Radiation risk reduced

Paul Boughton

The company responsible for safely cleaning up the Sellafield site in Cumbria, UK, has passed a significant milestone, after pumping two million litres of liquid out of one of the world’s oldest nuclear waste stores.

In doing so, Sellafield Ltd has halved the radioactive content of some of its historic liquid nuclear waste, significantly reducing the potential hazard posed by a fifty year old waste store at the site in West Cumbria.

Constructed in the 1960s, the Magnox Swarf Storage Silo (MSSS) was key to supporting Magnox nuclear power generation in the Britain. Spent fuel from plants all around the UK was sent to Sellafield so that it could be recycled in the Magnox Reprocessing Plant.

However, in order to avoid power shortages during the miners’ strikes nuclear plants around the country increased power generation – meaning that spent fuel arrived at Sellafield at a faster rate than the reprocessing plant could properly handle it.

Fuel cladding, known as swarf, was placed in storage underwater in MSSS with no real plan in place for how it would be removed and repackaged in the future.  The challenge would have been difficult enough then — but as the years have passed the condition of the storage silo, built of concrete more than 50 years ago, has deteriorated.

Since the inception of the Nuclear Decommissioning Authority (NDA) in 2004, focus has been on decommissioning the store as quickly as is safely possible. For that to happen, all the waste stored in the silo, including the water in which the swarf is submerged, needs to be removed so that the building can be demolished.

In order for that to happen safe routes for the waste that is removed must be identified, with new plants constructed to deal with waste as it is removed. All this takes time — but significant progress is being made. Most of the spent fuel is now in such condition that it will never be reprocessed, and is classed as nuclear waste.

The swarf is stored under water in the twenty two individual compartments within the silo, but the water, used as a radiation barrier to shield the workforce and the environment from the radioactivity from the swarf, had been in the store so long that it had itself become a nuclear waste.

Engineers designed a system to purge the water by pumping it out of the store, and then use a clever chemical process to remove the radioactivity from it, with fresh water replacing it in the store, making the plant safer and the job of receiving the swarf easier.

In less than five years since the programme to remove liquid effluent from the silo started, two million litres has been pumped out.

Head of MSSS, Chris Halliwell, explained: “Back in the day, the MSSS literally helped keep the lights on by supporting Magnox electricity generation, however it’s now been retired from active service. The radioactivity we’ve removed and treated arose from several hundred tonnes of uranium fuel which during its lifecycle would have generated enough electricity to power over eleven million homes for a year.

“It was never built with decommissioning in mind and safely removing the liquid and solid nuclear wastes requires some ingenious engineering. We have now successfully removed liquid waste containing 10,000 terabequerels of radioactivity from the store - which equates to locking away roughly the same amount of nuclear waste discharged to sea in the Japanese Fukushima accident. Completion of this liquor transfer from the MSSS is an important step towards emptying the silos, processing the waste and safely decommissioning this legacy plant.”

The next stage will be to remove the solid waste inventory from the facility, process it and encapsulate it for safe long term storage. Three silo emptying plants are being built, the first of which will be brought to the Sellafield in a few months time.  This will undergo testing before being available for solid waste retrievals in 2017.

Magnox Swarf Storage Facility (MSSS)

* The MSSS received Magnox fuel cladding or swarf from the First Generation Magnox Storage Pond and the Fuel Handling Plant along with a range of other items of intermediate level waste. The swarf was transported in purpose built flasks and tipped into the silo compartments.

* The MSSS stopped receiving bulk waste in 1991 and alternative modern storage facilities were provided in the form of encapsulation plants.

* The Magnox swarf, which is almost 100% magnesium, is stored underwater. The swarf then undergoes a corrosive process which results in temperature increases and the release of hydrogen gas. The plant design and operations ensure that the heat and hydrogen cannot build up and risk exceeding safe levels within the plant.

* Between 1994 and 1999, a significant quantity of metal swarf was retrieved from the facility, by use of the Swarf Retrieval Facility. The waste was exported to a modern plant for encapsulation and storage.

* A Becquerel is a measure of how radioactive something is in decays per second. A Terabequerel is a million million decays per second, which releases different forms of radiation. Removing 10,000 Terabequerels is equivalent to 10,000,000,000,000,000 Bequerels. If all of this 10,000 Terabequerels were concentrated into a single object it would be about the size of a can of paint, however would be hugely radioactive and at a distance of about six metres that object would deliver 50 lifetime doses in less than one hour.

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