Getting radioactive sludge out of the Sellafield legacy ponds is the mother of all spring cleans.
And, just like a household vacuum cleaner, there’s a variety of different ‘heads’ to attach to the machinery needed for sludge retrievals. All of them do different jobs… and they’re so ingenious, we think Sir James Dyson would be proud.
You may not have seen it in all of the ‘Reviews of the Year’ for 2016 (small matters such as Brexit, the Olympics and Donald Trump hogged the column inches) but for us [Sellafield Ltd] 2016 was the year of sludge. We started firing on all cylinders in the task of removing this material – a mixture of corroded fuel, broken-down organic matter and waste particles – from the two legacy ponds.
In the First Generation Magnox Storage Pond, we started bulk sludge retrievals for the first time in March – marking the start of about six years of systematically removing 1,500m3 of silty radioactive waste (the equivalent volume of around 15 double decker buses).
At the end of the calendar year a fortnight before Christmas, the first ever exports of sludge took place from the Pile Fuel Storage Pond. The volumes and radiation levels of the material in this older pond are less challenging, but it’s still a mammoth undertaking that will take until 2022, according to current programme estimates.
Now that we’ve started the process of emptying both legacy ponds, it’s time to pay tribute to some of the different bits of kit which are making these facilities less hazardous day by day by getting the sludge into a safer place. The Pile Fuel Storage Pond has a different set of tools as the sludge there is thicker, so here we’re focusing on equipment in the First Generation Magnox Storage Pond.
Operating in a radioactive environment, they all have to be ultra-reliable, require minimum maintenance and get on with the job at hand.
Bulk Sludge Removal Tool
How it works:
We place a 1m x 1.3m square metal ‘hood’ (a bit like a giant upturned biscuit tin) on top of an area of pond floor covered in sludge. The hood is 30cm deep. Two pumps then agitate the sludge within the hood by sucking in water and squirting it out to create a slurry. The main discharge pump in the middle of the hood sucks up the slurry and transfers it over to storage tanks in another plant to settle out. It takes 15-20 minutes to clear each area and then the hood is picked up by a crane above and placed on the next area to do the same again – this is called a ‘hop’. A normal transfer of 4-5 hops sends the equivalent of around 1,000 household baths (80m3) of contaminated water over to the storage tanks – although it only yields about 10 baths (800 litres) of actual sludge once settled.
Manufactured by James Fisher Nuclear Ltd, we started trials in 2013. Learning from these trials led to modifications to reduce blockages and improve pump seals. TIS in Workington, which is now building a second tool, helped with the modifications too. The tool was first installed in the First Generation Magnox Storage Pond in April 2016 and was first used to transfer real sludge on 11 November 2016.
Crevice Sludge Removal Tool
How it works:
This gets into the parts the bulk sludge removal tool can’t reach with its 1m long hose able to hang down into crevices between skips – just like when we take the main attachment off the vacuum to run the hose under the skirting boards. It is ‘driven’ by a crane above and is raised and lowered each time to suck up a small area of sludge below in a movement called a ‘stitch’. It stitches its way along a skip so the sludge can be removed before it is lifted clear.
Manufactured by James Fisher Nuclear Ltd, we started trials in 2013. It has built-in cameras at the top of the hose so the driver can accurately line up the tool with the gap between skips. It is due to start being used to transfer sludge from the First Generation Magnox Storage Pond in March 2017.
Deluge Skip Wash Box
How it works:
It’s a bit like an underwater dishwasher which sits on the floor of the pond – except it’s about the size of a Transit van. Skips of fuel are picked up by the overhead crane and carefully lowered inside. The wash box is then closed underwater by the crane placing its lid on top. Six powerful jets of pond water, each with the equivalent force of a fireman’s hose, are then shot into the skip to agitate the sludge, while a discharge pump draws off the sludgy water inside it. Each sludge transfer delivers the equivalent of around 500 household
baths (40m3) of contaminated water to the settling tanks – although it only yields about 3 baths (250 litres) of actual sludge. Just like a dishwasher running without pots and pans, it has a self-clean cycle to remove accumulated debris inside.
We started working with National Nuclear Laboratories in the early 2000s to develop this tool, with the detailed design carried out by North West Projects. Manufactured by Darchem in Stockton on Tees, test trials started in 2013 and it was first used in the First Generation Magnox Storage Pond in March 2016. One of its first sludge transfers not only revealed previously unseen fuel bars in a skip, it also washed animal bones out of the skip – most likely from a seagull.
ROV-Controlled Retrieval Tool
How it works:
This is a development of the crevice sludge retrieval tool, but with a pump positioned on the pond floor and a much longer suction leg deployed into the sludge by a swimming Remotely Operated Vehicle; think of an old-fashioned vacuum cleaner with a long hose used to clean the stairs. This tool allows access to sludge that can’t be reached with the other tools, in particular under fixed furniture in the pond. It allows us to keep on de-sludging when the skip handler (the main pond crane) is unavailable.
The remote operated vehicle controlled retrieval tool was designed and developed in early 2016. Some very simple trials were carried out in an off-site facility to ensure that the current in pond remote operated vehicles could deploy the tool and allow the pilots to practise picking up and using the tool. The first tool was installed in the pond in June 2016 and so far has carried out a number of sludge recoveries from both the pond floor and the surface of skips.
This article was commissioned by Sellafield Ltd (http://www.sellafieldmagazine.com/suck-it-and-see/) and has been reproduced by kind permission.