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Application of an ultrasonic data recording and processing system to reactor pressure vessel examination
Int. J. Pres. Ves. & Piping 35 (1988) 307-320
Application of an Ultrasonic Data Recording and Processing System to Reactor Pressure Vessel Examination
S. R. Buxbaum, R. B. Pond Jr Baltimore G & E Co., FSRC-22, PO Box 1475, Baltimore, Maryland 21303, USA
& A. J. Willetts Heavy Section Inspection, EPRI, NDE Centre, 130 Harris Boulevard, PO Box 217097, Charlotte, North Carolina 28221, USA
A BSTRA CT Pre-service and periodic in-service nondestructive examinations of nuclear reactor pressure vessels in the USA are mandated by law and are performed in accordance with Section X I o f the American Society o f Mechanical Engineers ( A S M E ) Boiler and Pressure Vessel Code and Nuclear Regulator)' Commission ( N R C ) requirements. Although acceptable to Code and regulatory requirements, conventional examination procedures and techniques may not meet utility needs, such as providing an adequate technical basis.[br plant license extension arguments and responding to concerns for nuclear plant safeo'. A multichannel Ultrasonic Data Recording and Processing System ( U D R P S ) was developed to address these concerns and was applied to the inspection o f reactor pressure vessels in parallel with the Code-required inspection. The inspection requirements and the U D R P S and Code-required conventional N D E data acquisition and analysis are described. Results .[?om the Calvert Cliff~" R P V inspections demonstrated that the U D R P S examination was more sensitive than the Code-required examination, and caused only minimal impact on the Code-required examination and overall inspection schedule. 307 Int. J. Pres. Ves. & Piping 0308-0161/88/$03"50 ((~ 1988 Elsevier Science Publishers Ltd England. Printed in Great Britain
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
INTRODUCTION Pre-service and periodic in-service non-destructive examinations of nuclear reactor pressure vessels are mandated by law and are performed in accordance with rules of Section XI ~of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and Nuclear Regulatory Commission (NRC) requirements. Additional guidance for performing examinations is contained in the NRC Regulatory Guide 1.150 'Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations'. 2 These rules require examination of only certain parts of nozzles and all of the pressure retaining welds in the vessel. Although acceptable to Code and regulatory procedural requirements, conventional examination procedures may not meet utility needs such as providing an adequate technical basis for plant license extension arguments and responding to concerns for nuclear plant safety. Recently developed examination technology and methods of validating examination effectiveness are now available to plant owners that can be used to meet these extended needs. The reactor pressure vessel (RPV) maintains a unique place among the nuclear steam supply system components because its failure is unacceptable. The assumption of incredibility of vessel failure is a requirement of nuclear plant design and operation. Therefore, accurate detection and characterization of vessel flaws are essential. Several of the older RPVs in pressurized water reactor (PWR) systems have experienced unexpectedly high neutroninduced embrittlement in the beltline region due to high copper content in the welds. National and international co-operative programmes have shown that examinations performed to current minimum Code requirements are inadequate to detect many of the defects of interest, particularly underclad cracks. In response to this problem the NRC issued Regulatory Guide 1.150. After the Regulatory Guide was issued, several plants experienced incidents in which the emergency core cooling system injected cold water into the RPV while the vessel was still under substantial pressure. This Pressurized Thermal Shock (PTS) can cause thermal and pressure tensile stresses near the inside surface of the vessel which are non-negligible. This possible combination of embrittlement, operating transient, and flaws has provided a very strong incentive for the development of an improved RPV inspection capability. BACKGROUND In order to meet some of these needs for improved inspection, Pacific Gas and Electric placed a contract with Dynacon Systems Inc. to develop, test,
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and supply a multichannel ultrasonic data recording and display system, the Ultrasonic Data Recording and Processing System (UDRPS). a This system was designed to be optimally suitable for examination of reactor pressure vessels. In early 1983, funding for the project was continued by the Electric Power Research Institute (EPRI) so that (a)
development of the display and data evaluation software could continue, and (b) the system capability for detection ofunderclad cracks, a concern in a Pressurized Thermal Shock scenario, could be demonstrated.
This work was followed by a much more complete evaluation of a singlechannel system by the EPRI NonDestructive Evaluation (NDE) Center. This evaluation covered many aspects of defect detection in both the underclad region of reactor pressure vessels and the welds. Although defect detection was the primary aim of the evaluation, the ability of the system to size indications from the detection data was also evaluated. The results of an N D E Center evaluation of U D R P S were published in January 1986. 4 Concurrently, Baltimore Gas & Electric (BG & E) had contracted with Dynacon for the development and fabrication of a versatile multichannel U D R P S with the intention of using the system during the first 10-year InService Inspections (ISI) at their Calvert Cliffs Nuclear Power Plant (Units 1 and 2) to provide new baseline data for use in future license extension arguments. In preparation for the use of U D R P S for a Reactor Pressure Vessel examination, qualification testing of BG & E's system was performed at the N D E Center in May 1986, and the interface work with the Code Inspection vendor, Southwest Research Institute (SwRI) was conducted in September 1986. It was of particular importance that the additional U D R P S examination should have minimal impact on the legally required ASME Section XI examination and on the inspection schedule. In co-operation with SwRI and Dynacon, an approach was devised which allowed the BG & E U D R P S to acquire data from the SwRI transducers in parallel with the equipment used for recording the Code examination. This scheme enabled U D R P S to control the pulsing of the transducers and still allowed both systems to record data. Throughout the whole period of purchase of the U D R P S and planning for its use at the Calvert Cliffs 10-year ISI, the staff at the Nuclear Regulatory Commission (NRC) and Office of Nuclear Reactor Regulation (NRR) were kept very fully informed and involved. This process started with an information presentation at N R C Region 1 where the results of the EPRI evaluation were presented and discussed. Early plans for applying the U D R P S system at Calvert Cliffs were also presented. Later in the project
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
hands-on sessions were held at the BG & E laboratories at Fort Smallwood to allow N R C staff to become more familiar with the system and the graphics software. This activity ended with a demonstration of the combined U D R P S SwRI system to the N R C at the SwRI facility in San Antonio, Texas.
