Engineers in charge of dismantling a redundant nuclear reactor at Dounreay have unveiled the design of a giant robot, nicknamed Reactorsaurus, that cuts apart and removes equipment from the hazardous environment.
The team behind the 75-tonne Reactor Dismantler Manipulator (RDM) hope to award a contract in July to build a test model.
Work began two years ago on the RDM, which is designed to dismantle the liquid-sodium-filled prototype fast reactor that ceased operation in 1994. Once the sodium has been completely removed, the RDM will be used to cut up all equipment, including the pipework and the shield rods, into manageable pieces and lift them out of the reactor using two pincer arms that wield interchangeable cutting and lifting tools.
Jared Fraser, who leads the RDM design team at Dounreay Site Restoration Ltd (DSRL), described the environment the RDM will have to work in: ‘A lot of the fuel, the breeders and reflectors there are highly radioactive so we can’t do manned entry. It is also mostly below ground in a large tank and the only way to get into it is to remove the retaining shield, which then gives us a 3.7m diameter access port in the 60m vessel.’
Before dismantling begins, the sodium will have been drained out and treated in a sodium disposal plant, before a water-vapour nitrogen process is used to dissolve away the majority of the residue.
A containment cell will be built over the top of the reactor and the RDM will be deployed on a set of rails and dropped down into the port. Its manipulator arms will reach 16m to access the outer regions of the reactor. The first task will be to lift out the shield rods in the centre, using a large shear to cut the 30cm-thick solid-carbon rods into smaller sections.
Once the RDM has lifted the rods out, it will have access to the pipework, using one of its manipulators to grab the pipes and the other to wield a cutting tool.
A series of candidate cutting tools will be developed and tested at an off-site facility to select the one most suitable for the job. Potential tools include propane mixed with iron powder, as used in the WAGR (Windscale Advanced Gas Reactor) at Sellafield, and plasma cutting for steel parts. The engineers are also considering using diamond-wire encrusted disks and standard hydraulic shears and developing a diamond-wire cassette system. Water jetting could also potentially be used, but only when the operators are certain all the sodium has been removed, because of its intense reaction with water.
‘The facility that we build over the top will have two disposal routes for waste, attached to the containment cell,’ said Fraser. ‘We’ll drop in a waste basket, fill it with the size-reduced parts, lift it out, then it’ll go through a steam-cleaning process and an assay categorisation to determine what level it is. Low-level waste will go into half-height ISO containers and mid-level waste goes to three cubic-metre boxes that will be grouted for permanent storage.’
Operators will control all the motion on the system from the carriage to the hydraulically operated booms that extend in and out from a central control room. The RDM has dual redundancy built in, so if one system fails it has a back-up that can be retrieved in the event of an emergency.
All the equipment that the RDM uses, plus its entire horizontal boom, can be lifted out like a cartridge-assembly system. If maintenance is necessary, repair work can be made on a part outside the cell once it has been decontaminated.
Specialist materials will be used to withstand the high levels of radiation, including specially selected types of rubber, steel and hydraulic fluids. All the electronics will be radiation resistant, including the cameras used to guide the RDM, which cost in the region of £40,000 per unit.
‘Radiation levels will diminish as we take more out of there,’ explained Fraser. ‘As an ongoing process, we’ll change out cameras to put in lower radiation tolerance.’
The RDM will cost around £3m to build and will be commissioned in 2013. It will take around three years to complete its work, after which it will be decontaminated and disposed of as low-level waste and be decommissioned along with the reactor itself.
Engineers have designed a robot that will be able to dismantle and dispose of nuclear reactors. Berenice Baker reports