‘LaserSnake’: A New Approach to Nuclear Decommissioning

By Craig Wilson, Managing Director, OC Robotics, UK

Across the world there are high-hazard nuclear facilities which need to be decommissioned. Dismantling redundant equipment safely and cost-effectively is one of the biggest challenges facing the industry today, and an increasingly major issue for operators.

In 2014 the International Energy Agency said that almost 200 of the 434 reactors in operation around the globe would be retired by 2040, and estimated the cost of decommissioning at more than $100 billion. Some commentators felt even this was a great underestimate.

To meet a challenge of this scale, innovation will necessarily have a key role. With the engineering, safety and working time restrictions which apply to ‘active’ nuclear site environments, new technologies that can distance the human operator, be more efficient and save time can potentially play an important part in future decommissioning.

In summer 2016, OC Robotics led a demonstration project at the Sellafield site in northern England, to showcase one such new development which could transform the approach to dismantling redundant equipment.

For the project, called LaserSnake 2, a team led by OC Robotics integrated a high-powered laser cutter with a remotely-controlled snake-arm robot, and used it to cut up a 5.5 tonne steel dissolver vessel at the Sellafield site. While similar work has also been done in France, the LaserSnake 2 team was the first in the world to complete dismantling of a dissolver vessel using this technology.

The seeds of the LaserSnake 2 project were sown in 2001, when at OC Robotics we began to develop and build long, flexible, multi-jointed robotic arms. Software-driven and controlled by wire cables inside the arm, they are highly dextrous and can navigate through small spaces and cluttered environments, moving around and over obstacles. Depending on the equipment they carry, they can perform tasks such as inspecting structures, cleaning, sealing and welding.

Around 2011 we saw that there was potential to use such snake-arm robots in nuclear decommissioning work. Working with laser experts TWI (formerly known as The Welding Institute) we carried out a feasibility study, supported by the UK’s innovation agency Innovate UK, to integrate a snake-arm robot with a laser-cutter head.

In 2012, Innovate UK announced a competition for innovative ideas in decommissioning technology, with the winners receiving grants funded by Innovate UK, the UK’s Department for Energy and Climate Change , and the Nuclear Decommissioning Authority (NDA). Based on the success of the feasibility study, we applied and succeeded in winning a grant towards the costs of a full-scale demonstration. We called this project LaserSnake 2.

Marrying a snake arm and a laser cutter

The snake-arm robots developed by OC Robotics had already been used for inspection, welding and other remote tasks, for example in the aerospace and automotive industries. The concept was proven, but this would be an entirely new application.

Meanwhile our partners in the project, TWI (formerly The Welding Institute) had been developing laser-cutting processes since 1967, and successfully deploying the technology in many industries. TWI had previously tested laser cutters with conventional industrial robots to dismantle pond skips at Hinckley Point A in the UK – but only in a purpose-built decommissioning facility.

The question was, could a snake-arm robot effectively be fitted with a laser cutter for in-situ dismantling in a nuclear facility? The 2011 feasibility study showed that potentially it could.

To create a workable integrated system for this ‘live’ environment required both innovation and collaboration. OC Robotics and TWI teamed up with the UK’s National Nuclear Laboratory, and specialist companies ULO Optics and Laser Optical Engineering.

The LaserSnake’s cutting head is integrated with the flexible robotic arm. Photo courtesy: OC Robotics

For the project, we spent three years improving many of the mechanical, electronic and software aspects of our existing robots. The result was a new snake-arm robot with up to 4.5m (14.7 ft) of flexible length – double that of previous models – greater arm curvature and a payload increased from 5kg (11lb) to 20kg (44lb). This meant it could carry not only the laser cutting head but also a navigation camera, HD inspection camera, lighting and sighting lasers. Proprietary software which we developed allowed precise navigation and positional control of the laser cutter, working from a CAD vector file, or programmed by an operator.

The laser used for the dismantling task was a commercially-available unit producing 5kW of output power. The laser beam travelled via a 200 micron core diameter armoured optical fibre inside the snake-arm to the cutting head.

The laser-cutting head itself was developed by specialist company ULO Optics The cutting head, designed specifically for decommissioning, was configured to require little adjustment on site and weighed less than 2kg (4.4lb). The optical system focused the laser beam to a spot approximately 0.4mm (0.01 inches) in diameter, 15mm (0.6 inches) beyond the tip of the cutting nozzle. In trials, we found that this setting gave good cutting speeds, given the material to be cut, as well as tolerating variations in the distance from the cutting tip to the surface of the material being cut. This was important as it reduced the precision needed, and therefore the programming time, when setting the cutting paths.

ULO Optic’s laser cutting head also included a compressed air feed, with two roles – to cool the optics and to provide a stream of air alongside the laser to blow away the molten material and help the cutting process.

The challenge

At Sellafield, as part of their Future Decommissioning Strategy, the site operators continually seek new solutions for decommissioning challenges.

LaserSnake 2 fitted this brief very well, and the decision was taken to work with the project team to carry out a live demonstration.

The task chosen for the demonstration was the dismantling and removal of a steel dissolver vessel in Sellafield’s First Generation Reprocessing Plant. This was a real live problem facing the Sellafield decommissioning team. The vessel, situated on the eighth floor of the building, was 1.3m (4.25 feet) in diameter and weighed approximately 5.5 tonnes (6 tons). It had a dual-wall construction consisting of a 12mm (0.5 inch) thick outer shell and a 32mm (1.25 inch) inner shell, separated by a 40mm (1.5 inch) air gap.

