Advancing SG inspection

STEAM GENERATOR INSPECTIONS HAVE been conducted using robotic technology since the late 1970s. But they have been radically changed by advances in manipulators, inspection probes and delivery systems, along with the software that supports them. Developing these systems aims to improve the quality and speed of inspections, while reducing dose. With today’s technology, these inspections are more efficient and safer than ever.

Technology advances

Early steam generator inspections involved hand-delivered inspection probes. This was a manual and time-intensive process that required personnel to insert the probe into each tube, one by one. To reduce dose, engineers developed robotic delivery systems that could be installed into the steam generator and then operated remotely.

Over time, there have been 10 to 15 different types of robotic delivery systems.

In early versions staff mounted the probe delivery systems on the tubesheet, and an electric motor and remote control allowed them to move the system on its X/Y axis. However, personnel had to manually move the robot to different locations on the tubesheet. It also could not perform tube repairs or plug inspections.

This was further developed into manway-mount robots that could be inserted into the steam generator channel head-access hole. This improved tube access, but the robots faced challenges related to cable routing, stability and alignment accuracy. Personnel often had to intervene to straighten cable and add or remove sections of conduit to allow the tool head to be moved.

Meanwhile, engineers were also developing tubesheet- mounted robots. With a stationary mount to the tubesheet, these also had access limitations and had to be repositioned by personnel during the inspection. At this time robots gained the ability to perform remote tube repairs.

The next generation of robots was known as ‘arm and mast’. These robots anchored into a tube hole and extended a foot to the bottom of the channel head to gain a firm stance. This allowed the arm to locate tubes and deliver inspection probes. However, these robots also required personnel to reposition them periodically. Later advances in this style of robot included a device at the top that could remotely reposition the robot without human intervention. These arm-and-mast robots were the workhorses that supported heavy maintenance of the first generation of steam generators. As alloy 600 mill-annealed tubing began to show its age, large scale repairs including tube plugging, tube removal, shot peening and sleeving were conducted to extend steam generator operating life.

This work required a strong, and therefore heavy, robot. Many current robots for steam generator inspections are known as tubesheet walkers. They have fingers or grippers that can reach into the tube holes to grip the tube and hang suspended from the tubesheet. These robots can release several grippers at a time, and reposition them to “walk” about the tubesheet and complete the work with no exclusion zone. In addition, they can turn and move in a path to manage their cables and conduit to keep them untangled.

After extensive steam generator replacements, these robots have largely changed from being used for repairs to inspections in today’s reactor fleet. Lighter and nimbler robots conduct inspections more efficiently. For instance, Areva NP’s Trident steam generator inspection system has an autonomous, triple-probe system that allows personnel to monitor tube conditions throughout the steam generator and map secondary side deposits, foreign material and tube bundle support integrity. The Trident system inspected more than 17,000 steam generator tubes in less than 77 hours at a US nuclear plant during an outage in spring 2017.

Newer manipulators and probe delivery systems offer easier installation, removal and movement, which lowers dose for the personnel who manage these activities. These systems can also access tubes more easily, better align with tube holes and move more quickly across the tubesheet, increasing inspection productivity. Today’s robots can carry the necessary tooling and payloads for both inspections and repair. They are also more reliable and have lower maintenance costs. Collectively, these improvements help to shorten the schedule and lower personnel dose.

Software development

Just as the manipulators and probe delivery systems used for steam generator inspections have come a long way, so too has the software that manages these robots’ actions and the data processing that supports the inspection.

Once a robot is installed and made aware of its environment, today’s steam generator inspections are largely hands-off. This is possible due to thorough and comprehensive inspection plans that comprise guidance on thousands of tubes with different inspection requirements. This information is loaded into the robot so that it can determine the best path and tube sequence, as a car’s navigation system or a mobile app directs a driver on their best route. The software optimises the inspection, based on given parameters and information.

Once it determines the best path, the software ensures the robot arrives at the proper location and locates the inspection probe over the correct tube using mechanical encoders and a camera that counts tube holes as the robot moves. This provides independent verification that the robot has arrived at the correct location. The software also receives inspection probe data to determine whether the probe is in or out of the tube. These safety features help ensure the tubes and probes are not damaged.

In addition to telling the robot where to go and triggering the probe, software helps the robot better manage the flexible umbilical connected to the probe. Because the robot understands where it is positioned on the tubesheet, it knows how much umbilical is needed to push and pull the probe into and out of the tube. Software helps the robots do this automatically, without staff actions.

Software also verifies the quality of inspection data before the robot moves to the next tube. Commonly, in seconds it will verify 18 to 22 items of inspection data before the manipulator moves on. This helps to ensure the quality of the inspection data so the robot does not have to return to the same tube.

Because today’s tubesheet-walker robots are smaller than previous robots, two can be in the same steam generator channel head performing inspections at the same time, increasing productivity. Integrated collision avoidance software manages the movements of two robots, so each robot is aware of the other’s location. They can also share and optimise a joint inspection plan.

Once the inspection data are acquired, they are analysed to determine whether there are any flaws in the tube. This can be done during acquisition by streaming data immediately to the analysis software. The software reports back to the manipulator if there are other inspections needed with a different probe, and this is included in the inspection plan.

Inspection plans also allow robots to coordinate and conduct a range of different activities. For instance, the most common inspections conducted on tubesheets are of plugs. If a tube has a defect, both ends must be plugged, so water no longer flows through it. These plugs are inspected every time the steam generator is inspected. Previously, personnel would direct the robot around the steam generator to video the tubesheet. Then an inspector would watch the video until it went over a plug, stop the video, inspect that plug and report the findings.

With current technology, eddy current testing and plug visual testing can be combined into one inspection plan. This allows the robot to understand whether it is delivering a probe into a tube to inspect it or simply focusing on the plug and capturing the necessary visual information. As a result, the robot only needs to move across the tubesheet once to complete multiple actions.

Looking to the future

Technologies for steam generator inspections are constantly evolving on several fronts. New manipulators are largely used for conducting inspections, supported by continually advancing acquisition and analysis software, and higher-performing inspection probes. For example, earlier probes could see that a flaw or defect existed, but could not determine what it was. More sophisticated technology can now characterise the defect during the inspection.

Not only that, but advanced eddy current instrumentation offers faster processing time. Personnel can use a range of frequencies in the probe to see different things in the tube, collecting more than 4,000 samples per second. These high digitisation rates of the inspection data require advances in the electronics of the eddy current instrument. Whereas initially probes used to scan a tube at a rate of 12 inches per second, now they commonly inspect at speeds in excess of 80 inches per second.

Ultimately, advances in steam generator inspection technology allow these inspections to be performed completely remotely, with the critical benefits of improving dose, safety and performance. Personnel can rely on the manipulator, the probe delivery system, the inspection probes themselves and the software to manage the inspection with minimal human intervention, making these inspections safer and more efficient than ever before.