TABLE OF CONTENTS

Abstract Introduction The Ringhals Examination Concept for the RCP Inservice Inspection Outline of the Combined UT and ECT Procedure Design of the NDT System Qualification and Certification Inservice Inspections at the Ringhals Site Conclusions References

ABSTRACT

A new concept has been implemented for the inservice inspection of PWR main coolant pump welds, based on a robot system that allows for a NDT examination from the inner surface. This paper shall address the design of the NDT system, and the experience gained from both the qualification exercises and the inservice inspections performed at the Ringhals nuclear units 2 and 4.

Commercially available UT and Eddy current data acquisition systems were used. Customized UT transducers and ECT probes were manufactured to account for the coarse grain structure typical of the statically cast stainless steel pump casing. Dedicated scanning plates and electrical cabling were designed to interface the various probes and the data acquisition system to the inspection robot.

NDT procedure qualification (open trials) and NDT personnel qualification (blind trials) were performed to comply with the requirements of the Swedish nuclear authority. Relevant examples from the open trials shall be presented to illustrate the excellent examination capability of the method.

The first inservice inspection according to the fully qualified new concept was performed in the Ringhals 4 unit, in September 1997 : the scheduled 7-day period for the intervention was respected. For the second inservice inspection, performed in May 1998 in the Ringhals 2 unit, the examinations were completed in less than 5 days. In both cases, the detailed interpretation of all NDT examination data was finalised and reported less than 48 hours after completion of the last scanning sequence.

INTRODUCTION

Inservice inspection of primary loop and reactor coolant pump welds of pressurised water reactors (PWR) has over the years been performed to a limited extent, due to the difficulties encountered when performing ultrasonic examinations on heavy wall cast stainless steel components. Indeed, the propagation of acoustic waves is strongly disturbed by the coarse-grain austenitic metal structure : the major problems occurring are beam skewing and distortion, high background noise and uncertainty on the propagation velocity (1).

In January 1995 however, the Swedish regulator SKI imposes new rules for the inservice inspection of nuclear power plants, including more stringent requirements regarding the volumetric inspection of certain areas for remaining manufacturing flaws and service induced defects. In addition, a performance demonstration is demanded for all NDE activities within the framework of the inservice inspection programme. A formal qualification of the NDT method, the equipment and the personnel is now to be performed, based on the demands of the utilities and approved by the Swedish Qualification Body (SQC). In order to comply with these new requirements, the Ringhals plant initiated a project, aiming at performing inservice inspection on a reactor coolant pump in the summer of 1997, and on the complete primary loop in a second phase (2). Qualification and in service inspection performed with a NDT examination method for the RCP inspection shall be adressed hereunder.

THE RINGHALS EXAMINATION CONCEPT FOR THE RCP INSERVICE INSPECTION

A new concept was chosen for the inservice inspection. The main objective of inservice inspection is to detect service induced defects, which most often occur as surface breaking defects on the inner surface of the considered component or weld. Therefore, taking into account the difficulties induced by the statically cast austenitic structure, non-destructive examinations performed from the inner surface of the pump casing should provide improved detection and sizing capability.

Over the past decade, efforts have been made to reduce radiation exposure for NDT personnel, according to the so-called ALARA principle. This strategy inevitably results in the use of remotely operated systems. Thus, a dedicated robot system was manufactured for inspecting the reactor coolant pump from the inside (3). Operated from a graphically programmed workstation this robot system allows access to all areas to be examined, while avoiding collisions with the pump casing.

Based on the outcome of a round-robin test involving several NDT companies (4), AVI was chosen to develop a combined UT and ECT procedure for the RCP casing welds and the RCP to inlet elbow weld, covering the inner third of the wall thickness.

OUTLINE OF THE COMBINED UT AND ECT PROCEDURE

figure 1 : Pulse-echo examination plan for parallel flaws in RCP casing weld

figure 2 : Pulse-echo examination plan for parallel flaws in RCP to inlet elbow weld

The procedure is largely based on a recent design of twin-crystal (TRL) search units, developed subsequently to the capability studies on austenitic steel components conducted in the PISC III programme (5,6). As the beam focusing technique yielded good results on heavy section components made from cast or wrought austenitic structures, an attempt was made to obtain contact TRL probes generating acoustic beams similar to those of focusing transducers, so as to avoid the field implementation difficulties associated with the size of these. Curved piezoelectrical elements were found to provide a solution by adding an optical focusing effect to the pseudo-focusing resulting from the convolution of the transmitter and receiver beams. The efficiency of this new concept was successfully tested during the Ringhals round-robin test and was maintained in the procedure prepared for the qualification exercises.