Inspection requirements Adequate examination methods are needed to detect and size flaws which might compromise vessel integrity and to address license extension concerns. Current rules for predicting the failure probability over the service life of the vessel specify a flaw distribution that is probably overly conservative in specific cases. Accurate data on the actual flaw distribution must be a component of a reliable evaluation of the integrity of a vessel. The additional U D R P S inspection was intended to provide data which would enable a more realistic and better characterized flaw distribution to be used for safety assessments. To this end, it was important to inspect to the highest possible standard in order to increase the probability of detecting even very small discontinuities. This emphasis on a high quality examination was evident in three key areas of the ultrasonic testing procedures used: (1) T h e scan plans for the examinations were designed so that, whenever possible, the scan motion was parallel to the transducer beam direction so that target motion could be used as an analysis tool. (2) The minimum requirements of the A S M E code allow a stepover between scan lines of 90% of the crystal size. This was felt to be inadequate to meet the requirements of a high quality inspection. Accordingly, this value was reduced to 50% of the crystal size for this examination. (3) Regulatory Guide 1.150 requires a recording sensitivity of 20% DAC. The U D R P S procedures require a sensitivity which allows background noise to be observable. For the examinations at Calvert Cliffs the sensitivity was set at a level approximately equivalent to 2% DAC.
UDRPS description Although some of the data acquisition hardware is proprietary to Dynacon Systems Inc., the U D R P S is built mostly from standard items and uses software to provide the operating configuration. For the sake of simplicity, this description will be of a single channel system, shown diagrammatically
Reactor pressure vessel examination
I
UT Scope
Automated I Scanner
Front
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1 Terminal
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I
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Fig. 1. Blockdiagram of single-channelUDRPS device. in Fig. 1. The differences between single- and multichannel operation are noted. The data acquisition system can be configured to operate with almost any scanning and ultrasonic system. Most aspects of the digitizers are software configured, and all other hardware is completely software configured. Whatever configuration is used, the U D R P S must always provide the trigger signal for the ultrasonics so that the digitizers have a known time reference. Scanner system integration can be achieved either by assuming a fixed scanner speed, or by using scanner encoder signals. In either case, start/stop synchronization is required to tell the U D R P S when to start and when to stop taking data. Data from the ultrasonic system enters the U D R P S through a custom designed front-end digitizer and signal processor. Digitization of the data commences at the system trigger pulse, which also triggers the ultrasonic instrumentation, and continues for as long as required. Some pre-processing of the data, known as recirculation, occurs in the front end. This technique, which is similar to a linear approximation to the Synthetic Aperture Focussing Technique (SAFT), was originally included to improve signal-tonoise (S/N) ratios. This processing is also used to advantage during multichannel set-ups to balance data processing loads later in the chain of hardware by reducing the amount of data to be considered. Data flows from the special front ends to a standard Analogic Array processor, where a moving window algorithm considers the local S/N ratio and target motion in distinguishing target returns from background noise. Data points within each and every recorded 'A' scan, which meet these requirements, are flagged for later display.
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S. R. Buxbaum, R. B. Pond Jr, A. J. Wilh, tts
From the array processor data is passed to a host Hewlett-Packard minicomputer. During data acquisition, the minicomputer serves primarily as a programmable data buffer between the array processor and disk storage. Storage can be either direct to archival medium, which is normally optical disk, or, if data rates are higher, a Winchester disk provides intermediate storage for later archiving to optical disk. The minicomputer has sufficient speed and capacity to buffer three front ends, each with three input channels. Data from one of the input channels can be selected for display on the color graphics system as it is being recorded to disk. The system, which is owned by BG & E, has some additional external hardware associated with the interface to the SwRI inspection tool. This allowed for a constant rate pulse stream to be sent to all channels of the SwRI ultrasonic system so that their data recording system functioned correctly. The U D R P S is then able to select from the returning analog data stream the channels which it needs, in the order in which it needs them. To maintain position reference, this process of selecting % ' scans is synchronized to the encoders on the inspection tool. Start/stop signals for the U D R P S are also derived from the inspection tool encoders and are typically set so that they occur at 100 counts, or 25.4 mm, inside the SwRI scan limits.
P R E P A R A T I O N F O R RPV I N S P E C T I O N S Since some very major changes had been made to the front-end hardware by Dynacon during the development work on the BG & E contract, a series of repeat tests was conducted at the N D E Center. These were designed to demonstrate that the performance of the new system, in single-channel mode, was at least as good as the earlier system which had been subjected to EPRI evaluation. This performance demonstration was a BG & E contract requirement. Following successful completion of these tests the U D R P S was transported to the SwRI facilities in San Antonio for system integration work (Fig. 2). The objectives of this work were: (1) (2) (3)
to establish the interfacing of the mechanical scanning system and conventional ultrasonic system with the U D R P S ; to assure that the U D R P S interface did not interfere with the conventional A S M E code examination: demonstrate both of the above to representatives of the Nuclear Regulatory Commission and the Authorized Nuclear In-service Inspector (ANII).
