By David Seery
In August, personnel from WesDyne Sweden AB, along with colleagues from WesDyne International LLC (both fully owned Westinghouse subsidiaries), and Westinghouse Electric Company LLC, used specialized robots to inspect all required boiling water reactor core shroud weld seams at the Kernkraftwerk Mühleberg (KKM) Nuclear Power Plant in Mühleberg, Switzerland.
To successfully perform this inspection, the team used WesDyne’s T-crawler robot family, together with a novel time-saving nondestructive testing solution. This solution contributed to completing the work several hours ahead of the planned outage window. This was the fourth such inspection the global team has performed for KKM since 2011.
Meeting the inspection requirements for this core shroud’s weld seams is an accomplishment that speaks to the collaboration between utility, WesDyne and Westinghouse personnel to share needed information, plan, design and implement a solution to an access puzzle that is both effective and economic.
During the 1990s, the weld seams of some boiling water reactor core shrouds were found to have intergranular stress corrosion cracking. This discovery prompted organizations around the world, including the Electric Power Research Institute Boiling Water Reactor Vessel Integrity Program and the Swiss Federal Nuclear Safety Inspectorate ENSI to require that the weld seams of some boiling water reactor cores be thoroughly inspected. For KKM, this requirement must be satisfied every two years.
Determining the extent of indications in the weld seams of core shrouds requires that inspection tools are able to maneuver in and around the reactor core to access the weld seams and acquire accurate and thorough data. The indications can include both axial and transverse (off-axial) intergranular stress corrosion cracking. KKM added a requirement in 2015 to detect transverse indications.
The inspections are performed under water – to depths of up to 80 feet (~25 meters).
The reactor design at KKM also includes internal parts that make access to portions of the welds in the already narrow annulus region of the reactor even more difficult. The reactor internals include jet pumps, instrumentation sense line piping, vertical riser pipes, and the structures that support each of these components. Adding to the challenge are tie rods that were installed as mitigation against potential weld seam intergranular stress corrosion cracking, a modification made to the core shrouds of some boiling water reactors.
The general requirement at KKM is that inspection tools are able to navigate gaps less than 2 inches (5 centimeters) wide, and in certain limited areas, to deliver probes in gaps as narrow as 1 to 1.25 inches (2.5 to 3 centimeters). In addition, the inspection robots are off-camera during significant portions of scanning and movements between areas, requiring careful planning to achieve accurate and full data acquisition.
WesDyne had considerable experience performing underwater inspections within the restricted space of the annulus region in other boiling water reactor designs. For other inspections of this nature, WesDyne had developed a family of modular design inspection tools – the T-crawlers – and had been applying these robotic tools since 2006. However, the reactor models for which they were originally designed did not have nearly as many internal structures and, therefore, WesDyne’s existing inspection tooling was unable to sufficiently maneuver around some of the reactor internals presented by KKM’s core shroud to complete the inspections.
WesDyne leveraged the modular T-crawler design to grow the number and variety of these robust and complex robots to meet KKM’s needs, allowing several manipulators to be realized to meet the specific requirements of each examination area, including the areas with very narrow gaps.
WesDyne personnel worked closely with KKM personnel, who facilitated pre-job walkdowns and provided details that allowed the WesDyne team to develop the additional robotic manipulators and plan for the off-camera inspection durations.
Knowledge gained the first time WesDyne conducted these inspections for KKM in 2011 prompted the creation of an additional type of tool for the T-crawler robotic family, the T-hin.
Ultimately, four separate robotic manipulator designs were created, with each tailored to examine one or more weld groups. Two of the designs, the T-horizontal and the T-hin, have both left and right configurations. This feature facilitates parallel examination, working in different directions, along like welds as needed. Some of the robotic manipulators can freely roam the core shroud using suction cups and a well-developed control system. The suction cups are also important for the other designs – the T-horizontal and T-vertical, where they are used as part of attitude (tilt) control mechanics, as part of at least one motion axis, or simply for providing a solid anchor point against an otherwise featureless surface.
While all free-movement, remotely operated robots pose a challenge in determining and tracking accurate and repeatable positioning, WesDyne made procedural adjustments to accommodate the inspection durations during which the robotic scanners would be out of visual camera range at KKM. To ensure the full welds were captured, WesDyne developed procedures for establishing landmark positions using several different techniques at multiple points around the entire weld areas. The quality of the data and weld coverage of the inspections improved with each inspection, allowing highly accurate indication characterization.
The easy-to-handle system requires only a small crew onsite and proved to be a good choice for the job: Not only could the family of robots be grown to accommodate inspection needs, these compact T-crawler robots allow inspections of different areas of the core shroud to be performed concurrently with each other and in parallel with other reactor vessel outage activities.
Once submerged under water, the lightweight robotic inspection systems do not require use of a crane even for larger movements between inspection areas; they can be handled manually with poles and ropes. Additionally, they have a small footprint on the refueling floor and do not require a separate rail system or other bulky support equipment to be installed in the reactor.
Careful planning using multiple probe solutions allowed the field team to configure each T-crawler manipulator so that a single setup in most cases accommodated performing inspections of all of the welds intended without the need to lift for reconfiguration. Once the manipulators were attached and positioned, the scanning moved along quickly, with two systems working in parallel. Since this work was performed concurrently with other preparatory work being conducted on land before immersion in water, the T-crawler manipulator technicians and operators worked closely together. Direct communication and tight coordination was maintained between data acquisition personnel and machine operators, and extended to other groups that performed examination and maintenance activities in the pool and on the refueling floor. Teamwork was very important for staying within the timeframe and completing the work safely and accurately.
Operators using the numerous members of the T-crawler manipulator family interchangeably benefit through use of a single motion control software interface, reducing the risk of human performance errors when switching between the robotic manipulators. The software interface is highly flexible to support the free-roaming movements of the T-scanner robots and is capable of performing the complex scan patterns required for data acquisition. Using code the company had already developed, WesDyne tailored it as needed for KKM’s landmark sightings and special movement patterns.
In preparation for the 2015 inspection, the team continued nondestructive examination technique development, adding and qualifying a novel nondestructive examination solution to cut the time required for data acquisition while also meeting the new requirement to inspect for transverse indications. The approach combined the existing advanced phased-array ultrasonic testing with the more conventional time of flight diffraction technique (TOFDT). Advanced phased array greatly reduces scanning time because it effectively provides very fast additional scan axes by electronically “steering” the ultrasonic testing beam using precisely controlled delays to an array of transducer elements, whereas only single elements would be used in conventional forms of ultrasonic testing requiring slower mechanical scanning. The same dynamic beam-steering also allows different approach angles to illuminate indications, which provides useful detection and characterization information using a single probe, where several would have been needed previously. The use of TOFDT provided an extra degree of accuracy for certain types of characterization needs. Combining the best of these two existing nondestructive examination technologies allowed inspecting the welds for both lateral and transverse indication detection and sizing, including accurate depth determination, within a single scan.
By 2017, following the companywide continuous improvement goals, the team has achieved many refinements to accommodate the access challenges of the weld seams of the KKM reactor core, as well as improvements to optimize data acquisition and inspection efforts. The series of examinations have shown repeatability of the system, providing the data needed by KKM to satisfy regulatory requirements for continued operation of the plant.
The T-crawler system is compatible with all major nondestructive inspection data collection systems and can be adapted for other components with challenging inspection scenarios.
David Seery is a lead engineer for Control Systems at WesDyne Sweden AB.