Bespoke machine cuts station down to size30 July 2001
The very nature of decommissioning means that the planners have to deal with a number of specialised circumstances, which call for specialised tools and techniques. Cutting a reactor into manageable sized pieces was one such operation that engineers have had to deal with.
Decommissioning of a European 250MWe nuclear power station began in the early 1980s with careful planning and the development of specialist techniques. Now, in 2001, the success of this decommissioning operation is demonstrated by a greenfield site.
To achieve the complete decontamination of the plant and surrounding area without exposing those working on the project to high doses of radiation, numerous complex bespoke processes were used. One area of interest was the dismantling of the reactor which required the development of specialist bespoke machinery.
A number of firms were called in to offer advice and expertise on how this might be achieved.
In response, several possible solutions to cut up the redundant reactor were presented. Investigations into the proposed solutions were carried out regarding the costs that each solution would incur, the aerosol release levels, cutting times and waste quantities. Mechanical cutting using a bandsaw was shown to be the best and most cost-effective solution, and Furmanite consequently designed and developed the bandsaw machine.
One of the most demanding aspects of the design was that all of the machine adjustments, configuring, blade replacement, workpiece loading and bandsaw control would need to be carried out remotely in the nuclear storage pool under three metres of water. This was done in order to minimise man-dose and to contain the contaminated swarf which would be generated during the cutting.
Furmanite developed a prototype machine that was tested at its premises in Derby. The final machine was then manufactured and shipped over to the site, where engineers provided on-site technical support and training to local operators.
The scale of the plant dictated dismantling in sections to enable the removal and decontamination of individual parts. The reactor, as one of these sections, was transferred to a storage pool, where the bandsaw had already been installed on a turntable in preparation.
The saw was designed to reduce the reactor vessel internals to sections of no greater than 500mm cube to facilitate its disposal. To perform such a task, a number of requirements would need to be met and challenges overcome. The biggest challenge was operating the bandsaw underwater in a reactor pond, within restricted space. Moreover, access through the airlock into the containment area was limited to just 1000mm by 1500mm. Subsequently the bandsaw was used to assist with other equipment, in the dismantling of the reactor pressure vessel itself.
The machine needed to be hydraulically operated, capable of precision underwater cutting and adjustable to enable cuts to be made at three radial angles. To achieve these requirements, careful design and testing processes were used and the machine was mounted on a turntable for radial adjustment and able to raise hydraulically for vertical cuts. To avoid contamination of the pond the hydraulic medium used was a demineralised water/glycol mixture.
All assemblies needed to be water-tight and all hydraulic pipes were shrouded in PVC sheaths to prevent water ingress and contamination. All screws were sealed and lockwire was used to prevent vibration allowing them to undo.
For safety, pressure sensors were built in to detect blade stall and limit switch control powered vertical travel. Blade speed, tension and feed rate were adjustable and displayed on the remote control panel.
Carbide wear pads, through which the blade travelled, were designed to be adjustable to allow for angling the blade. The blade could be remotely moved through 90° to maximise the adaptability of the equipment.
All components of the machine required a high quality surface finish (0.8mm Ra) and low porosity for ease of decontamination. In response, Furmanite created components from polished stainless steel or hard anodised aluminium with a PTFE anti-porosity treatment.
In total, about 200m of cutting was carried out, resulting in around 160 individual cut components. These were analysed for activity and then electro-polished, reducing the remaining activity by a factor of 300, permitting controlled recycling of the total shell.
This is an example of just one small part of this extensive project which has taken almost 20 years to fully complete. The techniques used for the decommissioning of this plant and the specialist machinery developed proved that it can be done with low radiation exposure to workers, little or no impact on the environment, low amounts of reactive waste and with reasonable and predictable costs.
The demineralised water/glycol hydraulic propulsion developed on this project has since been used extensively in other projects carried out by Furmanite in nuclear environments around the world, whilst the bandsaw has found spin-offs in a wide range of non-nuclear plant modifications and dismantling projects.