Twin benefits10 March 2021
A combined tool for removal of pressure tubes and calandria tubes can help reduce outage duration, cut cost and improve safety in Candu refurbishments
THE NUCLEAR INDUSTRY HAS A long history and culture of pushing boundaries of design and bringing innovation to complex projects in challenging environments.
As the original equipment manufacturer of Candu reactor technology, SNC-Lavalin is no different. Our unique understanding of the reactor design and our constant involvement in addressing challenges on refurbishment projects gives us a unique line of sight. Using this expertise over the years, we frequently spot opportunities to develop tools and approaches that make projects safer, save our clients time and reduce the costs associated with reactor outages without sacrificing quality. An outage can cost a utility up to $1 million per day, presenting a strong incentive to accelerate critical steps to return to service and restart the revenue stream.
Discussions and preliminary assessments using historical data made it clear that new tooling that enabled Candu pressure tubes and calandria tubes to be removed in one extraction instead of two would significantly improve the retube process. It would also affect the release and removal of calandria tube inserts (CTIs) and the volume reduction of pressure tubes and calandria tubes.
To protect these innovations, many of the methods and tooling have either been patented or have patents pending in Canada and other jurisdictions (eg Canadian Patent, CA 2766583, “Methods and Apparatus for Handling Materials for Retubing a Nuclear Reactor” and Canadian Patent, CA 2766459, “Calandria Tube, Pressure Tube, and Annulus Spacers Removal Apparatus and Method for Nuclear Reactor Retubing”.) The new tooling cuts five per cent of the total retube time off the critical path of a refurbishment project (nearly a month) and saves the client up to $30 million per unit. But while reducing the project timeline was the initial driver, it also increased safety.
Knowledge from previous refurbishments was used to understand the new constrained operating envelope of the tooling. Changes, including different analysis methods and the addition of a monitoring system, were considered and mitigated. The unknowns of the system were tested, and additional contingencies put in place to address the new risks to the project. The design was supported by theoretical analysis, proof of concept and mockup tools testing.
The new tooling and modified process will be applied to Ontario Power Generation’s (OPG’s) ongoing refurbishment at Darlington.
Understanding the Candu design
Candu reactor cores have an array of horizontal fuel channels, each forming part of the heat transport pressure boundary. The fuel channel is a thick-walled zirconium pressure tube which contains the fuel and water for heat transport. Stainless-steel end fittings provide connections for heat transport water circulation and fuel changing.
In order to minimise heat transfer from the pressure tubes into the surrounding moderator, each pressure tube is encased in a second thin-walled tube called a calandria tube. The space between the tubes, known as the annulus, is sealed, at both ends, by stainless-steel bellows, which are filled with carbon dioxide gas acting as insulator. The bellows provide flexible connections, which allows for axial differential thermal expansion, radiation growth of the pressure tube and an access point for the annulus gas system.
The large vessel known as the calandria contains fuel channels and calandria tubes. A Candu reactor contains either 380 or 480 fuel channels and calandria tubes, depending on its power output. The calandria tubes are secured at each end to the calandria vessel by mechanically roll expanded joints. The calandria tube wall is sandwiched between the calandria tube sheet bore and an insert ring.
A Candu reactor undergoes a refurbishment, including safety upgrades, as it approaches the mid-point of its operating life, readying it to continue delivering safe, clean and reliable power for another 30 years. The refurbishment operations to remove and replace the components that make up the reactor fuel channels and calandria tubes are broken down into sets of activities, known as work series.
Two of the work series, which have the longest durations and are in the refurbishment critical path, are the series to remove and replace the pressure tubes and calandria tubes. Each takes approximately one month to complete, per unit.
Safety and control improvements
Depending on the tooling configuration used and constraints within the reactor vault, some Candu refurbishment projects require the removed tubes to be hoisted from the work platform at the reactor face to the vault floor in a shielded flask.
The materials handling and hoisting operation is complex, because of the weight of the multi-tonne flasks and the space constraints. In a 480-tube unit, removing the two tubes together cuts the number of lifts in half, from 1920 to 960.
The tooling innovation also improves employee safety through improved radiological protection and control of hazards from the annulus spacers.
In the previous process, the annulus spacers were typically destroyed and dislodged by the pressure tubes as they were pulled through the calandria tubes. Pieces of annulus spacer ended up in unplanned locations and were difficult to handle in subsequent operations. Workers had to physically retrieve the spacers in hazardous environments. Removing the tubes together, the annulus spacers remain locked in place.
Personnel training and development
SNC-Lavalin’s Retube, Inspection, and Maintenance Systems Group (RIMS), a multidisciplinary group consisting of over 200 engineers and technical resources, is responsible for the developing this system and the associated training programme. Members of RIMS are experienced in the design, build, qualification and operation of tooling systems that are deployed in highly radioactive environments, particularly on projects at Candu nuclear facilities around the globe.
