Other Major Component Inspection II

Transcription

1 Other Major Component Inspection II Ultrasonic Inspection Technique for BWR Shroud Support Plate Access Hole Covers S.W. Glass III, B. Thigpen, Areva, France BACKGROUND Access hole covers (AHC) are found in Boiling Water Reactor (BWR) plants with jet pumps. The AHCs are located in the shroud support plate (SSP). The function of the SSP is to provide a structurally rigid connection between the reactor vessel and the shroud support ring, as well as create a flow-barrier to provide proper reactor coolant flow through the core. Typically the SSP contains two AHCs approximately 180 degrees apart; some BWR/6 plants have only one AHC. During the installation phase of the reactor internal components, construction personnel required access to the lower plenum area. Holes were cut through the SSP to provide an entrance and egress point. When entry to the lower plenum was no longer required, the access hole was sealed with a welded plate; the AHC. Typically the SSP and the AHCs are fabricated from Alloy 600 and joined with an Alloy 82/182 weld. Alloy 182 is considered susceptible to stress corrosion cracking (SCC) in the BWR environment. In 1988 visual inspections performed at a U.S. plant identified cracking in both of the AHCs. In response to the recommendations contained in a Service Information Letter (SIL) issued by the original equipment manufacturer, periodic inspections of AHCs have been performed since that time. Several different AHC designs exist depending on the type of BWR, the fabricator and the time period of construction. For the purpose of this paper only AHCs classified by the BWR Vessel and Internals Project (BWRVIP) as Thin AHC and Conventional SSP With Ledge shall be considered. Figure 1 provides an illustration of the AHC location in relation to the core shroud and RPV wall. Figure 2 provides a side-view illustration and weld joint design. Figure 1 - AHC, Top View

2 Figure 2 - AHC, Side View NDE CHALLENGE Recently released inspection requirements from the BWRVIP, BWRVIP-180: Access Hole Cover Inspection and Flaw Evaluation Guidelines [1] place a heightened emphasis on AHC inspections. BWRVIP-180 [1] was written to address long term potential degradation due to Stress Corrosion Cracking (SCC). Due to the as-welded condition of many AHC configurations, traditional fixed angle contact. Ultrasonic testing methods can encounter significant challenges related to data quality and examination volume coverage limitations. Figure 3 is a photograph of an in-service AHC. Figure 3 - In-Service AHC NDE APPROACH The AREVA AHC inspection approach consists of an innovative immersion phased array ultrasonic inspection technique coupled with a simple remote delivery system. The combination of this phased array technique and efficient delivery tool design allows for rapid data acquisition and increased examination volume coverage as compared to previous inspection methods.

3 Access Hole Cover Inspection Tool The inspection tool consists of an active suction-cup to securely attach to the AHC coupled with a 4 degree of freedom manipulator. Once attached, the tool provides four independent axis of motion; theta, radial, pitch and roll. The pitch and roll movements are used to position the transducer for optimum inspection angle generation. The theta and radial movements are used during the data collection process. The radial movement has an auto-tracking feature that can be programmed to perform discrete radial movements during the examination. These real-time radial adjustments compensate for inaccuracies in scanner placement and thus greatly reduce the initial tool placement time and accuracy requirements. The inspection tool is pole deployed with handling poles that include offsets and positive buoyancy adjustments to facilitate rapid installation. Innovative approaches to tooling design were employed to create a robust, easy to deploy, sophisticated yet straightforward manipulator with low foreign material exclusion (FME) hazards. Figure 4 provides an illustration and picture of the AREVA access hole cover inspection tool. Figure 4 - AHC Inspection Tool Ultrasonic Technique The ultrasonic system consists of a 64 channel phased array instrument and a 64 element linear phased array transducer approximately four inches long. The phased array technique employed for flaw detection and characterization is typically referred to as linear scanning or an electronic raster. With this technique a sub-set of the transducer s full aperture is excited to generate the desired inspection angle(s). This sub-set of elements is then indexed forward one element at a time along the full length of the transducer aperture. Figure 5 provides an illustration depicting linear scanning. The four red elements on the far left side of Figure 5 represent the active elements in the first index position. In the next illustration to the right, the first element is now deactivated and the fifth element is now included in the four active element group. This process of adding the next element and dropping the last is continued along the full aperture. Figure 5 - Linear Scanning Technique