The vessel was inside a concrete walled cell. In earlier work it had been taken off its mounting and cut into three sections using a diamond wire saw, but to remove it from the cell it needed to be reduced to much smaller pieces, each weighing no more than about 25kg (55lb).

The LaserSnake in action dismantling the dissolver vessel. Photo courtesy: OC Robotics

The conditions in the cell were categorised as C3/R4 with contact dose readings within the vessel up to 12 milliSv/hr.

This vessel was representative of the many challenging items likely to be encountered in decommissioning at Sellafield Site. In many of the cells on the site, due to radiation levels, access is either very limited or impossible without significant work to reduce hazards. Where manual dismantling of equipment is still possible, traditional cutting methods take a long time – exacerbated by the challenges of avoiding contamination spread and working in restrictive protective clothing.

In the past, bespoke remote working systems using traditional cutting tools have been created for such facilities, but they have often been expensive and unreliable. Plasma cutting was very challenging due to the precision needed, the length of time the process would take, and the number of cell entries.

In all, this made an excellent test case to demonstrate the potential of LaserSnake technology.

Deploying the LaserSnake

In the first half of 2016, Sellafield’s Active Demonstration team carried out a programme of work to prepare for deployment of the snake-arm robot. This included making the cell safe for laser light and installing a filtration unit, compressed air supply, electrical distribution, CCTV and lighting. The team also provided the necessary assessments, planning, documentation and logistics.

Working together, we then installed the system in its position. The body of the snake-arm system, with the laser generator and electro-mechanical systems, was outside the concrete cell containing the dissolver vessel, with the arm positioned to enter the cell through a hole drilled in the 5ft thick concrete wall. From arrival on site the system was installed in one week. We worked closely with Sellafield Ltd to create a modular containment system. When the snake-arm was pushed into or withdrawn from the cell, the negative air pressure in the cell prevented release of contamination. During cutting the arm was covered by a sleeve sealed to the wall, isolating the housing from the cutting process. The rail drive mechanism used to push the snake-arm into the cell was also sealed and its rack and pinion were left unlubricated to reduce the likelihood of trapping contaminants.

The controls and display screens were positioned at a workstation outside the cell, from where the operators would manage and monitor the cutting process.

Making the Cut

With everything in place, the cutting programme began in July 2016.

The LaserSnake arm was moved into the cell, directed remotely and monitored on screen. Tackling each of its two steel skins in turn, the cylindrical dissolver vessel was cut into man-handleable sections. The angle of the cutting beam was carefully managed so that the beam lost most of its energy as it passed through the steel, and the process was planned so that any residual energy struck other parts of the dissolver vessel rather than the structure of the cell. In this way no additional laser beam absorbing materials needed to be used until the very last cuts. Planning the cut angles was important to ensure the cut pieces of the vessel wall fell away rather than being trapped by their shape.

In 45 hours of cutting, over 66 metres (656 feet) of cuts were made at a typical speed of 80mm (3.1 inches) per minute. Due to the angles, the laser was frequently cutting through up to 60mm (2.3 inches) of steel, and at one point, on a flange, the thickness of material to be cut reached 75mm (2.9 inches)

By the end of the process in August 2016, the vessel had been successfully reduced to around 175 pieces, each weighing approximately 25kg (55lb) as planned, which could then be removed from the cell.

With installation, programming and cutting time and complete removal of the LaserSnake system, the team had spent a total of 48 days on site.

The LaserSnake is operated remotely from outside the cell. Photo courtesy: OC Robotics

Chris Hope, Sellafield’s decommissioning capability development lead, says that in this trial remote-controlled laser cutting has proved to be a far better solution than the alternatives. And for him, the most impressive aspect of the project was the speed of implementing this advanced technology on site. By adopting a ‘fit for purpose’ attitude, and through great teamwork, the partners planned and completed a potentially game-changing project in around a year from Sellafield’s first involvement. On the actual delivery and set-up phase, Chris Hope says “we had three objectives: to get the laser onto the site, to install it and to fire up the laser and do some cutting. We did all of that within a week.”

The challenges for the project partners were not just about engineering but about regulatory compliance and logistics – from nuclear safety courses for OC Robotics and TWI staff, to detailed planning to co-ordinate the work of the different teams.

To quote Chris Hope again, “This has been the best collaborative project the team have delivered. To bring new technology onto the site and enable a potential step change in decommissioning operations has been really rewarding.”

The project went on to win two awards in 2016; one an internal ‘Business excellence – people’s choice’ award from the site staff at Sellafield, and the second a Technology/Innovation Implementation award from the Nuclear Decommissioning Authority as part of the 2016 NDA Estate Supply Chain Awards.

Encouraged by the success of the project, the NDA is now funding a new competition, together with Innovate UK and Sellafield, to encourage further collaborative innovations in the decommissioning sector.

Meanwhile Sellafield Ltd are now looking at opportunities to make this kind of laser-cutting technology ‘business as usual’ around the site, to help dismantle other redundant items of plant.

They expect that in the foreseeable future the technology will be used routinely. Rebecca Weston, Technical Director for Sellafield Ltd, said “robotics is becoming an important part of our daily activities. The ability to support our clean-up mission by accessing areas that are too radioactive for human entry makes new robots, like the LaserSnake, an essential part of the team.”

With a typical remote cell estimated to cost around £15m ($18.7m) to decommission, the cost of decommissioning cells at Sellafield amounts to billions of pounds. The signs are that systems such as the LaserSnake, which can significantly reduce the time and personnel input required for dismantling tasks, have the potential to deliver major savings for nuclear decommissioning operators wherever they are in the world.

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