Nuclear industry site Sellafield announced that it had safely removed the first batch of waste from the Magnox Swarf Storage Silo (MSSS) facility. Standing in the oldest part of the Sellafield site in Cumbria, the MSSS has stored nuclear waste in its water-filled chambers for the past 60 years. Once a vital part of the nation’s nuclear energy generation requirements, the building stored casings removed from the used fuel rods of Magnox reactors in first-generation nuclear power plants so that the fuel inside could be reprocessed.
Today, due to the age of the building, the contents it held, and the fact it was never built with decommissioning in mind, it is one of the most hazardous nuclear facilities on the Sellafield site. Meticulous planning at every stage of the project has been central in the project.
“This is the culmination of decades of preparation by hundreds of people across Sellafield and our supply chain,” states Chris Halliwell, head of programme delivery for MSSS at Sellafield Ltd. “As well as maintaining the original concrete structure of the building, we’ve designed purpose-built retrieval machines. The first of three such machines has now started extracting the waste, which will take another 20 or so years to complete.”
Wolverhampton-based Ansaldo Nuclear Ltd designed and engineered the retrieval machines – or silo emptying plant (SEP) machines. The work initially involved engineering a number of bespoke tools, including: a hoist and hose unit (which stores enough rope without it twisting or corroding); a silo rake; first and second stage deployment units; a heavy-duty hydraulic manipulator; a transfer bogie; and a thermocouple cropper.
GRAB MECHANISM
The SEP machines operate on a very simple premise, not dissimilar to a fairground grab mechanism. According to Halliwell, the machine locks over a silo and seals itself around a hole. A hydraulic grab then descends from a hoist drum into the waste – corroded Magnox alloy that has degraded into a sludge – before closing and retracting.
“The corrosion of the swarf creates highly flammable hydrogen, so we constantly have to ventilate each compartment to avoid unnecessary build-up,” says Halliwell. “The reaction that’s taking place is also exothermic, so it naturally wants to heat, which accelerates the generation of hydrogen. As a result, it’s really important that we maintain temperatures and continuously cool the silo liquors.”
Once the grab retracts into the operator bulge, it becomes possible to see what it has collected before dropping it into a 1 m3 stainless steel skip.
“We don’t select specific elements of waste, so it’s really just lucky dip,” states Halliwell. “We repeat this cycle until we have sufficient waste to fill the skip, which then withdraws along a tunnel and is lifted into a bottom-opening flask that sits on top of the machine. The flask shields the waste but also acts as a transport container for transfer elsewhere on site. It’s a very hazardous environment without compare anywhere in the world, as far as we know.”
By 2024, the site will have all three SEP machines working alongside one another, each weighing nearly 400t and containing almost 14,000 different working parts. These “shielded caves on rails” will be operational for the next 20 years, so they are built with maintenance in mind, even if the moving parts have been endurance-tested for thousands of cycles.
“It’s a very complex machine,” states Sellafield Ltd maintenance engineer Andrew Tinkler. “There are 23 individual modules that are designed to be unplugged, isolated mechanically and electrically, and removed for repair if need be. During commissioning, we had an open gamma gate so we could walk inside the machine and undertake repairs internally. But now it’s active, there’s no way we can go through the transfer tunnel; everything is contained. One of the interesting characteristics of the machine is its capability to move from compartment to compartment. We can jack the machine up and drag it to the next compartment to retrieve future waste.”
MANY TASKS
The nuclear supply chain has been working alongside Sellafield to manufacture and install the SEP machines, although this enormous project has many more aspects. For instance, preparing the silo for operations has been a huge programme of work, including seismic strengthening of the building, installing new ventilation and inerting systems, building new cranes, and keeping the old buildings safe and secure throughout. “We had to look after the original asset, which has obviously been degrading over time,” explains Barbara Kincade, head of operations at Sellafield Ltd. “One of our main requirements was to ensure the building could withstand the time required to empty the waste material. We try to open up as many work fronts as possible on a daily basis, while making sure not to overly congest the working areas from either a conventional or radiological safety perspective. It’s our job to manage the legacy that we’ve been handed.”
After the waste has been picked, it is carried out of the plant in a shielded transport container and placed in highly-engineered 3m3 boxes for safer storage. Thousands of these UK-manufactured 3.5t duplex stainless steel boxes will be required to complete the project. They are double-lined and able to vent any hydrogen that might build up inside. Each one is sent to an existing store on site where it will remain until the completion of a new box encapsulation plant (BEP). The BEP will handle the drumbeat of the three waste retrievals machines, each sending a skip of waste every day.
Clearly, the scale of the task is huge. There are 22 concrete compartments in the MSSS, each holding waste with the equivalent volume of six double-decker buses, about 11,000 m3 in total. Halliwell likens the task to emptying 22 wheelie bins using three teaspoons, which is why it will take around 20 years to complete. All of the waste will eventually reside within a geological disposal facility.
The reason that many people object to nuclear is partly because of its noxious remains, but this innovative engineering project provides an example of how the UK is at least getting to grips with its oldest nuclear waste.
Andrew Prince, retrievals machine operator, says: “We’re taking waste out of an ageing silo, placing it in more fit-for-purpose storage packages and sending it downstream to more modern facilities. That can only be a good thing, not just for the site but also the local and wider communities.”
The development coincides with the closure of the Magnox reprocessing programme after many years, and is perhaps symbolic in the shift in emphasis towards cleaning up rather than reprocessing, concludes Halliwell.
On 18 July 2022, Sellafield announced that the Magnox Reprocessing Plant had safely reprocessed the final box of spent fuel from the UK’s fast reactor programme being stored in the facility’s ponds. It seems the UK has truly commenced a new era of decommissioning and clean-up.