Two in one9 May 2017
A new tool can identify cracks that are both parallel and perpendicular to core shroud welds. Craig Ranson explains how the technique was used the first time at a US boiling water reactor.
Ultrasonic testing plays a key role in maintaining the safe operation of nuclear energy facilities, both in the USA and around the world. Many of the plant areas are difficult or impossible to reach for direct visual or physical inspections, so the ability of ultrasonic testing to interrogate inaccessible regions is an essential tool for evaluating the integrity of plant components. As the physical configurations of some plant components create inspection challenges, innovative non-destructive examination technologies are required to help operators and engineers perform the required periodic inspections. In ultrasonic testing, operators use concentrated sound waves to detect defects or age-related degradation in a plant’s equipment and components. By identifying and monitoring these conditions, operators are able to maintain primary system safety and availability, reduce operating and maintenance costs, and extend plant life.
Ultrasonic testing is especially useful for monitoring intergranular stress corrosion cracking (IGSCC) and off-axis flaws (OAF) in the welds in the core shrouds of boiling water reactors (BWRs). As BWRs age it is fairly common for their core shroud welds to show signs of IGSCC and OAF, even considering the robust stainless steel used in their construction. Nuclear plant operators carefully monitor these conditions, as this area of the plant is crucial to maintaining the safety and reliability of the core containment and the coolant flow through the core. For plants in the USA, the Electric Power Research Institute’s (EPRI’s) BWR Vessel Integrity Program (BWRVIP) requires that core shroud welds in BWRs undergo ultrasonic and visual inspections at specific intervals.
Until recently, the ultrasonic testing techniques used for BWR core shroud welds focused on the detection and characterisation of cracking that is primarily parallel with the weld axis. This type of cracking is primarily related to IGSCC.
In addition, visual examinations of the core shroud welds are performed on a routine basis. Evidence of cracking perpendicular to the weld axis, or OAF, has been identified during these visual examinations. Evidence of OAF prompted the industry to generate a new guideline for inspection that focuses on detection and characterisation of this type of cracking.
Where IGSCC-related cracking is primarily oriented parallel to the weld, OAF indications primarily form perpendicular to the welds and are believed to be related to irradiation-assisted stress corrosion cracking (IASCC). To effectively monitor both conditions, operators would probably have to conduct one or two tool removals and installations, reconfiguring equipment to switch the tooling for the different inspections (horizontal weld, vertical weld, and OAF detection and flaw sizing exams). These changes are typically time consuming, and dose-intensive to inspection personnel, effectively doubling the inspection time and forcing operators to extend costly plant outages and downtime.
Moreover, the area of the plant where the core shroud welds are located poses some unique challenges for ultrasonic testing. These include tight gaps – less than 2 inches wide – between jet pumps and the core shroud, water depths of the order of 80 feet, and areas with high flow rates due to shutdown cooling operations. BWR units with repair clamps and tie rods present additional challenges with respect to tooling interference. Inspection requires complex, robust tooling that can remotely perform the required inspections, underwater and in high radiation fields. Personnel exposure to elevated temperatures and radiation exposure is also a critical concern for individuals working with the equipment.
A new solution
In 2016, challenged to complete a set of ISGCC and OAF inspections for a US customer during the spring scheduled outage, Areva NP’s engineers developed an innovative solution that improves the effectiveness of ultrasonic testing for core shroud welds. Their goal was to properly identify and characterise cracking parallel and perpendicular to the weld axis (IGSCC and OAF), without affecting the plant’s outage schedule and while maintaining personnel exposure as low as reasonably achievable.
To achieve this, Areva’s non-destructive evaluation development team created a new inspection tool and ultrasonic testing technique specifically targeted to the application. This innovative approach helps to accurately detect and characterise all flaws surrounding the weld, both perpendicular and parallel. It pairs an advanced dual-matrix phased-array ultrasonic testing technique with a specialised multi- axis delivery manipulator. This new manipulator can be rotated and extended to enable both horizontal and vertical weld examinations to be peformed without removing the tooling from the reactor for orientation changes. The new dual-matrix phased-array transducers house two 8x4 matrix arrays (64 elements per transducer) in a 4.0cm x 5.2cm x 2.21cm (1.6in. x 2.06in. x 0.87in.) package. It is important to note that only one probe design was ultimately required, since the tooling was able to deliver the probe in different orientations.
