TIME-MIDAS: all in one inspection of PWR pressure vessels30 July 1998
A new automated system for carrying out a complete volumetric examination of PWR reactor vessels has been developed by Tecnatom. The system integrates the two main elements of an automated inspection system: a manipulator, called TIME, and a Data Acquisition System, called MIDAS. Together they provide new possibilities in reducing times and improving reliability for in-service inspection of reactor pressure vessels.
Nowadays in-service inspection (ISI) contractors for nuclear power plants have to face up to continuously increasing competition as a result of the deregulation of the power market. The reduction in ISI costs cannot be achieved only by dropping service prices. Of more potential value, ISI contractors can help their clients by increasing plant availability through two drivers: first, providing a high quality, reliable inspection will ensure the continued good operation of the plant; and secondly, minimising intervention time for the inspection will keep down the length of the outage and reduce radiation doses.
In its pursuit of a major improvement, Tecnatom set out the components of an advanced automated examination system with a multiple technique capability (eg ultrasonic, eddy current and visual) which will provide a more effective ISI service for PWR pressure vessels:
• A reliable, fast and versatile data acquisition system (DAS). This will be validated on a mock-up containing realistic flaws.
• A manipulator which uses the most advanced technology available, able to support the capabilities of the DAS. Such a device will have to meet demanding safety requirements, as well as to function quickly to minimise vessel occupation time and shorten the critical path of the outage.
In addition, the company also sees the need to establish a team of people skilled in the use of such sophisticated equipment to ensure a very high quality service.
Having been performing inspections of reactor vessels since 1981, Tecnatom made substantial use of its accumulated experience and knowledge in launching an internal project to develop a new integrated RPV inspection system to meet its aims. The new system includes a DAS, called MultItechnical Data Acquisition System – MIDAS – and a mechanical scanner, called Tecnatom’s Inspection Mechanised Equipment – TIME. Tecnatom’s TIME-MIDAS will provide automated examination of reactor vessels for the new millennium.
MECHANICAL EQUIPMENT: TIME
The objective pursued for the mechanical equipment was to develop a system that operates effectively in all areas required in the inspection of the reactor vessel and that reduces the costs of inspection preparation, system transportation, assembly, operation and decontamination.
TIME is composed of an extendible central mast of squared cross-section, supported by three feet that rest on the vessel flange. The equipment is centred on the vessel flange using guide pins on two of the feet (shown in figure); the third foot rests on a special pneumatic system.
The feet have an adjustable wedge system for connecting to the mast, making the system adaptable for use on vessels of wide diameters.
The mast consists of five tubes, the outer is fixed and the other four telescope outwards with a guide system to a maximum extension of 11 200 mm, making possible the inspection of all types of PWR vessels, eg Westinghouse, KWU, VVER, and other designs.
The inner most tube of the mast has a flange on which is mounted the boom rotating system, made up of three different sub assemblies:
• The rotation drive system.
• The cable guide system.
• Four extendible inspection arms (two by two) which are interchangeable.
The two upper arms support the modules for inspecting the vessel inner radius welds and the nozzle (safe end) to primary pipe welds as well as the threaded areas and the flange to shell weld. The two identical arms are interchangeable and can be extended radially to handle different vessel diameters without the need for costly modifications.
The arms are constructed from square tubes that provide a high enough rigidity to ensure that the maximum deflection of the telescopic arms is less than 5 mm with a load of 50 kg at its maximum extension of 2000 mm. This is more than adequate to inspect the nozzle to primary pipe welds.
Below these are the other two arms that carry the modules for the inspection of circumferential, longitudinal and meridional welds of the vessel wall; they also permit the inspection of the nozzle to vessel weld from the vessel wall. Similar to the upper arms in construction, they have a smaller extension as the areas to be inspected do not require as long a reach. One of them has a pivoting device (shown below) that permits the inspection of meridional welds at the bottom of the vessel using appropriate tools, robotics arms, etc.
There are specific modules for the inspection of each area of the vessel. The flange to vessel weld inspection module is the only one that does not rotate as this motion is not necessary. All other modules have their own rotational system.
The module used for the inspection of the threaded zones is mounted on the same arm as the flange to shell weld module. The opposite arm carries the module used for the inspection of the vessel inner radius welds, the nozzle to vessel weld and the nozzle to primary pipe weld.
Finally, two identical modules are mounted on the lower arms. These perform the inspection of circumferential, longitudinal and meridional welds of the vessel shell. The meridional welds are inspected using the vertical pivoting arm.
The control system is based on the SIROCO VME which is the current standard instrumentation Tecnatom uses for applications that require high performance of the mechanical equipment. This system is interconnected through an ETHERNET line to the operating computer, a PC with a user graphical interface for operating the mechanical equipment and also monitoring the operating conditions.
The SIROCO VME controller performs the low level control in real time of the mechanical equipment. A specific card set connected to the VME bus provides the basic control of motors and the management of all input-output signals.