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Fig. 2. Combinedsystemtesting activities at SwRI. The first two items took nearly three weeks of intense work by both Dynacon and SwRI staff. A long succession of minor problems was solved to provide a solid interface between the two systems. The EPRI NDE Center provided one of their underclad crack test blocks for use during the demonstration of the system. After formal presentations had been made to the NRC representatives and the ANII to describe the scope of the examinations and the concept of the piggyback connection of the UDRPS, the set-up procedures were demonstrated. This was followed by a scan of the test block using SwRI's Fast-PaR tool and a set of typical vessel examination transducers.
APPLICATION OF UDRPS AT CALVERT CLIFFS The reactor vessels of both units at Calvert cliffs are generally configured as shown in Fig. 3. Access to some of the beltline longitudinal seam welds is restricted on the inside surface because of the presence of surveillance capsules used for long-term monitoring of material properties. The vessels were designed and fabricated by Combustion Engineering Co. Plate material is SA533 Grade B Class 1, and forgings are from SA508. Calvert Cliffs Unit 1 results
In November 1986, field use of the UDRPS system for the Calvert Cliffs Unit 1 RPV examination took place. The scanning and ASME Section XI examination were performed by SwRI, and the UDRPS examination was made by BG & E and Dynacon Systems personnel, with the assistance of
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
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Diagram of reactor vessel construction.
NDE Center personnel. During the set-up and the first few days of scanning, there were several problems with the UDRPS, including frequent stops due to a timing problem. The cause of the timing problem was traced to a bad circuit card in the interface to the PaR controller. Once this was corrected, scanning proceeded smoothly. Although the UDRPS vessel examination procedure is not directly related to the amplitude-based ASME Section XI procedures, we feel that it is essential to compare the examinations as part of the process of gaining more general acceptance for the new technology and for quantifying the relative sensitivities of the examinations. The ASME procedure requires calibration from cross-drilled hole reflectors in a clad calibration block of the same nominal thickness as the vessel being examined, whereas the UDRPS sensitivity is based upon the noise level present when moving the transducer over the vessel surface. Typically, using these procedures, the
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UDRPS examination has been measured 12-20dB more sensitive than the equivalent ASME procedure. Even at this enhanced sensitivity, the only features of interest noticed during the examination at Calvert Cliffs Unit 1 were in the longitudinal seam welds. Each of these welds produced a complex and distinctive pattern of reflectors. The observed signals were characteristically in parallel lines on the display, with each line being subdivided into a series of short and distinct target motion lines. Observable signals were seen on all of the angle beam inspections. The circumferential separation of the lines was approximately 35 mm, with vertical separation between the centers of target motion lines being in multiples of roughly 12mm. Using conventional dB drop methodology to characterize these signals would have been extremely laborious. The UDRPS graphics displays, together with copies of the weld drawings, allowed a fairly rapid assessment of these signals to be made. It was clear that these signals originated from points at or close to the side fusion face of the weld. Many of the signals have extremely low amplitudes, and could have been caused by minor acoustic impedance mismatches along the scallops of the weld fusion zone. Some signals persisted for several successive increments along the length of the weld with amplitudes approaching 5% DAC and were most probably caused by small pieces of slag. A very few signals, which also had length along the weld, had higher amplitude responses which approached 15% DAC. These could conceivably be from a series of very small lack-of-fusion defects. Some support for this analysis is given by a section of similar manufacture and vintage RPV weld which was available at the NDE Center. This was ground and etched to provide a better picture of the weld structure and several of the small defects present. ( c f Fig. 4.) Calvert Cliffs Unit 2 results When the Unit 2 vessel at Calvert Cliffs was examined in April 1987, a similar pattern of signals was observed in the vertical seam welds. On this vessel, one signal from the near-surface transducer exceeded the reporting level on the Section XI examination. The indication was located in the beltline region in the vertical seam weld at 30 ° circumferential location and started at 216-6 in below the vessel flange. Since the indication was within 25 mm of the inside diameter (ID) surface, it was outside the gated region for the 45 ~' transducer and 60 ° transducer Code examination. However, the additional UDRPS examination, which did not restrict the data in this way, showed clear signals from the 45 °, 60 ~ and 70 ° transducers from both directions. Analysis of the UDRPS data conservatively bounded the reflector size as 35 mm long by 20 mm through-wall located 15 mm beneath the ID surface and at or close to
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
(a)
(b)
(c) Fig. 4.
(a) Entire weld; (b) s[ag; (c) lack-of-fusion/hot tear.