For embedded flaws, the procedure prescribes 45° and 60° TRL probes with a frequency of 1 MHz for both weld types. For the pump casing welds, a supplementary 45° TRL probe at 0.5 MHz is required for examination of the depth range between 35 and 80 mm. In addition, straight beam TRL search units with various depth ranges are required. The examination plans for pulse-echo techniques searching for flaws parallel to the weld are shown schematically on figure 1 (RCP casing weld) and figure 2 (RCP to inlet elbow weld) . As transverse flaws must also be searched for in the pump casing welds, additional transducers of the same types are to be oriented parallel to the weld centre line.

The TOFD "obstruction method" is a complementary but essential part of the NDT examination procedure. It is based on the interruption of the acoustic energy transmission by a surface flaw that would lie between the transmitter and the receiver probes, and proved to be very efficient for detection and accurate sizing of near-surface defects in highly attenuating materials. Single crystal compression wave probes have been selected for the implementation of this technique.

Eddy current testing is prescribed to confirm the presence of surface-breaking flaws, and to accurately measure their length. A dedicated Ghent style magnetically biased driver pickup probe was developed. Actually, this probe type contains two separate receiver coils, for the detection of both parallel and transverse flaws. Its performance was assessed on 1 mm deep and 0.15 mm wide spark-eroded notches introduced in test specimens representative for each of the weld types.

DESIGN OF THE NDT SYSTEM

The NDT equipment to be provided for the inservice inspection must fulfill a number of specific requirements. First of all, the examination is to be conducted from the inside of the pump casing, while it is completely filled with water. Furthermore, it was decided that all equipment should be controlled from outside the containment building. Moreover, the NDT system must be fully compatible with the OASIS inspection robot, and efficient troubleshooting procedures are required to reduce delay times to maximum a few hours in case of any equipment failure.

NDT equipment


figure 3 : Schematical representation of NDT system design

A portable Tomoscan system (R/DTech) is used for recording and storage of examination data. The system is equipped with a remote 16-channel pulser/receiver unit, mounted in a dedicated splashproof housing, located in the robot housing. The eddy current probe is connected to a TC5700 unit (R/DTech) serving as a front end, and the analog X-Y output signals are routed to the Tomoscan system. This allows for a coherent data recording of both UT and ECT signals. Position information from the various probes is obtained by connecting the encoder inputs of the Tomoscan system to a robot interface unit generating pulses compatible with incremental encoder signals. A schematical representation of the NDT system design is shown on figure 3. Data analysis is carried out on a PC-based workstation, equipped with the Tomoluis software. Tomoluis can provide compatible graphical images of UT and ECT data, thus simplifying the integration of the examination results of both techniques.

Probe design and fabrication
Only contact probes are used. The surface of the probe shoes is machined to comply with the scanning surface in the best possible way. All probes are prepared to be used in complete immersion : the cables are moulded into the housing, leaving no connector at the probe side.

Mechanical interface


figure 4 : Scanning plate mounted on inspection robot for upper weld examination

Dedicated scanning plates were designed and manufactured to interface the various transducers to the inspection robot (see figure 4). Each scanning plate contains between 4 and 6 probes, each of them mounted into a double gimbaled support. The supports are individually spring loaded in order to insure correct mechanical contact with the pump inner surface. The vertical compliance of 35 mm should be sufficient to cope with the existing tolerances on the geometrical shape of pump casings. The use of quarter-turn fasteners allows for easily mounting of the scanning plates onto the arm of the inspection robot, even for an operator wearing protective clothing and gloves.

QUALIFICATION AND CERTIFICATION

To comply with the rules imposed by the Swedish regulator, a formal qualification programme was setup and witnessed by SQC. In a first step, the capability of the OASIS inspection robot was demonstrated, and positioning accuracy and repeatability were found satisfying.

The second stage consisted of the qualification of the NDT procedure and the equipment. During this stage, the examination capability of the system in terms of flaw detection, length sizing and height sizing was assessed on a limited number of flaws in "open" testblocks. All qualification exercises were performed on the Ringhals RCP mock-up, consisting of a real pump casing with an inlet elbow welded to it. A limited zone of the mockup is considered as an "open" testblock, while the largest part is used as a "blind" specimen. Obviously, the latter part was kept sealed during all preparation work.

figure 5 : Real flaws in "open" pump casing qualification specimen as detected by the TOFD technique; rough cluster of small cracks (left) and smooth lack-of-fusion (right)

Figure 5 shows ultrasonic data obtained with the TOFD obstruction technique during the procedure qualification exercise: all near-surface flaws were reliably detected, and through-wall sizing was performed with errors smaller than 4 mm. In addition, the pulse-echo examination revealed all embedded flaws within the volume to be examined, without reporting any false calls.

