ULTRASOUND & SUBMARINE ADVANCEMENTS FOR RPV INTERNALS BAFFLE BOLT INSPECTION. S.W. Glass, B. Thigpen, AREVA, USA A. Bleuze, E. Brau, AREVA, France

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1 More Info at Open Access Database ULTRASOUND & SUBMARINE ADVANCEMENTS FOR RPV INTERNALS BAFFLE BOLT INSPECTION S.W. Glass, B. Thigpen, AREVA, USA A. Bleuze, E. Brau, AREVA, France ABSTRACT As many nuclear plants approach the end of their initial 40 years license period, inspection or replacement of their reactor internals bolts must be considered. This is consistent with the Materials Reliability Program (MRP 227/228) guideline for plant life extension. Assurance of the internals structural integrity is essential for continued safe operation of these plants. If there is no suspicion or indication of bolt failure, simple inspection is normally more cost-effective than replacement. Inspection specialists have inspected thousands of internals bolts with conventional or Phased Array UT. In some cases, complex bolt and head geometries coupled with counter-bore and locking bar interferences render classical UT inspections difficult. Additionally, different bolt head configurations and bolt capture mechanisms mandate specific qualifications for each bolt type. AREVA has used advanced modeling approaches to optimize transducer designs to efficiently inspect these complex bolt shapes and has improved the SUSI submarine delivery platform for bolt inspection. This paper firstly discusses advances in the UT inspection methods for particularly challenging bolt designs including: the ID Hexagonal socket bolts characteristic of the WESTINGHOUSE 2 loops plants, the locking bar configuration characteristic of WESTINGHOUSE and FRAMATOME 3 and 4 loop plants and the B&W button head bolts. Secondly the results of the corrosion crack growth study engaged for UT inspection qualification is presented. At the end, advancements in position control of the SUSI submarine to assure correct identification of the internals bolts during the inspection are discussed. INTRODUCTION The reactor internals baffle plate assembly channels cooling water through the fuel assemblies. The assembly is comprised of rectangular plates nominally parallel to the vessel wall bolted into edge plates nominally aligned with the vessel radius and former rings located at multiple elevations within the vessel (Figure 1). Bolt shank diameters are typically 2 to 3 cm. The internals baffle assembly operates at elevated temperature ( ºC) and is highly irradiated due to its close proximity to the nuclear fuel. These conditions are known to increase stainless steel material susceptibility to inter-granular stress corrosion cracking. Normally this component is not readily observable. The lower internals are rarely removed from the vessel except to allow access for vessel wall ultrasonic examination. In the 1980 s Visual Test (VT) inspections of several reactor internals Figure 1: Internals Assembly highlighting baffle bolt locations discovered indications of baffle bolt cracking [1]. Most of these bolts were made of 316 cold worked stainless steels. 735

2 Head-to-shank Bolt Head Stress Corrosion Cracks Figure 2: SCC can present various orientations with a tilt angle of up to 30 degrees from the head face. Temperat ure and the water environment alone coupled with the relatively low stress levels on these bolts should not have been sufficient to cause cracks. Most of the cracked bolts though were located in the second and third rows from the bottom, corresponding to the highest neutron irradiation region. This indicated that the neutron irradiation coupled with stress and elevated temperatures increased the bolts crack susceptibility and in some cases led to cracking. The failure mode was identified as irradiation assisted stress-corrosion cracking (IASCC). Typically cracks are located in the head-to-shank region and along the shank even to the first thread in the body of the bolt (Figure 2). The cracks propagate along grain boundaries and therefore present a complex surface for UT signals to reflect from. This can be significantly different from EDM notch flaw representations. Although the specifics are quite varied, most regional codes and regulatory bodies now require a bolt management program that includes VT, a prudent volumetric inspection (ultrasound) sample (typically every 5 to 10 years), or limited replacement of accessible bolts. Inspection vendors have invested heavily to automate and reduce the cost and schedule for bolt inspection thereby encouraging utilities to select the lower-cost inspection option. This gamble usually pays off. Moreover, the baffle structure is tolerant of a large number of failed bolts without compromising the structural integrity but there is still concern for loose parts. The global statistics are difficult to verify but on the AREVA built French CPO plants, between 1 and 11% of inspected bolts are confirmed cracked. Bolt inspection frequency has increased in the US with plant-life extension programs performed in accordance with MRP 228 guidelines [2]. This document provides numerous internals inspection recommendations but the major automated inspection focuses are the guide-card channel wear measurements and baffle-bolt inspection Sound Bolt 1 2 Notch Depth 3mm Notch Depth 4mm 4 3 Figure 3: Conventional UT bolt integrity 736

