Computer simulation aids recovery28 July 2000
Nuclear power plants pose great challenges for inspection and repair. This is especially true for the early power plants.
Each of the UK’s Magnox reactors is unique. As the world’s oldest power reactors, they were products of the pioneer spirit which dominated the nuclear industry in the 1950s and 1960s. Being unique, their designs pose engineers with great maintenance and repair challenges. The sophistication of the solutions, involving computer simulation and the use of remotely operated manipulators, robots and vehicles, has given the BNFL Magnox Generation team a great deal of knowledge and experience of working in hazardous conditions with limited access, using remote techniques and computer simulation.
Barnaby Beere is a member of BNFL Magnox’s Remote Operations Branch. He has been working on manipulator control systems and computer graphics for use in outage operations for six years. At Dungeness A, he has been involved in reinforcing crown top hats within the reactor pressure vessel.
“The structure of the reactor pressure
vessel is such that there are insulation plates on the inside surface bolted to the pressure vessel,” says Beere. “To prevent heat loss through the bolts, they are covered with components known as crown top hats which are welded to insulation plates.”
The steel to which the crown top hats are attached has been exposed to high temperature CO2 for over 30 years. It has reacted to form magnetite, which occupies a greater volume than the metal it displaces, leading to a problem known as ‘oxide jacking’. As a result, some of the crown top hats have become detached from the surface of the reactor pressure vessel. In itself, this does not pose a safety problem, but without the insulation the hats provide, the reactors’ lose efficiency. Furthermore, should a hat fall off, it could block a fuel channel. A technique had to be developed to repair the hats.
Engineers decided that the best technique would be to drill a hole through the crown top hat and attach a retention plate to it and the reactor pressure vessel with a winged bolt. With over 240 crown top hats in each reactor, this was a complicated task that had to be carried out many times, although it was only necessary to repaired the failed crown top hats. The quicker it could be done, the shorter the outage.
A custom-built heavy-duty manipulator with ten degrees of freedom is used to carry out the repairs. It has a reach of at least 3m and a payload capacity of 90kg. It is designed so that it can enter the reactor through a fuelling standpipe 200mm in diameter. Access to the interior of Magnox reactors is only possible through the fuel standpipes.
Until recently, targeting the repair package onto the crown top hat has been extremely difficult because the hats are secured to a sphere rather than a flat surface,
making the mathematics of the movement more difficult. The fine positioning of the tip of a ten degree of freedom manipulator is a conceptually complex operation. BNFL Magnox recently solved this problem, and fine movement can now be calculated. This has improved the process greatly, and the final targeting that used to take three hours can now be done in 20 min.
The Envision computer simulation package used by BNFL Magnox Generation also played a vital role. It allowed the engineers to plan and rehearse the work beforehand, establishing the most sensible paths to deploy the manipulator. During the work itself, the computer simulation was also useful in providing the operator with virtual points of view that could not be obtained using a remotely operated camera.
As with all Magnox outage work, the repairs were rehearsed first using the computer simulation software and a mock-up of the reactor at the BNFL Magnox Generation facility at Littlebrook, near Dartford in Kent.
At Wylfa in North Wales, the reactor has a reinforced concrete cylindrical pressure vessel. Pipes designed to detect burst fuel cans within the reactor pass out of the reactor and are secured to the thermal shield wall. The pipes are attached to the wall using both fixed and free hangers, which provides the system with the flexibility to deal with thermal expansion. During the 1995 and 1996 outages, small cracks were observed within the welds of the free hangers. This posed no risk of a radioactive release, but the welds had to be repaired, and a remote procedure had to be developed to do this.
“It took nine months to design, commission, build and rehearse the repair methodology,” said Beere. “Access to the area was through refuelling standpipes and we used our Snakes manipulator. It has 15 degrees of freedom, a 90kg maximum payload and a 5m reach.”
The cracks were again caused by long-term exposure to high temperature CO2, and the aim of the repair work was to ensure that the free hangers will maintain their integrity until the plant is shut down, which BNFL recently announce will take place by 2021 at the latest.
The repair process involves three main operations; removal of the oxide from the weld site using a high speed burring process, attachment of an earth-return clamp, and then overwelding with a high quality, pulsed metal inert gas weld.
Using the original construction drawings, it was possible to create a computer graphics simulation of the reactor and plan the route the Snakes manipulator would take. Welding parameters were established during a comprehensive set of welding trials which took place at Littlebrook.
The Snakes manipulator is a hydraulic robot arm with electrically driven azimuth and vertical drives; it sits within an outer tube which rotates as a whole.