4 Prior to the performing any inspection sequences the effects of temperature must be considered. This is particularly important due to the immersion phased array technique being employed. Temperature sensors are deployed to the inspection location and remain in place during the examination to permit constant temperature monitoring. System calibrations are performed in a vessel which is heated to the examination site temperature. Adjustments to sound velocity based on temperature are made during the creation of the phased array focal laws. Figure 6 depicts the effect of temperature on sound velocity in water. Water Velocity vs. Temperature Velocity ("/µs) Degrees (F) Figure 6 - Temperature Effects on Velocity in Water The inspection sequence consists of performing several scan types. Each scanning sequence requires approximately four minutes. The first scanning sequence is referred to as the profile scan. This profile scan differs from the flaw detection and characterization technique in that each of the transducer s 64 elements is excited simultaneously creating individual 0º angles. Performing the profile scan provides several important pieces of information. First, the data is used to map the location of the weld toes and this information is used to program the auto-tracking feature of the delivery tool. Second, the information is used to measure the water gap under the transducer around the component circumference. This information is used to calculate the adjusted water gap dimension when programming the phased array focal laws for flaw detection scanning. Third, this data provides surface details, including weld width and general surface condition information. This is used to develop the final scan plan parameters to assure maximum coverage of the required examination volume and is useful to verify that the tool has attached itself to the cover correctly. Additionally, this data is useful if indication depth sizing is required. The second scanning sequence is used for the detection of circumferentially oriented flaws and is referred to as radial scanning. This scanning sequence uses probe pitch to direct the UT beam towards and away from the center of the cover in the radial plane. Inducing the physical probe pitch reduces the amount of electronic beam steering necessary to achieve the desired inspection angles and reduces the negative effects associated with over-steering. This scanning sequence is performed from both sides of the weld; once looking in towards the center of the cover and once looking out from the center towards the shroud support plate. If an excessively wide weld crown is present, additional radial scans can be performed to achieve adequate coverage of the inspection volume. These scans are performed using the linear scanning method. The water gap information obtained from the profile scan is used to calculate the adjusted water gap under the first element of the array based on the degree of probe pitch and the pitch direction. The radial scanning sequence uses phased array techniques to generate four focal law groups. These focal law groups create 0, 45, 60 and 70 inspection angles.

5 Figure 7 provides an illustration of the radial scanning technique. For radial scanning in areas where full access to the examination volume is limited due to interferences from the core shroud or reactor pressure vessel, a sector scanning technique is used. This sector scan technique uses a rather small sub-set of the full transducer aperture to generate a range of inspection angles from 40 to 70. This small active aperture is positioned as close as possible to the scan limitation and the sound energy is directed towards the weld region. The addition of this sector scanning technique allows additional examination volume coverage in areas with limited access to the weld. Figure 7 - Radial Scanning Technique After the radial scan sequences have been completed, the next scanning sequence performed is circumferential scanning. The circumferential scan sequence is used for detection of flaws with radial orientation (perpendicular to the weld axis). This scan sequence uses a probe roll to direct the UT beam in the clockwise and counter-clockwise directions around the circumference of the cover. Performing the radial scanning sequences in this manner reduces the need for multiple transducers, reduces the complexity of tool design and maximizes coverage of the examination volume. If the weld to be inspected is of moderate width, coverage of the examination volume can be achieved in one clock-wise and one counter clock-wise scanning sequence. For welds with greater widths, multiple scan sequences can be used to provide the required coverage. These scans are performed using the linear scanning method and generate neutral, negative and positive beam skews to enhance coverage and provide increased detection capabilities for miss-oriented flaws. The active aperture is indexed along the full probe aperture creating an electronic raster scan across the inspection volume. The transducer roll is established to induce a fixed inspection angle. The degree of roll required is calculated using Snell s Law, plus adjustments to consider temperature influences on the sound velocity at the component. The water gap information obtained from the profile scan is also used to calculate the adjusted water gap under the array. Figure 8 illustrates the basic principle of the circumferential scanning sequence.

6 Figure 8 - Circumferential Scanning Technique CONCLUSION Access hole covers (AHC) found in Boiling Water Reactor (BWR) plants with jet pumps have been periodically inspected since the discovery of cracking in a U.S. plant in Inspections have consisted of visual and ultrasonic testing. Due to the as-welded condition of many AHC configurations, traditional fixed angle contact ultrasonic testing methods can encounter significant challenges related to data quality and examination volume coverage limitations. Recently released inspection requirements from the BWRVIP, Access Hole Cover Inspection and Flaw Evaluation Guidelines BWRVIP-180 [1] have place a heighten emphasis on Access Hole Cover inspections. BWRVIP-180 [1] was written to address long term potential degradation due to Stress Corrosion Cracking (SCC). AREVA s proprietary AHC inspection approach consists of an innovative immersion phased array ultrasonic inspection technique coupled with a simple yet efficient delivery system. The combination of this phased array technique and efficient delivery tool design allows for rapid data acquisition and increased examination volume coverage as compared to previous inspection methods. This inspection technique has been demonstrated through the BWRVIP program with specific focus on the thin creviced AHC design. REFERENCES 1) BWRVIP-180: BWR Vessel and Internals Project, Access Hole Cover Inspection and Flaw Evaluation Guidelines