|NDT.net - October 1999, Vol. 4 No. 10|
|TABLE OF CONTENTS|
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.
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.
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.
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
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.
|figure 3 : Schematical representation of NDT system design|
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.
|figure 4 : Scanning plate mounted on inspection robot for upper weld examination|
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.
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.
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