This configuration makes possible the separation of the operational and inspection zones. The operating computer communicates with the SIROCO VME controller through an optical fibre line. An additional communication line between the controller and the data acquisition system provides position data of the manipulator in real time. The control of the manipulator is performed via a terminal (portable computer or a manual console).
As a safety measure, mechanical equipment is provided with reference (guide) points and physical limits (to limit all arm motions, eg extension or rotation movements). The control system also has a logic program to check movement before the motion reaches the physical or software programmed limits, allowing work to be undertaken with a greater safety margin.
All movements that require greater attention have additional safeguards to prevent undesired motion.
In the event of failure of plant air supply, there is an auxiliary line attached to two bottles of nitrogen which are automatically switched on to prevent the loss of pressure in the watertight box with the equipment. This allows sufficient time for equipment extraction if necessary.
The mast and the inspection arms also have a manual retraction system for emergency situations, such as the loss of plant electrical supply, etc.
A number of auxiliary systems are incorporated, including a calibration block with side drilled holes to provide the periodical calibration checks as required by the relevant Code.
A minimum of two colour cameras are also carried. These have a zoom and motorised focus and are mounted on a pan and tilt device to allow movement in all directions. This provides permanent visual images on the screens of the DAS allowing a full monitoring of the movement of the equipment.
A MULTITECHNIQUE DAS
As the choice of architecture will lead to a set of specific equipment requirements, this was the most important consideration in developing the multitechnique data acquisition system. We also considered if any DAS presently available on the market is close to the desired one and what would be involved in developing a fully integrated system.
To obtain the desired goal of a full integration of techniques, the data acquisition system must have the following:
• The capability to integrate data from different techniques, from both the hardware and software points of view.
• Specific tools for each technique.
• Common tools for every technique used.
The first requirement is clear: a multitechnique system must be able to meet the data acquisition needs for every technique used with all the hardware needed contained in the same “box” and under unified control. This means that a global approach, which takes account of the requirements of all techniques, is adopted.
Normally, systems developed for a specific technique do not take account of the needs of others, as designers concentrate on their own systems. This normally leads to non-general solutions, making it quite difficult to achieve the desired “open” system for many techniques and also to avoid duplication of hardware and software. It was therefore quite a challenge to use several of these as modules to create a multitechnique system, including hardware and software, without duplication.
The multitechnique approach should provide many benefits including:
• A decrease in total time of inspection. Many of the functions and steps to be performed need be done only one time.
• A decrease in amount of equipment, data links etc. Communications between different modules are minimised.
• An improved quality of inspection with fewer possibilities of errors. A single central control capability over all the system also facilitates this.
We also took account of how actual inspections are carried out in nuclear power facilities. Typically, nuclear plants have controlled and non-controlled areas. All hardware directly involved in signal generation and acquisition are placed in controlled areas in order to decrease the length of cables and, therefore, noise generation. Computers providing control, data storage and analysis are normally placed in non-controlled areas. The equipment in controlled areas must have links for data, video, voice and general communication with computers outside. Usually there is no easy access for installing the cables. The ideal solution is to have only a single link between the acquisition hardware and computers. This approach: decreases the total time and cost for hardware installation and increases reliability of the system – having central control by one system decreases the possibility of error, making this configuration the most robust one.
The resulting architecture is implemented in MIDAS as shown in the diagram. Only one link between data acquisition hardware and the computer ensures noise free communication.
As mentioned above, the general philosophy of the design is to obtain an open system, that is, one with no restrictions to adding new hardware. Many of the commercially available systems cannot be easily developed to meet new needs.
And another important benefit, integrating all acquisition hardware within a single control system leads to full integration of data and a unified data format which will also facilitate development and planning of software, calibration and analysis.
In addition, to obtain the high level of performance needed, the computer must have considerable power to process data and provide complete control of the system. This requires:
• Fast communication between acquisition hardware and the computer to gather the maximum amount of data.
• High capacity for storage and processing of data to ensure that there is no limitation to components inspected.
• A capability to perform many functions in real time, eg data reconstruction, preprocessing and backup, which will decrease total time needed to complete all steps of the inspection.
All these requirements have been taken into account in the design of the MIDAS system, in order to integrate ultrasonic, eddy current and visual techniques.
The system has been used in the field, with excellent results, not only from the point of view of evaluation results but in the reduction of total time of inspection. Inspection was carried out with ultrasonic and eddy current simultaneously, and fault indications found with ultrasonic techniques on surfaces were also validated using eddy current inspection.
Developing the TIME and MIDAS systems required the experience and knowledge accumulated by Tecnatom over many years in the design and the manufacture of several tools for different inspection applications in the nuclear industry. The result is an advanced integrated multitechnique system that is a powerful tool for carrying out a high quality pressure vessel inspection and for providing a better evaluation of results.
TIME-MIDAS will be used for the first time in a ten-year ISI next year. The time needed to perform the full inspection is estimated at three days.