Diving into nuclear ponds
Steve Franks talks to Khai Trung Le regarding the use of divers to clear the Sizewell A nuclear fuel storage pond.
Sending people directly into nuclear fuel storage ponds may not have been the most obvious solution to clearing radioactive waste, but after completing a first run at Dungeness A, UK, nuclear power station in 2016, the UK Nuclear Decommissioning Authority (NDA) is looking at repeating that success with clearing skips in the Sizewell A nuclear fuel storage pond.
Typically, pond clean-out involves lifting whole skips out of the water using remotely operated equipment where they can be cut up prior to decontamination, storage, and disposal. The process is slow, and without the water acting as shielding, the risk of airborne radiation exposure is increased. The use of divers working underwater where they have greater access to awkward areas, separating the waste with plasma cutters, has lead to a faster, safer, and more productive process.
Steve Franks, Sizewell A Ponds Programme Manager, told Materials World, ‘Taking the items into the air means having to set up a rigorous environment due to the airborne hazard and dose rate associated with doing this work. By putting someone in the water, there are fewer burdens to control radioactive discharges, and the water acts as an excellent shield, absorbing most of the gases that come from the cutting process.’
The strategy was originally put in place as Cavendish Fluor Partnership (CFP) was bidding on NDA Magnox decommissioning activities. Although the NDA has decided to terminate its contract with CFP early in 2019, Sizewell decided to move forward with nuclear divers. It has instead worked with USA contractor Underwater Construction Corporation due to the team’s experience of working in underwater environments. On the choice of a US firm, Franks said, ‘It is a very specialist field. There aren’t any UK companies with sufficient nuclear diving experience. We’ve gone with a company with a reputation throughout the world for diving in challenging nuclear environments.’
The primary issue with sending people into nuclear ponds is, unsurprisingly, ensuring diver safety. However, Franks noted that there are advantages to working in the pond environment. ‘In commercial diving, there are more hazards to deal with, including tides and visibility. Diving in a nuclear pond is a relatively calm environment.’ Radiation remains the focus, and divers are equipped with protective dry suits and are continually monitored. Franks continued, ‘Radiation is well understood and controlled, so they’re well protected from a monitoring perspective. We use probes that measure radiation and give a constant readout to a supervisor.’
Since the 250 dives conducted at Dungeness A, the Sizewell team has refined the use of plasma cutters in the decommissioning process. Franks said, ‘We’ve learnt how to control the environment that occurs outside of the ponds. Essentially, we’ve had to ensure any airbone contamination that comes off as a result of dive activity is controlled. The off-gases that come from plasma cutting have a potential to cause increases in airborne activity. But with controlled ventilation, we have a good arrangement at Sizewell.’ Although the team are only decommissioning one pond, the inventory at Sizewell is significantly larger than at Dungeness.
Sizewell A operated from 1966 to 2006, with defueling beginning in 2009. The first dive was conducted in February 2018, surveying the pond floor, transferring sludge into a purpose-built tank, setting up cutting equipment, and size-reducing the first of 35 skips. Sizewell is currently around 30% through the work programme, and should be complete by the end of 2018.