3 ULTRASOUND TECHNIQUES Conventional ultrasound (UT) techniques have long been known to be able to detect cracks and flaws in bolts. Transducers are coupled to the bolt head allowing sound to be transmitted through the shank of the bolt. If a strong reflection is observed from the bottom of the shank with minimal additional reflections, the bolt integrity is confirmed. If however, reflections are observed before the bottomof-shank reflection is detected and/or the bottom-of-shank reflection is reduced, a compromised bolt is indicated or suspected (Figure 3). A typical transducer suitable for inspection of a flat-head bolt may have transmit and receive elements distributed in four quadrants. Transmit and receive interrogations can be from circumferentially adjacent or circumferentially opposite elements (Figure 4). In some cases, the bolt shank is made of highly attenuative materials so use of a back-wall echo is not possible. The most likely location however for cracks to occur are in the head-to-shank area, through the shank and into the bolt down to the first thread. The sound does not have to travel as far to interrogate this bolt region as it does to reach and return from the end of the bolt. QUALIFICATIONS Figure 4: Four quadrant UT probe Although the general inspection approach is straight-forward, each particular bolt geometry requires a specific qualification and technique adaptation. Technique development and qualification typically begin with a 3-D bolt model coupled to a planned UT transducer or transducer set (Figure 5). The UT and coupling configuration parameters are then optimized to produce a prototype probe. A series of bolts are then prepared with verification flaws to confirm the model predicted inspection capability. Flaws are determined in accordance with the client and sometimes the regulatory body but generally they include elliptical cuts or straight cuts in the locations of greatest concern and with a distribution of depths and orientations including the minimum target detectible flaw depth. Elliptical cuts are most representative of real flaw growth but straight cuts are easier to manufacture and typically yield similar results. Controlled fatigue cracks or weld-embedded cracks are also becoming more wide-spread as the costs for these more realistic cracks are becoming more manageable. Figure: 5. CIVA modeling used to optimize frequency, element size, and angle prior to fabricating transducer 737

4 Through the years, several vendors have acquired a substantial portfolio of bolt inspection qualifications. In 2007, a comprehensive catalog of bolt designs associated with AREVA s key clients was assembled. The information included design geometry, material, access considerations, and an indication of the inspection technique and qualification status. This encompasses a high percentage of the possible bolt configurations but certain bolts still represent a challenge and/or availability of newer techniques coupled with an interest to improve the inspection quality have prompted continued advancement of techniques. Recent AREVA advancements have been focused on the B&W button head bolts, the Westinghouse 2- loop ID Hex bolts and the external hex head bolts with a locking bar typical of AREVA French plants (Figure 6). B&W BUTTON HEAD Figure 6: Recent advancements have focused on 3 separate bolt types For the B&W button head bolts, the challenge was to couple ultrasound into the curved head with a sufficiently controlled beam angle to interrogate the shank area of interest. Following the CIVA model optimization, transducers were manufactured and verified to detect all target flaws. This system was successfully deployed in 2013 using the SUSI submarine delivery tool. WESTINGHOUSE ID HEX SOCKET BOLT This configuration poses a particularly interesting challenge. The ID socket is made by drilling the basic socket hole then placing the hexagonal socket faces within the head using an EDM machine. The intuitively obvious face for introducing ultrasound into the bolt shank is the bottom of the socket. This however was not a controlled feature of the bolt design. Efforts to use a nominal profile of this surface have resulted in a poor percentage of successful inspections. Underwater dental molds of the hex socket bottom surface confirmed the variability of this surface. AREVA has developed an alternate technique using the well-controlled faces of the ID hexagonal socket. The ultrasound is reflected off the OD cylindrical side of the head sending the ultrasound into the bolt shank. The transmit and receive elements are located on a single face, adjacent faces, and opposing faces of the Figure 7: ID hex socket inspection from a single face, hexagonal socket. All combinations of adjacent, and opposing socket faces detect all target flaws transmit and receive elements are near head. 738