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the weld preparation face. Since the signal amplitude only just exceeded 20% DAC, Code sizing techniques, as allowed by the Regulatory Guide 1.150, sized the reflector as a singular point. Characterization from the initial inspection data was difficult, but from the location, lack of sidewall fusion was suspected. Since the b o u n d i n g dimensions and location were unacceptable using the code tables in A S M E Section XI lWB-3510, further evaluation was required. Suitable transducers for this purpose were available at the N D E Center, as was a clad reactor vessel test block containing lack-of-fusion defects of approximately the same size and location as the postulated defect. Accordingly, a programme of work was outlined which would demonstrate the performance of the intended transducers and allow them to be used to rescan the indication volume. A large variety of potentially suitable twin-crystal longitudinal-wave (Lwave) transducers, with different angles and focal lengths, were available at the N D E Center for use in rescanning this indication. The two of most interest were a 45 ° 2 M H z f65, and a 60 ° 2 M H z f33. As further backup, a 70 ° twin-crystal L-wave unit, designed for underclad crack detection, and a 0 ° twin crystal were also available to try and improve definition of the innermost location of the indication, should that be required. All of the transducers to be used were manufactured by R T D in Rotterdam. Scans were made with each of these transducers over a lack-of-fusion defect in a clad longitudinal seam weld specimen defect block. The 45 ° and 60 ° transducers were also s c a n n e d o v e r a slightly deeper defect of a b o u t the same size. The clad on the N D E Center defect block was automatic strip clad as opposed to the automatic three-wire clad on the Calvert Cliffs vessels, but this was not thought to be significant when using these L-wave transducers. The defects were intended to be correct for a weld with a 15 ° angle rather than the parallel-sided weld on the reactor vessels at Calvert Cliffs. Very clear data were obtained from the defects in the N D E Center block. As expected, the 60 ° transducer performed very well on the defect closer to the surface but gave very indistinct signals from the deeper defect. The 45 ° transducer gave very poor signals from the top tip of the close defect and gave excellent top and b o t t o m signals from the deeper defect. Other work at the N D E Center had indicated that the 70 ° L-wave transducer was not ideal for sizing of defect. 4 The assumptions made about the indication in the vessel suggested that it was deeper than the optimum operating depth for the 70 ° transducer. Since very clear signals were obtained from the closer of the two lack-of-fusion defects in the N D E Center block, the 70 ° transducer was included in the list for possible use, to clarify the presence or absence of reflectors which may have been closer to the surface than originally thought. Zero degree transducer results were not encouraging. It was barely possible
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
to distinguish the top of the closer lack-of-fusion defect in the N D E Center block, confirming that this transducer should probably not be used. After the work at the N D E Center was completed, the transducers were taken to C C N P P where technicians from SwRI manufactured a special holder so that the 45 ° and 60 ° transducers could be mounted onto the P a R tool. The refined examination of this indication required scanning with the circumferential motion and incrementing with the hoist motion of the P a R tool. Adequate length sizing could be obtained by taking data on a sweep increment of 2.5 mm. The scan plan required a hoist increment of 1.25 mm, which was the lowest value which SwRI were comfortable with achieving. It was agreed that the increments could be made + 0.5 mm without pausing to try for a more accurate position. Data would be recorded on every sweep, so that analysis could be made from two separate data sets, each of which was recorded in the same direction on a 2.5 mm increment. Any doubts about the capability of the scanning system were quickly dispelled. Only 1 out of 100 sweeps across the indication area missed its position by 0.25 mm. The other 99 were correctly positioned, a remarkable achievement.
Conclusions of analysis The new data from the L-wave transducers showed remarkable consistency between the two angles and the two examination directions. After a careful analysis of all this new data it was concluded that the indication was a series of small slag stringers. Some of the data points considered during the analysis are plotted on a three-dimensional diagram in Fig. 5.
Fig. 5.
Plot of selected data points; Calvert Cliffs 2 indication.
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The axes on this plot show Xin inches below the vessel flange, Yin inches from weld centerline, and Z in inches below the vessel surface. The slag stringers identified during this analysis fell within the bounds of the region identified by UDRPS from the analysis of the original search data. Preservice radiographs were located in the archives at Combustion Engineering and transported to Calvert Cliffs for re-examination. These clearly showed slag inclusions, and multiple shots had been taken to help resolve the disposition of the slag. Angled radiographic shots showed the distribution of the slag in depth, and measurements of individual slag lines corresponded to UDRPS measurements. The maximum size identified for each stringer together with their proximity allowed each to be treated as an individual flaw. Using this information the flaw size was acceptable according to the ASME Code. Less than three days elapsed between the time the decision was made to perform a refined examination of the indication volume and the completion of the examination. During this time preliminary transducer work at the NDE Center was completed, the transducers were shipped to Calvert Cliffs, and the transducers were mounted and installed on the Fast PaR. These activities paralleled the continued RPV inspection. Only the time required to establish UDRPS parameters for the new transducers and complete the actual scans affected the outage schedule. This amounted to a little less than six hours. The analysis of the data took a further day and a half before being presented to the NRC.
SUMMARY The UDRPS system had been used earlier at other nuclear plants to inspect specific regions of reactor pressure vessels. This application at Calvert Cliffs was the first time the technology had been combined with a vessel inspection tool to perform a full 10-year ISI. This represents a major advance in the use of this type of inspection system. The short-term objective, to apply UDRPS to a full-scale of ISI of the Calvert Cliffs RPV, was met. The additional UDRPS examination had no impact on the ASME Code examination and had minimal impact on the outage schedule, in spite of some problems encountered in achieving this, particularly during the preparation for and inspection of the Unit 1 RPV. During the pre-inspection briefing meetings the NRC expressed genuine concern about the large quantity of data which would be generated by the UDRPS. Each reactor vessel inspection at Calvert Cliffs resulted in approximately 5 gigabytes of ultrasonic data which required review and archive storage. The use of optical disk technology was critical to the
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s. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
logistics of handling this data, and allowed very easy storage of data during scanning and transfer of data for analysis. Due to the relative ease with which very large amounts of data can be transferred using this technology it was possible for the review and analysis of the data to proceed in parallel with data acquisition and, in most cases, for the analysis time to be shorter than the acquisition time.
A C K N O W L E D G E M ENTS The authors wish to acknowledge the contributions of Dynacon Systems Inc. and the Southwest Research Institute to this work, and we thank Dr Gary J. Dau and the Electric Power Research Institute for their continued support.
REFERENCES 1. American Society of Mechanical Engineers. A S M E Boiler and Pressure Vessel Code: Section X I Rules.for Inservice Inspection of Nuclear Power Plants. New York, 1983. 2. US Nuclear Regulatory Commission, Office of Nuclear Regulatory Research. Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations. Washington, DC: Government Printing Office, Regulatory Guide
1.150 Revision I, February 1983. 3. Real-time signal processing in an ultrasonic imaging system. Moore, M. J. & Dodd, F. J., Materials Evaluation, 40 (1982) 976 81. 4. Evaluation ~f the Ultrasonic Data Recording and Processing System (UDRPS).
