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IN FOCUS Magnox Swarf Storage Silo

Decommissioning a fifty-year-old radioactive waste store that’s past its sell-by-date is not easy

Decommissioning a fifty-year-old radioactive waste store that’s past its sell-by-date is not easy, especially when there are no instructions for the job. The store in question was built in the early 1960s to hold cladding removed from spent nuclear fuel arising from the UK’s first generation Magnox power stations, to prepare it for reprocessing or recycling into new fuel.

The Magnox Swarf Storage Silo was originally actually built as a temporary store and became operational in 1964 however three further extensions built in the 1970s and 1980s providing in total twenty two individual compartments within the silo. The store is still going strong, although no one can pretend that it’s been built to modern engineering standards.

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of historic waste (approx.)

Our priority is to empty the store of its estimated 10,000 cubic metres of nuclear waste as soon as possible. The man in charge of decommissioning the historic store – Chris Halliwell – is realistic about the size of the job he faces.

Chris explained “The job is unique – simple as that. Removing decaying radioactive waste from the 22 concrete silos that are some 16 metres deep is not easy. All sorts of wastes were disposed of into the silos, which included not just fuel cladding but an estimated 60,000 items of miscellaneous radioactive metal waste. It’s hugely complicated and our workforce has had to come up with a way of doing the job safely in what is a challenging radioactive environment, with the constant build-up of hydrogen gas.”

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It’s a wonder that Chris sleeps at night, as there are so many things that could go wrong! The store is old and wasn’t designed with decommissioning in mind, so Chris’s team has to come up with some clever engineering solutions to physically get the waste out of the silos. The waste was tipped into the compartments through individual shielded hatches on top of the 22 compartments via an air-locked machine which prevented any radioactive material escaping into the atmosphere. This machine has long since been decommissioned and it was only designed to tip waste not remove waste and that’s been the big problem – how to get the waste back out of the silos.

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items of Miscellaneous Beta Gamma Waste to be removed from the twenty two underwater MSSS compartments

An additional headache is the fact that the Magnox fuel cladding was predominantly made of magnesium. The nuclear gurus of the time decided to store this waste underwater to keep it cool and prevent any potential fires – you need to cast your mind back to school chemistry lessons here when strips of magnesium ribbon burned oh-so brightly and oh-so easily! However, although the magnesium fuel cladding cannot set on fire because of the water, it does chemically react with this same water to produce hydrogen gas. Back to school and you’ll remember that hydrogen is a very light and very explosive gas, which is why the hydrogen gas in early Zeppelin flying machines was soon replaced with helium. Magnesium may have done a good job in the reactor but it’s not without its problems and it is pretty clear why the UK’s Magnox reactors – which were the nuclear workhorses of the time – never really took off internationally.

In 1992, swarf from the Fuel Handling Plant
was then routed to the Magnox Encapsulation Plant
where it was encapsulated into drums

Routine waste tipping into the silo was stopped in 1992 when a new modern waste store was built to encapsulate the waste into individual drums, which are safe and secure for long term storage and even suitable for disposal into an underground store, should one be built. The decaying radioactive waste will be pulled out of the compartments and packaged into high-integrity boxes.

So Chris and his team of 500 engineers, operators, maintainers, project managers and technical experts have got quite a job on their hands as Chris explained: “Simply put we’re concentrating on three work faces. We’re working to safely manage the historic waste on a day to day basis whilst simultaneously treating the radioactive silo liquid in which the solid waste is stored to reduce the radioactivity. We’re preparing for the physical installation of the solid waste retrievals equipment by decommissioning redundant equipment and upgrading the facility to last the extensive lifetime of the retrievals operation. In parallel, we’re building and testing the waste retrieval equipment itself at an off-site supplier’s factory in preparation for commencing waste retrieval.”

The final waste was tipped into the silos in June 2000