Cutting the cost of cleaning up nuclear waste

Posted on 27 Jul 2016 by Jonny Williamson

The Nuclear AMRC is working with Sellafield Ltd to cut the cost of making future designs of nuclear waste container boxes, potentially saving hundreds of millions of pounds in decommissioning costs.

'3m3 box robot' – the new robot photogrammetry cell at the Nuclear AMRC, used to inspect waste containers and other large assemblies.
‘3m3 box robot’ – the new robot photogrammetry cell at the Nuclear AMRC, used to inspect waste containers and other large assemblies – image courtesy of Nuclear AMRC.

The clean-up programme at Sellafield and the other sites managed by the Nuclear Decommissioning Authority (NDA) will need tens of thousands of special steel boxes over the next 30 years to safely store and dispose of hazardous nuclear waste.

The current design is a standardised three sqm stainless steel box which can be stacked for long-term storage, but cost costing tens of thousands of pounds to manufacture.

Sellafield Ltd is driving a project to significantly reduce that cost, and tasked engineers at the Nuclear AMRC to help come up with solutions which could save the taxpayer hundreds of millions of pounds over the lifetime of the programme.

Nuclear AMRC operations director, Stuart Dawson explained: “This is a challenging project requiring a fully multi-disciplinary approach, bringing together many areas of research, with the potential to deliver significant savings to industry.

“With our world-leading capabilities and expertise, the Nuclear AMRC is uniquely positioned to address such complex manufacturing problems for demanding sectors like decommissioning.”

The research focuses on the two most promising routes for cost reduction identified by Sellafield Ltd – optimising and automating welding of the container; and producing the lid flanges by casting instead of machining.

Automated welding

Nuclear AMRC research engineers worked alongside specialist welding engineer from Sellafield Ltd, Jade Leonard, to investigate new approaches, including fully automated welding and inspection.

The complex design of the boxes means that many joints are not easily accessible to current mechanical welding tools, so the team investigated small flexible welding heads that can fit into tight spaces and weld in several directions using a range of welding technologies.

'3m3 casting lift' – a prototype top flange emerges from the casting box at AMRC Castings.
‘3m3 casting lift’ – a prototype top flange emerges from the casting box at AMRC Castings – image courtesy of Nuclear AMRC.

The team have completed initial trials with a range of arc and laser welding technologies, using equipment at the Nuclear AMRC and at specialist welding partners, and identified the most promising for further development. In the ongoing second phase, the welding team are using the chosen technologies to produce more complex representative test pieces.

Duplex steel’s high strength also makes components difficult to machine. High residual stresses in the material can lead to changes in geometry when it’s machined or welded.

The Nuclear AMRC has installed a new robotic cell to develop automated inspection techniques which can ensure that boxes produced with new techniques meet specification. The six-axis robot arm carries a photogrammetry head, which rapidly builds up a detailed three-dimensional image of the box’s geometry.

The research is part-funded by the Civil Nuclear Sharing in Growth (CNSIG) programme, which aims to develop the UK manufacturing supply chain for civil nuclear with support from the Regional Growth Fund.

The results will be shared with UK manufacturers which can produce the boxes to the required specifications.

The automated cell then analyses this model and identifies any distortions or defects within minutes, rather than the days needed for inspection on a traditional coordinate measuring machine (CMM).

Cast, not machined

To investigate new casting techniques for the top flange, the Nuclear AMRC called on the specialist expertise of AMRC Castings – part of its sister centre, the University of Sheffield’s AMRC with Boeing.

The top flange, a large and complex hollow square with four corner lifting features, is currently produced by machining from a solid block, with most of the expensive high-grade alloy being cut away.

AMRC Castings investigated whether the complex shape could instead be cast as a single item.

Using the centre’s Replicast ceramic mould technology, the team successfully cast two highly accurate, one-piece prototype frames. The frames have passed material and metallurgical testing, and exhibited a superior surface finish.

The Nuclear AMRC’s machining group will now investigate how the cast part can be finished to the final precise specifications, while maintaining geometrical accuracy and surface integrity.