5 automatically interrogated to complete an inspection (Figure 7). This inspection system was deployed to one plant on a rush basis following schedule difficulties for a large-scale replacement plan resulting in significant savings in critical path time [3]. AREVA OD HEX WITH LOCKING BAR These bolts represent a significant portion of the overall bolt population and also represent a particular challenge due to the limited clean surface area available to insonify the bolt. Moreover, this kind of limited surface caused concerns about the relative amplitude for edm notch reflections vs. stress corrosion cracks. A series of bolts was developed with induced SCC cracks. These cracks were then examined with focused immersion UT (Figure 8) and with X-ray tomography (Figure 9) to confirm actual flaw locations in support of a new qualification effort. This qualification is on-going at the time of this paper's publication but the techniques used to demonstrate the true shape of induced flaws that can serve as inspection references represents a significant advance in the level of technology that can and has been applied to this overall inspection challenge. Figure 8: Reference un-cracked (left) and induced SCC samples (center and right) immersion UT showing full extend of induced SCC flaws. Figure 9: X-ray tomography by ERSF Synchrotron gives indication of crack morphology. ADAPTING TO SUBMARINE Internals bolt inspection requires precise delivery of the transducers to the bolts and a well-organized program to manage the data from the hundreds of inspections performed during each intervention. AREVA s Submarine System for Inspection (SUSI) has been adopted as a preferred delivery approach (Figure 10). A small neutrally buoyant cable is connected to a joy-stick controller manned by the trained SUSI operator. Video feedback of the view in front of SUSI aids the submarine driver to position the system. A lateral and rotational fine-position mechanism was added to the submarine base to facilitate transducer positioning. All of the added components are weight compensated so that the submarine is essentially neutrally buoyant requiring little energy to maintain depth and trim. 739

6 Figure 10: SUSI submarine with cameras plus linear traverse for higher productivity The submarine is launched and piloted from the side of the reactor and internals stand pool. The operator has an overview perspective either by eye, and/or from an overview camera. After driving near the first bolt to be inspected, the finepositioning mechanism with aid from the video camera focused on the transducer is used to position the transducer until a UT signal is recognized indicating proper coupling to the bolt. The UT data is captured and verified then the operator is released to go to the next location. The examination strategy is to position over the first baffle plate and inspects the top row (typically 3 bolts) then change elevation and repeat the process. This allows the operator to register which bolt is being inspected. The data is captured and managed with custom software that allows the bolt identification to be associated with the data. Vertical position is managed with vertical thrusters and monitored with a pressure sensor to better than +/- 3 cm. This is adequate to assure correct identification of the bolt being inspected. Speed of inspection is partially a function of the operator skill and the particular bolt configuration but typical inspection rates exceed 20 bolts/hour. This allows a typical full internals inspection intervention in less than 3 days. This Acoustic pinger/receiver positioned on SUSI plus internals top and bottom Figure 11: Acoustic location tool provides position feedback better than +/- 20 mm system has been used for more than 300,000 bolts over more than 10 years. Recent developments of the SUSI delivery platform include addition of a centrally located acoustic position sensing system. Acoustic chirpers are located on SUSI and are detected by the listening hydrophone. By triangulation on these signals, the absolute position of the submarine can be determined within the internals or vessel to within +/- 20mm (Figure 11). This is more than adequate to assure correct bolt identification and adds to the certainty and inspection verification completeness. 740

7 SUMMARY and CONCLUSION A range of bolt inspection techniques for reactor internals bolt examination has been developed and most RPV internals baffle bolts can be inspected to some degree. New techniques however are being applied in response to rising customer expectations and in some cases re-qualification efforts are being performed in order to introduce new and better methods. These techniques are deployed on the SUSI submarine inspection platform to maximize the inspection speed and minimize intervention schedules. This paper has addressed the particular case of the B&W style button-head bolt, the Westinghouse ID hexagonal head bolt, and the French style OD hex-head locking bar design where techniques have been or are being developed and qualified. In addition, an acoustic position verification system has been developed to allow the low-cost submarine mounted inspection approach to have confirmed and verified position feedback to assure the correct bolt identification. ACKNOWLEDGEMENTS Developments were performed by French, German, and US operational units of AREVA NDE-Solutions plus the AREVA NDE Technical Center (NETEC). REFERENCES [1] IAEA Tec Doc 1557 Assessment and Management of Ageing of Major Nuclear Power Plant Components Important to Safety: PWR Vessel Internals June 2007 Update [2] Materials Reliability Program: Inspection Standard for PWR Internals (MRP-228), EPRI, Palo Alto CA: [3] L. Friant and T. Richards; Innovative Solution to Baffle Bolt Inspection and Structural Integrity, 9 th International conference on Non Destructive Evaluation in relation to Structural Integrity for nuclear and pressurized components, May