RADWASTE MANAGEMENT |
Machining waste containers for MSSS
A new method of machining nuclear waste containers has been devised to increase production rate and reduce costs
IN ORDER TO DECOMMISSION NUCLEAR waste silos on the former UK nuclear site at Sellafield, a large number of bins have to be machined to tight geometrical tolerances to ensure they can be stacked safely. These 3x3x3m , double- walled box fabrications are made from 6mm thick Duplex stainless steel plate that tends to induce vibrations when it is milled.
One of the firms leading the Sellafield decommissioning work, Stillington-based Darchem Engineering, had been taking delivery of machined bins from contract machinists BEL Engineering in Newcastle-upon-Tyne since 2017. But as chief executive Jonathan Lamb explains, “We wanted to increase productivity but realised that further improvements to the machining process were impossible using conventional machinery and fixturing. “When we tried to raise the speed of machining,
Above: A graphic of one of the fixtures for machining a nuclear waste bin, showing the positions of the hydraulic swing clamps and one set of internal airbags that damp vibrations. A second set of airbags is inserted into the workpiece itself to provide an equal and opposite holding force before machining starts
chatter resulted and it was difficult to hold tolerance. So we engaged with Kingsbury to develop an entirely new machining concept in respect of both workholding and metalcutting to improve cycle time and increase production rate and reduce cost.” It was not an exercise for the financially faint-hearted. The two companies invested £500,000 in researching and developing an optimal solution for machining two variants of waste bin. These were for the Pile Fuel Cladding Silo (PFCS) and the Magnox Swarf Storage Silo (MSSS). Kingsbury is a machine tool supplier based in Gosport and managing director Richard Kingsbury, said, “Duplex will seek out any weakness in a machining system. We considered every conceivable source of vibration, from the machine bed to the column, guideways, spindle, the spindle-tool interface and the cutting tools including their grade and geometry. “We then turned our attention to a forensic analysis of how best to fixture the bins and it was here that we achieved a breakthrough. By inflating five airbags between the fixture and the box and five more inside the workpiece itself, we managed to muffle the entire unit so it could be milled at high speeds and feeds without vibration. “A final piece of innovative engineering involves
hydraulically rotating the 12 swing clamps around the face of the fixture one by one out of the way of the approaching cutter, so that at any time 11 are engaged to maximise rigidity.” To understand the natural frequencies of the MSSS bin,
Above: An MSSS Duplex stainless steel box being machined on the Waldrich Coburg Taurus 30 machining centre
34 | May 2021 |
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modal analysis was conducted in 12 locations around the fabrication. A tap test on a fully fixtured bin, with 32 touch clamps and three push clamps engaged in addition to the 12 swing clamps and with the internal and external airbags inflated, showed that the vibration amplitude fell to below 0.1g in under 17ms on average.
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