Palo Alto, CA: Electric Power Research Institute, January 1986, NP-4397.
Application of an Ultrasonic Data Recording and Processing System to Reactor Pressure Vessel Examination
S. R. Buxbaum, R. B. Pond Jr Baltimore G & E Co., FSRC-22, PO Box 1475, Baltimore, Maryland 21303, USA
& A. J. Willetts Heavy Section Inspection, EPRI, NDE Centre, 130 Harris Boulevard, PO Box 217097, Charlotte, North Carolina 28221, USA
A BSTRA CT Pre-service and periodic in-service nondestructive examinations of nuclear reactor pressure vessels in the USA are mandated by law and are performed in accordance with Section X I o f the American Society o f Mechanical Engineers ( A S M E ) Boiler and Pressure Vessel Code and Nuclear Regulator)' Commission ( N R C ) requirements. Although acceptable to Code and regulatory requirements, conventional examination procedures and techniques may not meet utility needs, such as providing an adequate technical basis.[br plant license extension arguments and responding to concerns for nuclear plant safeo'. A multichannel Ultrasonic Data Recording and Processing System ( U D R P S ) was developed to address these concerns and was applied to the inspection o f reactor pressure vessels in parallel with the Code-required inspection. The inspection requirements and the U D R P S and Code-required conventional N D E data acquisition and analysis are described. Results .[?om the Calvert Cliff~" R P V inspections demonstrated that the U D R P S examination was more sensitive than the Code-required examination, and caused only minimal impact on the Code-required examination and overall inspection schedule. 307 Int. J. Pres. Ves. & Piping 0308-0161/88/$03"50 ((~ 1988 Elsevier Science Publishers Ltd England. Printed in Great Britain
308
S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
INTRODUCTION Pre-service and periodic in-service non-destructive examinations of nuclear reactor pressure vessels are mandated by law and are performed in accordance with rules of Section XI ~of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and Nuclear Regulatory Commission (NRC) requirements. Additional guidance for performing examinations is contained in the NRC Regulatory Guide 1.150 'Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations'. 2 These rules require examination of only certain parts of nozzles and all of the pressure retaining welds in the vessel. Although acceptable to Code and regulatory procedural requirements, conventional examination procedures may not meet utility needs such as providing an adequate technical basis for plant license extension arguments and responding to concerns for nuclear plant safety. Recently developed examination technology and methods of validating examination effectiveness are now available to plant owners that can be used to meet these extended needs. The reactor pressure vessel (RPV) maintains a unique place among the nuclear steam supply system components because its failure is unacceptable. The assumption of incredibility of vessel failure is a requirement of nuclear plant design and operation. Therefore, accurate detection and characterization of vessel flaws are essential. Several of the older RPVs in pressurized water reactor (PWR) systems have experienced unexpectedly high neutroninduced embrittlement in the beltline region due to high copper content in the welds. National and international co-operative programmes have shown that examinations performed to current minimum Code requirements are inadequate to detect many of the defects of interest, particularly underclad cracks. In response to this problem the NRC issued Regulatory Guide 1.150. After the Regulatory Guide was issued, several plants experienced incidents in which the emergency core cooling system injected cold water into the RPV while the vessel was still under substantial pressure. This Pressurized Thermal Shock (PTS) can cause thermal and pressure tensile stresses near the inside surface of the vessel which are non-negligible. This possible combination of embrittlement, operating transient, and flaws has provided a very strong incentive for the development of an improved RPV inspection capability. BACKGROUND In order to meet some of these needs for improved inspection, Pacific Gas and Electric placed a contract with Dynacon Systems Inc. to develop, test,
Reactor pressure vessel examination
309
and supply a multichannel ultrasonic data recording and display system, the Ultrasonic Data Recording and Processing System (UDRPS). a This system was designed to be optimally suitable for examination of reactor pressure vessels. In early 1983, funding for the project was continued by the Electric Power Research Institute (EPRI) so that (a)
development of the display and data evaluation software could continue, and (b) the system capability for detection ofunderclad cracks, a concern in a Pressurized Thermal Shock scenario, could be demonstrated.
This work was followed by a much more complete evaluation of a singlechannel system by the EPRI NonDestructive Evaluation (NDE) Center. This evaluation covered many aspects of defect detection in both the underclad region of reactor pressure vessels and the welds. Although defect detection was the primary aim of the evaluation, the ability of the system to size indications from the detection data was also evaluated. The results of an N D E Center evaluation of U D R P S were published in January 1986. 4 Concurrently, Baltimore Gas & Electric (BG & E) had contracted with Dynacon for the development and fabrication of a versatile multichannel U D R P S with the intention of using the system during the first 10-year InService Inspections (ISI) at their Calvert Cliffs Nuclear Power Plant (Units 1 and 2) to provide new baseline data for use in future license extension arguments. In preparation for the use of U D R P S for a Reactor Pressure Vessel examination, qualification testing of BG & E's system was performed at the N D E Center in May 1986, and the interface work with the Code Inspection vendor, Southwest Research Institute (SwRI) was conducted in September 1986. It was of particular importance that the additional U D R P S examination should have minimal impact on the legally required ASME Section XI examination and on the inspection schedule. In co-operation with SwRI and Dynacon, an approach was devised which allowed the BG & E U D R P S to acquire data from the SwRI transducers in parallel with the equipment used for recording the Code examination. This scheme enabled U D R P S to control the pulsing of the transducers and still allowed both systems to record data. Throughout the whole period of purchase of the U D R P S and planning for its use at the Calvert Cliffs 10-year ISI, the staff at the Nuclear Regulatory Commission (NRC) and Office of Nuclear Reactor Regulation (NRR) were kept very fully informed and involved. This process started with an information presentation at N R C Region 1 where the results of the EPRI evaluation were presented and discussed. Early plans for applying the U D R P S system at Calvert Cliffs were also presented. Later in the project
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S. R. Buxbaum, R. B. Pond Jr, A. J. Willetts
hands-on sessions were held at the BG & E laboratories at Fort Smallwood to allow N R C staff to become more familiar with the system and the graphics software. This activity ended with a demonstration of the combined U D R P S SwRI system to the N R C at the SwRI facility in San Antonio, Texas.
