Decommissioning of a number of sites is currently taking place at Sellafield, each one posing unique problems. Finding solutions and achieving the aim of removing and treating the radioactive material is providing BNFL with valuable experience for the larger projects that need to be addressed in the future.
Two of the most contrasting projects involve work on the B212 caesium extraction plant (CEP) and the B205 pilot plant.
The CEP plant produced caesium sulphate sources for radiotherapy treatment and has been redundant since 1958. It is located in a series of highly shielded cells within void areas on top of a still operational plant. Radiation levels in the CEP and void areas preclude any manual decommissioning techniques.
The decommissioning machine (DCM) is housed in a 780 tonne mobile module which attaches at various locations along the length of the CEP. The DCM consists of a gantry mounted four degrees of freedom deployment system incorporating a slewing ring and telescopic boom, a tilt table with two heavy duty remote tool change units and a range of manipulators, from which a large range of tools are applied.
The DCM needs to be rigid enough to provide a stable platform for the disposal of tools but flexible enough to cover its full working envelope. Robust enough to remove cell shield walls and large waste packages but controlled enough to remove brittle laboratory size glass vessels intact. Whatever specific operation is being undertaken, uncontrolled removal of plant, collisions and the minimisation of air borne contamination are significant concerns which has meant that a comprehensive programme of inactive trials faithfully mimicking the active plant has been necessary.
Due to the mobile nature of the primary contaminant, one of the key design features of the DCM is the ability to easily decontaminate and maintain it. Whilst looking complex the DCM’s modular design, surface finish and many months of inactive trial work ensure maintainer dose is minimised.
The B205 plant is a laboratory scale facility used to aid design and early operation of the full scale Magnox reprocessing plant at Sellafield. The facility was constructed during the 1950s and became fully redundant in 1970. The pilot plant consisted of four heavily shielded main cells which, due to the significant radiological waste inventory, required remote decommissioning and dismantling.
The overriding issue has been to design a system sufficiently small enough to allow it to function in the confines of the laboratory. It has not been possible to design a single system capable of dealing with all the remote challenges associated with this project and a combination of dexterous manipulator and a more simple and robust Brokk machine, has been used to deploy the full range of tools necessary. For the dexterous work a rail mounted seven degree of freedom hydraulic manipulator was used to deploy a range of relatively light weight tools primarily associated with the size reduction of small gauge pipes and vessels. The Brokk Minicut ROV was used for the dismantling of cell walls, the beds of machine tools and the deployment of large hydraulic shears and spreaders.
Both systems were operated from a shielded control room and viewing was exclusively via camera systems.
It was possible to gain access to the general laboratory area and as a result specific secondary systems associated with the control of contamination and radiation shine paths could be installed at the point of source before and in some cases during operations. This has allowed for a more aggressive and therefore much less costly approach to remote operations than is possible on the B212 project, where radiological conditions are so severe they preclude any human access to the void areas around the caesium plant as well as the plant itself.
Due to the specific nature of the plant geometry and radiological hazards a complex control system was not required. Most of the design effort was associated with the various tools and more specifically, as there is no force feed back in the systems, the tool and deployment system interface.
This is one of the more advanced remote decommissioning projects and much of the experience gained, especially that associated with operations and tool performance, is being used to benefit many of the other remote decommissioning projects that BNFL are associated with.
