduction in equipment will also inevitably result in a reduction in the overall costs.
Fig. 7: Computer simulation showing SISTAR with two arms working simultaneously
SISTAR comprises a cylindrical platform suspended in the water which can move under water to its proposed position within the reactor pressure vessel either using propeller drives or with the aid of support cables. It then aligns itself horizontally and fixes itself in the center of the RPV using legs which are extended radially in a synchronous fashion. Depending on the task in hand, the robot is equipped with one or two pivot arms arranged on top and underneath the platform which can swivel concentric to the RPV. As in the CMM, the two robot arms can therefore reach large inspection areas over the entire circumference of the RPV at the closure head sealing area, at the nozzle flange, in the cylindrical area and in the bottom dished head and can inspect these in a single run without interruption. During the entire inspection the platform itself need only be repositioned vertically a few times. The two robot arms can perform inspections in two different areas simultaneously without hindering each other and without the risk of collision.
The measuring of the position of the robot platform in the RPV does not involve complex equipment. It is performed on the basis of UT measurement of distance from defined geometric areas and edges in the RPV which are in the respective work area of the robot arms. If sufficient benchmarks are not available in the RPV an alternative is the provision of measuring poles fastened to the RPV closure head flange which project far enough into the RPV for their bottom end to function as a reference point for the surveying.