Training will be carried out at the Darlington Energy Complex, where there is a full-scale nuclear reactor mockup training facility. The mockup represents the dimensionally accurate, physical environment of a Darlington reactor. It allows workers to practice their work tasks, perfect their techniques and perform dry runs in a controlled and realistic environment prior to executing them on the reactor face.
Skilled trades will be trained to operate the tooling system using the retube control system human machine interface (HMI). This computer program is displayed on multiple monitors and controlled via a keyboard and mouse. As the system is automated, operators will initiate sequences, review parameters and progress, and identify anomalies.
Innovations in the removal of pressure tubes and calandria tubes
The combined pressure tube and calandria tube removal process required modifications to the current tooling used to remove the calandria tubes.
The existing tooling was designed for empty calandria tubes and its ram extends into the tube to grip its internal diameter. A pulling and pushing force were simultaneously applied to its ends to extract the tube from the bore in which it is located. Since the diametric clearance between the pressure tube and the calandria tube is small, it is challenging to fit the mechanisms for gripping. If the pressure tubes were still inside the calandria tubes, the tooling could not access the gripping points within the calandria tube.
The tooling was redesigned to grip the calandria tube near its end and fit around the pressure tube. The increased mass from a pressure tube with the potential of up to nine dummy bundles inside it required a design solution to ensure the removal tooling would clear components not replaced during retubing.
Calandria tube insert release
The restrictive operating envelope required additional modelling to assess and limit any adverse changes to the reactor component state required for the work series that follows removal of the pressure tube. The modelling created a reference case based on design dimensions against nominal reactor lattice site conditions, with consideration for tolerances corrected to an individual case, based on available current operating and original construction data sets.
Information on pressure tube axial growth was obtained from OPG for evaluation to determine the extent to which mitigating action would need to be taken.
The data showed that the conditions imposed a new design requirement of shuttling the pressure tube back and forth on the CTI release series. This new challenge was overcome by reinforcing the load path with restraining collars and front cap, which were verified by finite element analysis (FEA). The findings of the FEA were used to confirm that the designed load path had the required strength and that the integrity of the existing components were not compromised by the required design changes.
An additional electromagnetic field analysis was performed to determine any parasitic effect on the performance of the tool caused by the location of the pressure tube. This assessment was used as input to the modification and for material selections.
Calandria tube insert removal
The original CTI removal tooling was affected by the presence of the pressure tube and could not perform the grip function on the calandria tube as it had in the previous design.
With a pressure tube present and axially elongated, space between the end of a severed pressure tube and the CTI is very small. Furthermore, gripping of the calandria tube had previously been found to cause pressure tube movement and could knock the CTI out of the opposite face. A modification now monitors the calandria tube for movement during CTI removal, and the redesigned tool head minimises pressure tube movement.
A comparison between pull forces at a Darlington 2 calandria tube and CTI pull forces. The assessment of required applied forces drove changes to the design of the process and gripper mechanism to ensure safe removal of the CTI. During the removal of the CTI monitoring detects any motion of the calandria tube with respect to the stationary tool body. The process stops if movement is detected.
A detailed analysis of failure modes and their effects (FMEA) was integral to the design of the system. The failure mode where calandria tube movement is detected during removal of the CTI resulted in planned contingency manoeuvres to ensure a safe, robust process.
Video processing equipment at the rear of the tool keeps digital components outside the high radiation fields. The video is automatically monitored by vision software and is available to operators as back up. It does not require operator input to stop the CTI removal process.
A fatigue analysis was performed to determine the number of cycles the modified tool head can endure.
Calandria tube insert reseat
An FEA was conducted to determine the maximum amount of force that can be applied to the CTI removal tool during the reseating, along with calculations regarding the maximum possible reseat force generated by the CTI removal tool. Results were used as basis for process decisions.
Post removal volume reduction
Following the removal of the pressure tubes and calandria tubes, their volume is reduced for intermediate storage and long-term disposal. This process had to be changed to handle the combination of pressure tubes and calandria tubes, to reduce both at the same time.
A contingency was built into the process to handle reduction of the two components individually, if the tooling failure rate turned out to be higher than expected.
An in-depth testing programme — proof of principle on the mock-up and design verification testing — was performed to validate functions and to modify software. The purpose was to develop techniques that would use the existing system and be able to implement the changes.
Improving outdated methods and tools has helped our clients drive down costs and speed up reactor operational readiness by implementing this innovative combined pressure tube and calandria tube removal. It made the processes more efficient, increased productivity and enhanced the safety of workers and sites.
Authors: Robert Mallozzi, Director of retube, Inspection and Maintenance Systems at SNC-Lavalin and his colleagues from the Candu Tooling Design Engineering Team