Inspection requirements Adequate examination methods are needed to detect and size flaws which might compromise vessel integrity and to address license extension concerns. Current rules for predicting the failure probability over the service life of the vessel specify a flaw distribution that is probably overly conservative in specific cases. Accurate data on the actual flaw distribution must be a component of a reliable evaluation of the integrity of a vessel. The additional U D R P S inspection was intended to provide data which would enable a more realistic and better characterized flaw distribution to be used for safety assessments. To this end, it was important to inspect to the highest possible standard in order to increase the probability of detecting even very small discontinuities. This emphasis on a high quality examination was evident in three key areas of the ultrasonic testing procedures used: (1) T h e scan plans for the examinations were designed so that, whenever possible, the scan motion was parallel to the transducer beam direction so that target motion could be used as an analysis tool. (2) The minimum requirements of the A S M E code allow a stepover between scan lines of 90% of the crystal size. This was felt to be inadequate to meet the requirements of a high quality inspection. Accordingly, this value was reduced to 50% of the crystal size for this examination. (3) Regulatory Guide 1.150 requires a recording sensitivity of 20% DAC. The U D R P S procedures require a sensitivity which allows background noise to be observable. For the examinations at Calvert Cliffs the sensitivity was set at a level approximately equivalent to 2% DAC.
UDRPS description Although some of the data acquisition hardware is proprietary to Dynacon Systems Inc., the U D R P S is built mostly from standard items and uses software to provide the operating configuration. For the sake of simplicity, this description will be of a single channel system, shown diagrammatically
Reactor pressure vessel examination
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Fig. 1. Blockdiagram of single-channelUDRPS device. in Fig. 1. The differences between single- and multichannel operation are noted. The data acquisition system can be configured to operate with almost any scanning and ultrasonic system. Most aspects of the digitizers are software configured, and all other hardware is completely software configured. Whatever configuration is used, the U D R P S must always provide the trigger signal for the ultrasonics so that the digitizers have a known time reference. Scanner system integration can be achieved either by assuming a fixed scanner speed, or by using scanner encoder signals. In either case, start/stop synchronization is required to tell the U D R P S when to start and when to stop taking data. Data from the ultrasonic system enters the U D R P S through a custom designed front-end digitizer and signal processor. Digitization of the data commences at the system trigger pulse, which also triggers the ultrasonic instrumentation, and continues for as long as required. Some pre-processing of the data, known as recirculation, occurs in the front end. This technique, which is similar to a linear approximation to the Synthetic Aperture Focussing Technique (SAFT), was originally included to improve signal-tonoise (S/N) ratios. This processing is also used to advantage during multichannel set-ups to balance data processing loads later in the chain of hardware by reducing the amount of data to be considered. Data flows from the special front ends to a standard Analogic Array processor, where a moving window algorithm considers the local S/N ratio and target motion in distinguishing target returns from background noise. Data points within each and every recorded 'A' scan, which meet these requirements, are flagged for later display.
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From the array processor data is passed to a host Hewlett-Packard minicomputer. During data acquisition, the minicomputer serves primarily as a programmable data buffer between the array processor and disk storage. Storage can be either direct to archival medium, which is normally optical disk, or, if data rates are higher, a Winchester disk provides intermediate storage for later archiving to optical disk. The minicomputer has sufficient speed and capacity to buffer three front ends, each with three input channels. Data from one of the input channels can be selected for display on the color graphics system as it is being recorded to disk. The system, which is owned by BG & E, has some additional external hardware associated with the interface to the SwRI inspection tool. This allowed for a constant rate pulse stream to be sent to all channels of the SwRI ultrasonic system so that their data recording system functioned correctly. The U D R P S is then able to select from the returning analog data stream the channels which it needs, in the order in which it needs them. To maintain position reference, this process of selecting % ' scans is synchronized to the encoders on the inspection tool. Start/stop signals for the U D R P S are also derived from the inspection tool encoders and are typically set so that they occur at 100 counts, or 25.4 mm, inside the SwRI scan limits.
P R E P A R A T I O N F O R RPV I N S P E C T I O N S Since some very major changes had been made to the front-end hardware by Dynacon during the development work on the BG & E contract, a series of repeat tests was conducted at the N D E Center. These were designed to demonstrate that the performance of the new system, in single-channel mode, was at least as good as the earlier system which had been subjected to EPRI evaluation. This performance demonstration was a BG & E contract requirement. Following successful completion of these tests the U D R P S was transported to the SwRI facilities in San Antonio for system integration work (Fig. 2). The objectives of this work were: (1) (2) (3)
to establish the interfacing of the mechanical scanning system and conventional ultrasonic system with the U D R P S ; to assure that the U D R P S interface did not interfere with the conventional A S M E code examination: demonstrate both of the above to representatives of the Nuclear Regulatory Commission and the Authorized Nuclear In-service Inspector (ANII).
