Taking up arms for decommissioning13 July 2004
The UKAEA’s decommissioning of the Windscale Advanced Gas-cooled Reactor is now in its final stages. Essential to the £80 million operation has been the remote dismantling machine and its manipulator. By Terry Benest
The United Kingdom Atomic Energy Authority (UKAEA) has built and operated a wide range of nuclear facilities since the late 1940s. The UKAEA’s mission is to restore the environment of its sites in a safe and secure manner, using currently available technology. This restoration includes the decommissioning of a number of redundant research and power reactors at five UK sites.
At the Windscale site in Cumbria, the UKAEA and its contractors started a project in 1998 to remove the core and pressure vessel of the Windscale Advanced Gas-cooled Reactor (WAGR). The project is currently in the eighth of ten ‘campaigns’ – the last of which is due to finish, within budget, in summer 2005 (see Table).
WAGR's distinctive dome
The principal technical difficulty associated with the removal of the activated components of the core and pressure vessel was that radiation dose rates of approximately 1Sv/h were anticipated in much of the reactor. In some cases this precluded manual dismantling operations and demanded the use of remotely operated equipment. In other areas, dismantling could be achieved by a combination of remote, semi-remote and manual operations.
The principal systems conceived to undertake the remote work comprised the following components:
• A remote dismantling machine (RDM) to deploy tools to dismantle the high dose components.
• A remotely operated manipulator, suspended from a mast that extends below the RDM, designed to carry out remote operations such as picking up debris, deploying cutting packages and positioning grabs.
• A wide range of decommissioning tools, connected to either the manipulator or a three tonne capacity hoist (also part of the RDM).
• A recovery and transport system to remove the dismantled sections.
• A waste route through which to move the components, sort them, take assay measurements, pack them in suitable containers, and treat them for disposal or storage.
REMOTE DISMANTLING MACHINE
In 1986, the UKAEA awarded an £8 million contract to construct the RDM to Strachan and Henshaw. The RDM, operated from a purpose built control room in the adjacent building, consists of two handling systems deployed beneath a turntable mounted at the reactor operating floor level: firstly, an extendable mast from which a remotely controlled manipulator is suspended and, secondly, a series of suspended crane rails enabling a 3t hoist to travel across the reactor vault into the adjacent cells. A two-storey building mounted above the turntable houses facilities for service provision and access for tool changing and maintenance. Operators are shielded from radiation by a lead shot filled shield floor within the turntable construction. Trial off-site erection was undertaken to test the RDM before it was finally positioned over the reactor in 1993.
Terry Benest in front of the upper parts of the RDM, during a tour of the site
The manipulator was specified to facilitate the cutting of the reactor vessel and its internals into manageable sections. It can reach all areas of the reactor vaults using a combination of:
• Rotation of the RDM turntable (370º coverage).
• The addition or removal of up to six mast sections.
• Vertical travel along the mast by means of a hoist and pulley system.
The original manipulator was a Taylor Hitec electromechanical device with a maximum reach of 2.5m and a maximum capacity of 35kg at full radius. This has been replaced by Schilling Titan II hydraulic manipulators, with the following key advantages:
• Continued service support with a good supply of spares.
• A considerably improved payload of 100kg.
• Its hydraulic system can provide power to tools through a custom made ‘wrist’ joint.
Since the main contractor, British Nuclear Group (formerly BNFL), already owned a number of the manipulator arms, it was possible to operate with at least three units: one installed on the RDM platform to undertake the work; one available as a spare for immediate changeover following any breakdown; and one kept radiologically clean to be available for off-site tooling trials and training.
Whilst being a highly adaptable and dextrous tool, and comparatively affordable at £250,000 per unit, operational experience showed some limitations. In particular, occasional losses of hydraulic power can occur, resulting in the manipulator ‘slumping’.
This prompted a radical rethink of the method of deployment of tooling. The general decommissioning regime is to use the 3t hoist as the prime method of tool delivery and services supply. The manipulator is used in a mainly supporting role to assist in tool placement, adjustment and, if necessary, emergency recovery.
The manipulator in use during Campaign 4
A major example of this is in the use of hoist and manipulator to deploy an oxypropane cutting torch for the size reduction of heavy steel sections such as the 75mm thick pressure vessel. The initial concept was for the manipulator to grip the torch and guide it through the required cut paths. However, the requirement to maintain fixed stand-off distances, which cannot be easily controlled by a manipulator, and the prospect of a sudden ‘slump’ and loss of control of a burning torch with the obvious safety risks could not be justified.
For the pressure vessel removal, therefore, a suite of tools that carry the cutting torch and control its movement across the vessel surface have been designed and manufactured. These tools are brought into the reactor vault with the 3t hoist and deployed onto the vessel wall. The manipulator is then used for final alignment, if required, using levelling adjusters located on the tool. The manipulator also deploys and connects the tools’ gas and electrical power services.
The tooling design will generally incorporate remote recovery features for the manipulator to operate, such as handwind attachments for motor drives or release latches. This allows the manipulator to be used to free a tool from the work piece so that it can be recovered by the hoist.
Tools: examples and evolution
Common tools employed using the manipulator and hoist include: parallel jaws; lifting pintles; plasma arc torches; reciprocating saws; drills, taps and dies; and shears. In the majority of cases these decommissioning tools are initially trialled on mock-up rigs nearby – for instance at the Egremont test facility run by British Nuclear Group. Most recently this included off-site trials of the remote operated cutting torch, which is currently in operation at WAGR.
The oxypropane cutting tool is tested off-site
Despite this careful development and testing, problems may still be encountered. For instance, in Campaign 5, problems were experienced during size reduction of restrictor sleeves using a specially procured internal pipe milling machine, suitably adapted for remote deployment. In trials, the machine failed to deliver the promised 40 cuts per cutting tip, achieving only ten cuts per tip. In the reactor itself, only one cut was achieved and the slow speed cutter had a tendency to rotate the tubes. To avoid delay, the project persevered with this equipment while developing an alternative solution. Because dose rates were low, manual intervention was used to cut the steel fuel channels using a modified 100mm angle grinder. The machine achieved production rates of over six cuts per hour. The same angle grinder was used again in subsequent stages of Campaign 5, both manually and modified for remote operation.
Still in Campaign 5, two layers of outer neutron shield graphite blocks, without any integral lifting features, had to be removed. A tool was developed to drill and tap a thread in three holes at the top of the graphite and bolt itself to the block. Using special drill bits, this was achieved in one action. After transfer to the waste basket the blocks were released by reversing the drill motor.
The control room: from here, operators can coordinate remote operations
THE WASTE ROUTE
The waste route was constructed through two of the heat exchanger bioshields to gain benefit from their shielding concrete. The waste is moved laterally from the reactor to the waste packaging plant for characterisation and encapsulation.
In the waste packaging building, all waste removed from the reactor vault is placed in WAGR concrete waste boxes and encapsulated in a cementitious grout. The anticipated high radiation dose rates dictate that the process has to be undertaken remotely. After encapsulation the dose rates on the external faces of the completed box reduce below transport regulations limits, mostly to <10µSv/h.
Low-level waste (LLW) boxes are transported by road to the LLW repository at Drigg for disposal whilst intermediate-level waste (ILW) boxes are sent to the WAGR ILW box store for temporary storage awaiting the provision of a national ILW repository.
Terry Benest, UKAEA Project WAGR manager, WAGR, Windscale, Seascale, Cumbria, CA20 1PF, UKTablesWAGR activities