Winfrith, located in Dorset UK, was constructed in the 1950s as a centre of excellence for nuclear reactor research which laid the basis for the further development of nuclear power. Seven of the site’s nine unique experimental reactors have been removed to date but the final two – Dragon and the Steam Generated Heavy Water Reactor (SGHWR) – are still being decommissioned.
Dragon is a prototype 20 MW thermal power output graphite moderated, helium-cooled reactor with an operating temperature of 1000°C. It tested nuclear fuel and materials to support high temperature reactor programmes at the Winfrith site from 1964 to 1975. At the time it was regarded as one of Europe’s most successful collaborations in applied science and the most important multi-national technical collaboration in nuclear energy. Dragon was the first demonstration of a high temperature gas-cooled reactor and was used as a material test facility to support worldwide high temperature reactor projects.
Nuclear Restoration Services (NRS) – a wholly-owned subsidiary of the UK Nuclear Decommissioning Authority – is decommissioning some of the oldest and most diverse nuclear reactor and research sites in the UK, including the Dragon reactor at Winfrith. All present their own unique and complex challenges – different reactor designs, operational functions and ancillary structures to name just a few of the technical issues.
Early decommissioning
Work began to remove and dispose of all the reactor plant outside of the reactor pressure vessel in 2011. This included the entire primary cooling circuit, the helium coolant purification system, the fuel loading and unloading systems, spent fuel storage, control rod mechanisms, the top of the reactor pressure vessel and half of the biological shield which was removed to facilitate access to the top of the reactor, as well as the purge gas pre-cooler (PGPC).
All the decommissioning that could be carried out without resorting to fully remote operations was completed. The highly irradiated reactor pressure vessel and its contents are the only significant item of radioactive plant remaining in the 100ft (30 metre) diameter cylindrical Dragon building.
Scaling the engineering challenge
Although the reactor has been offline for 50 years, the levels of radioactivity are still far too high to send workers in to remove the reactor manually. During construction of the building, a 1.8 metre-thick biological shield wall was created to house the 10 metre-tall reactor and shield building occupants from the radiation.
The only access to the reactor now is through the aperture in the top which had been created during earlier decommissioning work. Access is extremely limited, dark and constrained inside the reactor itself. Little if any consideration was given by the original reactor designers of the 1950s and 1960s as to how it would eventually be decommissioned.
The engineering design team had to solve the intricate complexity of safely removing over 300 tonnes of radioactive material piece by piece to remotely decommission the reactor, manage the nuclear waste and progressing one of the UK’s oldest nuclear sites towards site restoration.
Pioneering innovation
Safe, responsible and cost-effective disassembly of the Dragon reactor is a necessity if we are to prove the sustainability of nuclear fission as a power source of the future. Given the practicalities of both the accessibility and ambient radiation within the reactor itself, a first of a kind integrated, robotic solution was developed.
The first problem to solve was how to physically get close enough to all of the reactor components to segment and remove them. The solution was a remotely-positionable, telescopic mast with a six degree of freedom robotic arm. The arm is capable of handling the weight of a small motorcycle whilst still having impressive dexterity and flexibility to enable manoeuvre within the tight confines of the core.
The second part of the problem was how to practically cut out and remove components made of various metals and of different geometries ranging in thickness from 2mm to 75mm. This is where the work at Dragon has been breaking new ground.
Laser cutting is a well-established technology in the manufacturing industry, but its application in nuclear decommissioning is relatively uncharted. However, when used remotely for reactor dismantling, it offers several compelling advantages over traditional methods:
- Lower secondary waste compared to other hot cutting techniques
- Fast cutting speed
- Negligible recoil forces
- Very tolerant of stand-off distance from the workpiece
- Can cut through multiple material layers across air gaps
To harness these advantages, a laser cutting system was integrated with the primary core segmentation robot through a collaboration between NRS, Walischmiller GmbH, and TWI Ltd. This integration required extensive expertise in both robotics and laser technology to be drawn in from the supply chain to ensure seamless operation in the challenging environment of the Dragon reactor.