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Fig. 2. Combinedsystemtesting activities at SwRI. The first two items took nearly three weeks of intense work by both Dynacon and SwRI staff. A long succession of minor problems was solved to provide a solid interface between the two systems. The EPRI NDE Center provided one of their underclad crack test blocks for use during the demonstration of the system. After formal presentations had been made to the NRC representatives and the ANII to describe the scope of the examinations and the concept of the piggyback connection of the UDRPS, the set-up procedures were demonstrated. This was followed by a scan of the test block using SwRI's Fast-PaR tool and a set of typical vessel examination transducers.
APPLICATION OF UDRPS AT CALVERT CLIFFS The reactor vessels of both units at Calvert cliffs are generally configured as shown in Fig. 3. Access to some of the beltline longitudinal seam welds is restricted on the inside surface because of the presence of surveillance capsules used for long-term monitoring of material properties. The vessels were designed and fabricated by Combustion Engineering Co. Plate material is SA533 Grade B Class 1, and forgings are from SA508. Calvert Cliffs Unit 1 results
In November 1986, field use of the UDRPS system for the Calvert Cliffs Unit 1 RPV examination took place. The scanning and ASME Section XI examination were performed by SwRI, and the UDRPS examination was made by BG & E and Dynacon Systems personnel, with the assistance of
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NDE Center personnel. During the set-up and the first few days of scanning, there were several problems with the UDRPS, including frequent stops due to a timing problem. The cause of the timing problem was traced to a bad circuit card in the interface to the PaR controller. Once this was corrected, scanning proceeded smoothly. Although the UDRPS vessel examination procedure is not directly related to the amplitude-based ASME Section XI procedures, we feel that it is essential to compare the examinations as part of the process of gaining more general acceptance for the new technology and for quantifying the relative sensitivities of the examinations. The ASME procedure requires calibration from cross-drilled hole reflectors in a clad calibration block of the same nominal thickness as the vessel being examined, whereas the UDRPS sensitivity is based upon the noise level present when moving the transducer over the vessel surface. Typically, using these procedures, the
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UDRPS examination has been measured 12-20dB more sensitive than the equivalent ASME procedure. Even at this enhanced sensitivity, the only features of interest noticed during the examination at Calvert Cliffs Unit 1 were in the longitudinal seam welds. Each of these welds produced a complex and distinctive pattern of reflectors. The observed signals were characteristically in parallel lines on the display, with each line being subdivided into a series of short and distinct target motion lines. Observable signals were seen on all of the angle beam inspections. The circumferential separation of the lines was approximately 35 mm, with vertical separation between the centers of target motion lines being in multiples of roughly 12mm. Using conventional dB drop methodology to characterize these signals would have been extremely laborious. The UDRPS graphics displays, together with copies of the weld drawings, allowed a fairly rapid assessment of these signals to be made. It was clear that these signals originated from points at or close to the side fusion face of the weld. Many of the signals have extremely low amplitudes, and could have been caused by minor acoustic impedance mismatches along the scallops of the weld fusion zone. Some signals persisted for several successive increments along the length of the weld with amplitudes approaching 5% DAC and were most probably caused by small pieces of slag. A very few signals, which also had length along the weld, had higher amplitude responses which approached 15% DAC. These could conceivably be from a series of very small lack-of-fusion defects. Some support for this analysis is given by a section of similar manufacture and vintage RPV weld which was available at the NDE Center. This was ground and etched to provide a better picture of the weld structure and several of the small defects present. ( c f Fig. 4.) Calvert Cliffs Unit 2 results When the Unit 2 vessel at Calvert Cliffs was examined in April 1987, a similar pattern of signals was observed in the vertical seam welds. On this vessel, one signal from the near-surface transducer exceeded the reporting level on the Section XI examination. The indication was located in the beltline region in the vertical seam weld at 30 ° circumferential location and started at 216-6 in below the vessel flange. Since the indication was within 25 mm of the inside diameter (ID) surface, it was outside the gated region for the 45 ~' transducer and 60 ° transducer Code examination. However, the additional UDRPS examination, which did not restrict the data in this way, showed clear signals from the 45 °, 60 ~ and 70 ° transducers from both directions. Analysis of the UDRPS data conservatively bounded the reflector size as 35 mm long by 20 mm through-wall located 15 mm beneath the ID surface and at or close to
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(a)
(b)
(c) Fig. 4.
(a) Entire weld; (b) s[ag; (c) lack-of-fusion/hot tear.