After the formal qualification of examination procedure and equipment, each individual of the proposed NDT team for the inservice inspection had to pass a qualification exercise for the tasks within his activity scope. For both data acquisition and data interpretation (flaw detection and flaw sizing), NDT operators succeeded to meet the stringent qualification criteria during "blind" tests on real defects.

A three month period, from May to July 1997, was needed to complete the qualification process. Moreover, EN-45001 accreditation certificates were granted from both SWEDAC (Swedish accreditation body) and BELTEST (Belgian accreditation body). The former certificate is imposed for the performance of inservice inspections in the Swedish nuclear power plants.

INSERVICE INSPECTIONS AT THE RINGHALS SITE

Two inservice inspections have already been performed at the Ringhals site. A complete NDT team for this type of intervention consists of 5 persons : 2 operators for remote control and maintenance of the inspection robot, 2 qualified operators for acquisition and interpretation of the NDT data, and a team leader for supervision and coordination of the operations and communication with the plant staff. After the installation and the functional test of the system, only the operator performing the robot maintenance has to enter the containment building regularly. The complete inspection is performed by two NDT teams, working 12 h shifts. The quality assurance for the inservice inspection has been formalised in a Specific Organisation Plan, containing work procedures for both site organisation and technical activities. These procedures were extensively tested during the qualification exercises and the various audits conducted in the framework of the EN-45001 accreditation process, and were thus well assimilated by all operators involved.

A chronological checklist, mentioning all operations to be performed, is filled in by the team on duty. This allows for a smooth transition between successive workshifts, and is also used to monitor the progress of the intervention.

The detailed work procedures, with clearly identified tasks for the various team members, proved to be extremely useful for rapid problem solving in case of equipment failures or other unplanned events. Moreover, the existance of a "safety net" clearly reduced the stress on the individual operators.

Acquisition and interpretation of NDT data were performed according to the qualified procedures. Preparation, modification and identification of all documents and computer files related to the inservice inspection are submitted to strict rules. As a result of this, the detailed interpretation of all NDT examination data could be finalised, reported and archived on optical discs less than 48 hours after completion of the last scanning sequence. The first inservice inspection according to the fully qualified new concept was performed in the Ringhals 4 unit, in September 1997 : the scheduled 7-day period for the intervention was respected. For the second inservice inspection, performed in May 1998 in the Ringhals 2 unit, the examinations were completed in less than 5 days.

CONCLUSIONS

• A new concept was designed and implemented for the inservice inspection of PWR main coolant pump welds from the inside.
• Both the NDT system and the personnel passed the formal qualification exercise imposed by the Swedish nuclear authority.
• An EN-45001 accreditation certificate was granted for automated NDT in Swedish nuclear power plants.
• Two RCP inservice inspections were successfully performed, Ringhals 4 in September 1997 and Ringhals 2 in May 1998.
• The quality management and the specific personnel training induced by the qualification process were found to positively influence the conduct of the intervention : reduced occurrence and better control of unplanned events, reduced operator stress and improved cost effectiveness

References

1. Silk M.G., "Ultrasonic techniques for inspecting austenitic welds", in Research Techniques in NDT, Vol. IV, Academic Press, 1980.
2. Fernström H., "Inservice inspection of PWR reactor coolant loops - A challenging project", 14th Int. Conf. on NDE in the Nuclear and Pressure Vessel Industries, Stockholm (Sweden), September 1996.
3. Attaar M., Alford J.W., Davis J.B., Pratt W., Kwech H., "Robot for internal inspection of an RCP casing", 14th Int. Conf. on NDE in the Nuclear and Pressure Vessel Industries, Stockholm (Sweden), September 1996.
4. Martinsen H., "Results from Round Robin Test performed by Vattenfall Ringhals on cast stainless steel", Report nr. 0788/96, June 1996
5. PISC Report nr. 34, "Report on the evaluation of the inspection results of the cast-to-cast PISC III assemblies 41, 42 and weld B of assembly 43", EUR 15664 EN, 1995.
6. PISC Report nr. 35, "Report on the evaluation of the inspection results of the wrought-to-cast PISC III assemblies 51 and weld A of assembly 43", EUR 15665 EN, 1995.
7. Dombret Ph., Maes G., Cermak J., Davis J.B., Attaar M., "Development of a UT/ET procedure for the internal examination of PWR main coolant loop components", 14th Int. Conf. on NDE in the Nuclear and Pressure Vessel Industries, Stockholm (Sweden), September 1996.