The examination system is remotely controlled, so the tooling operators and ultrasonic testing technicians can remain outside the plant during the examination. The only personnel that have to be inside the plant are those installing, removing and maintaining the tool.
The new tool can be used to inspect most BWR plants with 3/4/5 configurations. Some specific weld geometries may require an extra demonstration; however, the tool and transducers should be the same.
Putting it to the test
The Areva team undertook a very careful process to develop the tool in compliance with guidance supplied by EPRI’s Core Shroud Focus Group and BWR Vessel Internals Project Assessment Committee. To ensure the tool worked properly, before it was used during an outage the Areva team developed and tested multiple concepts on a core shroud remnant mockup supplied by EPRI for just such a service. This data, as well as 3D modelling, was used to optimise the probe design with emphasis on the generation of L-wave and shear wave modes, beam skewing and probe size. Probe size was one of the main challenges and innovations of this product. The probe and the delivery tool had both to fit within the tight gaps between the core shroud and the jet pumps. The team worked with a transducer manufacturer to develop the final probe design.
The goal was to have one tool configuration for all welds. The designers packaged four transducers into a scan arm that directed the transducer beams perpendicular and parallel to the welds. Small robust air cylinders were designed to engage and disengage the transducers to the surface. Multi-axis transducer swivel mounts were designed to ensure they maintained proper contact with the inspection surface. The tool remotely engages the scan arm using a four-bar linkage, and the scan arm is rotated 90 degrees to scan the first side of the weld. Then it rotates 180 degrees to scan the opposite side of the weld. By using four properly positioned probes, the scan arm can acquire ultrasonic testing data along approximately 55 inches of weld length.
Areva NP demonstrated the new ultrasonic testing tool in September 2016, successfully combining IGSCC and OAF inspections for the core shroud welds in a US BWR. The first-of-a-kind tool configuration successfully completed the core shroud inspections for both ISGCC and OAF with excellent data quality, saving outage time and personnel exposure, resulting in a cost savings to the plant operator. The team’s successful demonstration proved that the phased array ultrasonic technique and new tooling could meet EPRI’s interim guidance, as well as current inspection requirements, with only one tool.
Having transducers oriented parallel and perpendicular to the weld axis, the generation of longitudinal and shear wave modes, combined with the ability to steer and skew the ultrasonic beam, provided a very robust technique capable of detecting and characterising cracking, regardless of the crack orientation with respect to the weld axis. The data were collected using a scanning sequence where the transducers were moved parallel to the weld axis and then indexed towards or away from the weld. In this manner, a very high level of data density was achieved in an efficient examination time.
Benefits to the plant operator
The primary benefits of this technique include significant customer savings in examination time and, more importantly, personnel radiation exposure. By deploying one tool for the entire inspection, the team minimised safety risks and radiation dose. During Areva’s September 2016 demonstration, a dose of 500 millirem was averted by the shortened process. Areva’s time on site was reduced by more than 10 hours and four hours of critical-path time was saved. All told, the reductions in time and personnel saved the plant operator more than $100,000.
The new tool also saved data acquisition time, and allowed the data to be examined more efficiently. The quality of the data and signals available to review allowed the ultrasonic testing data analysts to quickly interrogate and present the results. This is important, since a structural analysis is typically required for all relevant indications. The data analysis results were transferred to the appropriate engineering organisations for evaluation and analysis, and presented to the plant’s nuclear safety committee prior to plant startup.
Innovations such as this first-of-a-kind ultrasonic inspection tool are what help US utilities deliver on the nuclear promise: safely and efficiently operating the US nuclear fleet for decades to come. By finding ways to streamline processes and inspections, plants remain economically viable in the shifting energy market.
Author notes: Craig Ranson is senior vice president, Installed Base America at Areva NP