Fifty years ago this month, a fire raged through Pile 1 of Windscale nuclear power station in Cumbria, sending a plume of radioactive material as far afield as London, Germany and Scandinavia.
Now a new robotic tool based on conventional hydraulic technology more familiar in oil exploration is playing a key role in the decommissioning process.
TheUK Atomic Energy Authority
(UKAEA) is working with US automation companySpecial Application (SA) Robotics
and specialist decommissioning engineers C2HM Hill to return Windscale to a brownfield site.
Richard Roper, programme manager for the site, said: 'A new safety case was made in 2006 by the Nuclear Installations Inspectorate (NII). After an intrusive survey of the core, they found the state of the fuel was fine and the risk of criticality or explosion was not credible, and decided to accelerate the decommissioning process and reduce the cost.'
Unlike modern power plants, Windscale was not designed for decommissioning, so a great deal of site preparation and characterisation of the damaged fuel needs to be carried out before work starts.
Blocked by fuel
Dick Sexton, project manager at CH2M HILL, said: 'Windscale is much larger than current reactors. There are 3,500 individual channels for fuel and isotope channels, and 400 four-inch (10cm) chambers that are blocked by degraded fuel.'
During use, operators inserted fuel and controlled the process via the charge face of the pile. The fuel needs to be removed from the much smaller discharge face of the pile, requiring the use of remotely operated robotic machinery.
The team will cut a slot in the top of the reactor and install a carbon fibre mast that will position and manipulate the device, known as a fuel channel removal tool (FCRT).
The FCRT is like a circular Gatling gun fitted with 22 Kelly bars — arms that can be attached one after the other to reach the other end of the pile. Camera signals and control for the 'end effecters' — manipulators, such as grabbers, scoops and loosening tools — need to be sent though the Kelly bars.
At the edge of the pile, the fuel is mostly intact, encased in an aluminium finned container. Where the fire began, the fuel is in a variety of states: degraded, melted in place and burnt.
The team has a record of 52 of the channels through the iterative characterisation process by cable camera. The next characterisation campaign will examine 150 more.
When the operator knows what challenge lies ahead, he or she can select the appropriate end effecter.
'The retrieval tool is powerful - very "torquey",' said Sexton. 'If any aluminium is stuck, you can torque it off.'
Removed waste will be separated into intact fuel and debris using an industrial separator. It will be encapsulated in a polymer — various types are currently under test — and kept in a buffer store on the Sellafield site. Each unit of fuel needs to be kept separate from others to prevent the risk of heating. A different encapsulate will be needed for debris from that used for intact fuel.
Sexton said: 'We are thoroughly testing the FCRT. We don't want it to fail because it's a high radiation area we can't access. We have been doing decommissioning work for 20 years and find simple, off-the-shelf technologies work best.'
After the fuel and isotope is removed, the team will remove the control and shutdown rods, and modular core containment structures will be built. The heavy blocks that were installed on top of the pile to hold it down will be removed.
The graphite blocks will then be removed using a variety of industry-proven cutting, gripping and lifting tools. Finally, the bioshield will be demolished.
Although the new proposal aims to speed up decommissioning, it will take 10 to 15 years for all the fuel to be removed. Windscale will be a useable brownfield site by around 2030 at a cost of around £500m, funded by theNuclear Decommissioning Authority
According to site programme manager Richard Roper: 'Within three years, we hope to remove the west air inlet duct, test and re-test the fuel channel removal machine and buy the cranes and ventilation system we will need. But we will not have started removing the fuel by then.'
The Windscale fire sparked the genesis of the modern nuclear industry and influenced the building of future nuclear plants. It was designed and built in three years, but it could take another 23 to return the site to a useable condition.