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the weld preparation face. Since the signal amplitude only just exceeded 20% DAC, Code sizing techniques, as allowed by the Regulatory Guide 1.150, sized the reflector as a singular point. Characterization from the initial inspection data was difficult, but from the location, lack of sidewall fusion was suspected. Since the b o u n d i n g dimensions and location were unacceptable using the code tables in A S M E Section XI lWB-3510, further evaluation was required. Suitable transducers for this purpose were available at the N D E Center, as was a clad reactor vessel test block containing lack-of-fusion defects of approximately the same size and location as the postulated defect. Accordingly, a programme of work was outlined which would demonstrate the performance of the intended transducers and allow them to be used to rescan the indication volume. A large variety of potentially suitable twin-crystal longitudinal-wave (Lwave) transducers, with different angles and focal lengths, were available at the N D E Center for use in rescanning this indication. The two of most interest were a 45 ° 2 M H z f65, and a 60 ° 2 M H z f33. As further backup, a 70 ° twin-crystal L-wave unit, designed for underclad crack detection, and a 0 ° twin crystal were also available to try and improve definition of the innermost location of the indication, should that be required. All of the transducers to be used were manufactured by R T D in Rotterdam. Scans were made with each of these transducers over a lack-of-fusion defect in a clad longitudinal seam weld specimen defect block. The 45 ° and 60 ° transducers were also s c a n n e d o v e r a slightly deeper defect of a b o u t the same size. The clad on the N D E Center defect block was automatic strip clad as opposed to the automatic three-wire clad on the Calvert Cliffs vessels, but this was not thought to be significant when using these L-wave transducers. The defects were intended to be correct for a weld with a 15 ° angle rather than the parallel-sided weld on the reactor vessels at Calvert Cliffs. Very clear data were obtained from the defects in the N D E Center block. As expected, the 60 ° transducer performed very well on the defect closer to the surface but gave very indistinct signals from the deeper defect. The 45 ° transducer gave very poor signals from the top tip of the close defect and gave excellent top and b o t t o m signals from the deeper defect. Other work at the N D E Center had indicated that the 70 ° L-wave transducer was not ideal for sizing of defect. 4 The assumptions made about the indication in the vessel suggested that it was deeper than the optimum operating depth for the 70 ° transducer. Since very clear signals were obtained from the closer of the two lack-of-fusion defects in the N D E Center block, the 70 ° transducer was included in the list for possible use, to clarify the presence or absence of reflectors which may have been closer to the surface than originally thought. Zero degree transducer results were not encouraging. It was barely possible
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to distinguish the top of the closer lack-of-fusion defect in the N D E Center block, confirming that this transducer should probably not be used. After the work at the N D E Center was completed, the transducers were taken to C C N P P where technicians from SwRI manufactured a special holder so that the 45 ° and 60 ° transducers could be mounted onto the P a R tool. The refined examination of this indication required scanning with the circumferential motion and incrementing with the hoist motion of the P a R tool. Adequate length sizing could be obtained by taking data on a sweep increment of 2.5 mm. The scan plan required a hoist increment of 1.25 mm, which was the lowest value which SwRI were comfortable with achieving. It was agreed that the increments could be made + 0.5 mm without pausing to try for a more accurate position. Data would be recorded on every sweep, so that analysis could be made from two separate data sets, each of which was recorded in the same direction on a 2.5 mm increment. Any doubts about the capability of the scanning system were quickly dispelled. Only 1 out of 100 sweeps across the indication area missed its position by 0.25 mm. The other 99 were correctly positioned, a remarkable achievement.
Conclusions of analysis The new data from the L-wave transducers showed remarkable consistency between the two angles and the two examination directions. After a careful analysis of all this new data it was concluded that the indication was a series of small slag stringers. Some of the data points considered during the analysis are plotted on a three-dimensional diagram in Fig. 5.
Fig. 5.
Plot of selected data points; Calvert Cliffs 2 indication.
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The axes on this plot show Xin inches below the vessel flange, Yin inches from weld centerline, and Z in inches below the vessel surface. The slag stringers identified during this analysis fell within the bounds of the region identified by UDRPS from the analysis of the original search data. Preservice radiographs were located in the archives at Combustion Engineering and transported to Calvert Cliffs for re-examination. These clearly showed slag inclusions, and multiple shots had been taken to help resolve the disposition of the slag. Angled radiographic shots showed the distribution of the slag in depth, and measurements of individual slag lines corresponded to UDRPS measurements. The maximum size identified for each stringer together with their proximity allowed each to be treated as an individual flaw. Using this information the flaw size was acceptable according to the ASME Code. Less than three days elapsed between the time the decision was made to perform a refined examination of the indication volume and the completion of the examination. During this time preliminary transducer work at the NDE Center was completed, the transducers were shipped to Calvert Cliffs, and the transducers were mounted and installed on the Fast PaR. These activities paralleled the continued RPV inspection. Only the time required to establish UDRPS parameters for the new transducers and complete the actual scans affected the outage schedule. This amounted to a little less than six hours. The analysis of the data took a further day and a half before being presented to the NRC.
SUMMARY The UDRPS system had been used earlier at other nuclear plants to inspect specific regions of reactor pressure vessels. This application at Calvert Cliffs was the first time the technology had been combined with a vessel inspection tool to perform a full 10-year ISI. This represents a major advance in the use of this type of inspection system. The short-term objective, to apply UDRPS to a full-scale of ISI of the Calvert Cliffs RPV, was met. The additional UDRPS examination had no impact on the ASME Code examination and had minimal impact on the outage schedule, in spite of some problems encountered in achieving this, particularly during the preparation for and inspection of the Unit 1 RPV. During the pre-inspection briefing meetings the NRC expressed genuine concern about the large quantity of data which would be generated by the UDRPS. Each reactor vessel inspection at Calvert Cliffs resulted in approximately 5 gigabytes of ultrasonic data which required review and archive storage. The use of optical disk technology was critical to the
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logistics of handling this data, and allowed very easy storage of data during scanning and transfer of data for analysis. Due to the relative ease with which very large amounts of data can be transferred using this technology it was possible for the review and analysis of the data to proceed in parallel with data acquisition and, in most cases, for the analysis time to be shorter than the acquisition time.
A C K N O W L E D G E M ENTS The authors wish to acknowledge the contributions of Dynacon Systems Inc. and the Southwest Research Institute to this work, and we thank Dr Gary J. Dau and the Electric Power Research Institute for their continued support.
REFERENCES 1. American Society of Mechanical Engineers. A S M E Boiler and Pressure Vessel Code: Section X I Rules.for Inservice Inspection of Nuclear Power Plants. New York, 1983. 2. US Nuclear Regulatory Commission, Office of Nuclear Regulatory Research. Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations. Washington, DC: Government Printing Office, Regulatory Guide
1.150 Revision I, February 1983. 3. Real-time signal processing in an ultrasonic imaging system. Moore, M. J. & Dodd, F. J., Materials Evaluation, 40 (1982) 976 81. 4. Evaluation ~f the Ultrasonic Data Recording and Processing System (UDRPS).
Palo Alto, CA: Electric Power Research Institute, January 1986, NP-4397.