BNFL Magnox Electric has developed software to calibrate the manipulator, store the route information and drive it. The calibration software allows the operator to define accurately the joint positions and provide open- and closed-loop velocity control. It allows the routes to be driven with the joints moving at different velocities so that they arrive at the prescribed point simultaneously.
Each manipulator joint can be driven in fast or slow modes. Fast mode is for normal operation, while slow mode offers very fine control of each joint. This is particularly useful in restricted areas or when very accurate positioning is required.
Another successful operation took place at the Trawsfynydd reactor in North Wales, which was shut down a number of years ago and is undergoing decommissioning. The Trawsfynydd reactor offered engineers a unique opportunity to examine material which had actually endured a reactor environment rather than relying on laboratory testing and theory. By studying steel from the reactor’s south pole, steel which had undergone particularly intense neutron bombardment, engineers could gain real world data to add weight to the development of safety cases.
Remotely-operated vehicles were needed to cut six samples from the reactor pressure vessel. One carried a high pressure water jet to cut the steel; the other was a standard Schilling Gamma 2 teleoperations robot equipped with a clamp to retrieve the samples.
A cutting disc may have become stuck within the steel structure, and a high temperature cutting operation would have altered the steel’s properties. So that the steel was not changed in any way by the cutting process, a water jet was used to cut the samples. Once the samples had been removed, custom-designed rubber plugs were put in their place. The water jet had to cut through 15cm of steel and produced samples 21cm in diameter.
The Trawsfynydd reactor is placed on an ‘egg cup’ type structure. To gain access to the south pole meant insulation had to be cut away from outside the reactor. Before any work could be done, plant managers and nuclear inspectors had to be satisfied over the safety of the operation. Again, Envision software was used to plan the work, and in doing so, it was possible to produce a virtual ‘movie’, or story-board of the process, providing a clear visual description of the planned work.
The information obtained from examining the samples concurred with the
assumptions safety engineers had made with regards to the impact of neutron bombardment, fluctuations in temperatures and exposure to high pressure gasses on the ductility of the steel. This finding has enhanced confidence in the Magnox reactors still operating, as it suggests that safety assumptions built into decisions to continue running reactors beyond their original design lives are sound.
In all these operations, computer simulations, or virtual reality (VR) systems, are used throughout the life cycle of a project. This conceptual design tool allows for extensive off-line path planning, ensures that during rehearsals operators can concentrate on particularly difficult problems, and provides the operator with supplementary simulated points of view during rehearsal and implementation. It can also support the quality assurance and documentation which is a vital part of any repair or maintenance programme.
Desktop computers with the simulation displayed on monitors are used within the Magnox programme. Fully immersive VR systems are not necessary as all work is done remotely, and there are no situations in which an operator can come into direct
contact with the material to be inspected
During the recent outages at Dungeness, a specific challenge was to optimise the lengths of the two main limbs of the manipulator. This would normally be done by estimating a set of limb lengths, modelling the limbs, defining the inverse kinematics and checking whether the items that need to be inspected can be reached. It would normally be necessary to repeat the entire sequence to change limb lengths, and this takes a lot of time.
The computer’s inverse kinematics offers a much more flexible approach, with the possibility of including or excluding degrees of freedom from the solution. The length of the manipulator limbs can therefore be varied continuously while maintaining fully working inverse kinematics.
When the Littlebrook facility was first established in the 1980s, most work focussed on developing deployment routes for manipulators, and only a small amount of time was available for operator training and fine tuning. Graphical simulation means that manipulator routes are planned off-line, so far more time can be dedicated to operator training.
Other distinct benefits of the VR system are that the computer will only consider reactor components of particular interest; it
can give different reactor
components distinctive colours; it can display components fully shaded, translucent, or in wire frame; it can display non-existent features, such
as centrelines, pointers showing camera directions and field of view; it can place virtual cameras wherever required; it can provide multiple views simultaneously on the same screen; it can provide collision detection using visual and audio warnings; and it can display an intended next move.
Some of the Magnox reactors are likely to see operation lifetimes of 50 years. Not only were they the reactors that pioneered the development of nuclear energy, they will also help pioneer the developments necessary to extend reactor operation lifetimes, and in the process, outage management techniques. The BNFL Magnox Generation team has solved some extraordinary problems posed by the variations in design of the different Magnox reactors. In doing so, it has gained valuable experience in outage planning and operation, as well as working in conditions where access is extremely difficult.
With the increasing demand for skills and techniques associated with decommissioning and decontamination of nuclear sites throughout the world, the experience gained is likely to be invaluable.
”Our expertise and skills can applied outside the Magnox field,” says Beere. “We can offer services to any application involving remote handling, remote inspection, difficult access or hazardous environments.”