Performing keyhole surgery
The feasibility of using laser technology to cut the reactor core’s varying material types and thicknesses was initially tested on mock-up components at the Welding Institute, Cambridge. Further testing followed with a full-scale mock-up at the Winfrith site.
Technical data and learning from these trials were then applied to perform keyhole surgery using a snake robot (Lasersnake) to remotely cut a three-tonne heat exchanger vessel known as the Purge Gas Pre-cooler (PGPC) from Dragon’s highly radioactive reactor core in 2018. This was a first for the UK nuclear industry and its success proved laser cutting as the principal technique for reactor core dismantling over the use of alternative hot cutting techniques.
Phoebe Lynch, NRS strategic innovation programme manager, said: “Driving innovation into our mission is our greatest opportunity at NRS. All the learning from the initial operational phase of using laser cutting for the Dragon reactor core provided valuable insights into the feasibility, reliability and safety of this technique. These have been applied to refine the process to deliver this new phase of reactor core dismantling efficiently and pave the way for its broader adoption within the industry.”
The reactor dismantling sequence
The project has been divided into 11 distinct campaigns which individually deal with a different set of reactor components: cutting into the upper shield structures to reveal the core, removal of ‘lift out’ items, removal of graphite upper and lower reflectors, removal of steelwork structures, removal of the reactor pressure vessel and thermal shields.
Each campaign has a plan covering the:
- Cutting and handling of each component and structure, including a detailed sequence of cuts to be made
- Handling arrangements
- Use of bespoke waste baskets to aid in waste transfer from and/or handling in the core
- Use of waste storage furniture to facilitate packing of waste into HHISO containers or 6m3 box, and detailed packing plans for each package being produced.
Remote segmentation begins
NRS’s team of operators began remotely dismantling the upper shielding structures in July 2024. Although segmentation of the 50-tonne top ring thermal shield surrounding the top of the reactor has presented a unique and significant challenge to the Dragon team, the partial removal of this is nearly complete and will then allow access to be gained to the reactor itself.
Since the operational phase began, approximately 28 tonnes of waste have been recovered and exported from the facility. Segmentation of the top ring thermal shield and reactor pressure vessel neck required removal of over 120 individual items. Overall, the laser cutting system and reactor core segmentation machine have proven to be a remarkably capable engineering solution in the hands of the Dragon team.
It is anticipated that approximately 22 waste packages will arise during the lifetime of this project. These will be intermittently transported to the NRS Harwell site in Oxford for interim storage until the UK’s national Geological Disposal Facility is available for permanent disposal.
Operational excellence
Guided by the Engineering team the Dragon reactor laser operators are responsible for configuring the system to optimise each cutting operation. Their expertise has already been demonstrated with the successful removal of the reactor pressure vessel neck, marking a significant milestone in the reactor dismantling process.
The team continue to refine the application of the system in preparation to breach the reactor core later on this year. Significant challenges are anticipated as work continues, particularly with precise manoeuvring of the machine near core structures to cut components.
Additional tools have been developed to provide valuable insights:
- A virtual reality system that allows non-engineering staff to quickly grasp component shapes and dimensions.
- Digital twin functionality is now implemented which provides a real time graphical representation of the RCSM within the core environment based on real time positional feedback.
- Despite these advances, the success of the operation will heavily depend on the expertise of the team to navigate the confined core space without damaging or trapping the equipment.
Cutting edge decommissioning
The primary objective of the Dragon reactor dismantling project is to demonstrate how this process can be made more efficient and pave the way for the future use of laser cutting in nuclear decommissioning. Evaluating the experience already gained, it is clear that use of robotic laser cutting presents a transformative opportunity to streamline segmentation processes, increase operational efficiency, and improve safety in reactor core dismantling.
As the project moves forward, it is hoped that the insights gained from Dragon will not only contribute to the safe and efficient decommissioning of this reactor but also set a precedent for future projects. By demonstrating the effectiveness of laser cutting in this demanding field, the project team aim to establish it as a standard method in nuclear decommissioning, ultimately making the process safer, faster, and more cost-effective.
