Sellafield Ltd is leading the way in decommissioning highly radioactive nuclear facilities by using innovative techniques to both scan redundant facilities where man access is not possible and to stabilise these using a new foaming grout to allow safe dismantling.
These techniques have been specially developed to help decommission the Primary Separation Plant which was constructed in the early 1950s. The plant originally carried out the first stage of reprocessing fuel from the pile reactors which were developed to produce materials for the UK’s defence programme.
Decommissioning Director Russ Mellor said: ”Sellafield Ltd is applying both existing and new innovations to overcome technical decommissioning challenges, whilst at the same time making these work in highly radioactive environments. On top of this, we are committed to ensuring the safety of our workforce, the local community and the environment, whilst offering the best value to the UK’s taxpayers.”
Speaking on behalf of the Nuclear Decommissioning Authority (NDA), Sellafield Programme Director Ian Hudson said: “These projects are excellent examples of innovation, working with the supply chain to accelerate decommissioning on the Sellafield site. Innovation and technology development will help the NDA fulfil its mission to deliver solutions to the challenge of nuclear clean-up and waste management.”
Following the shutdown of the Primary Separation Plant, the 11 storey, 61 metre high process cell known as the Highly Active North Outer cell (HANO) was used as a ventilation route for 20 years. During this time, many of the mild steel beams within the cell were severely corroded by the acidic ventilation effluent and the cell needed to be stabilised to allow its safe decommissioning.
Sellafield Ltd has developed a special light-weight foaming grout – similar in consistency to that of honeycomb confectionery – which is being used to stabilise this redundant nuclear plant for its eventual decommissioning.
The use of a foamed grout in radioactive decommissioning projects is novel. Similar grouts have however, been used to backfill mines and voids and are normally used with higher densities and in environments where the performance of the material is less critical.
Jeremy Hunt, Senior Project Manager explained: “The use of this grout in nuclear decommissioning pushes the boundaries – it is an unconventional material in an unconventional environment.
“Following extensive research and development, the project team worked with the University of Dundee, Concrete Technology Unit and Westlakes Engineering to develop the lightweight foam grout.
“This light weight foaming grout is made of a cement grout base mix with a foaming agent added before deployment. It has an air bubble consistency to minimise the weight and seismic vulnerability on the building, and can be crushed to 30% of its original volume when full decommissioning of the building takes place.”
Deployment of the grout into the cell was carried in partnership with Kier Construction, who played an important role in undertaking proving trials prior to underpin the feasibility of the project.
The HANO cell in the former Primary Separation Plant has been successfully filled with 505m3 of light-weight foaming grout. Completion of this stabilises the internal vessels and pipework in the cell. It mitigates a significant safety risk of having vessels collapsing in the cell and leaves the cell in a fit state for care and maintenance pending full decommissioning.
The first phase of grouting to fill the cell shaft was completed in late 2007. After the development of an even lower density grout to minimise weight loading, grouting of the top 12 metres of the cell – consisting of floors 7 to 10 – started early summer 2010. The last of the 55 pours has now been completed, each with a maximum depth of 25cm taking 24 hours to complete and set.
Steve Slater, Head of Projects, Decommissioning and Demolition said “The HANO Stabilisation project is an important part of hazard reduction. We are proud to able to deliver on our commitments to the NDA and contribute to another Decommissioning success.”
Another innovation has been used in the Primary Separation Plant’s Shear Cave cell where man access is also not possible due to the high radiation levels. Here, laser and gamma scanning technology was used to characterise the condition of the cell. The N-Visage™ software and scanner system, which was developed by local firm REACT Engineering Ltd, takes data from the facility and constructs a map of where the radioactive sources are potentially located. This map can then be used to investigate the worker dose consequences of potential decommissioning plans.
Regarding the characterisation programme, Project Manager John MacGregor said: “We needed to establish whether concentrated high-dose hotspots could be removed and shielded to allow staff access to this part of the facility.
“Although the NVisage™ dose scenarios revealed that the ambient dose was still too high for personnel access, the information on radiological distribution is very valuable and will help us to plan future decommissioning programmes. We hope to use the technology for a number of other projects across the site.”