But a report published on Wednesday said slow progress had pushed back an estimated completion date into the late 2050s, with “significant safety and financial consequences”.
The ageing building sprung a leak in 2019 and is spewing about 2,100 litres of water into the ground every day, the National Audit Office (NAO) said.
However, due to the extreme hazards involved in accessing the building, engineers are unable to fix the leak or even determine its exact cause – meaning it is poised to continue for another 30 years.
The amount leaked daily could fill about seven standard-sized bathtubs. At that rate, it would take just over three years to fill an Olympic-sized swimming pool.
Sellafield Ltd, the taxpayer-owned company tasked with cleaning up the silo and the wider Sellafield site, insists the risk posed to workers and the public from the leak is “low”.
But the NAO said the leak underlined the serious consequences of failing to keep up with scheduled works at the site, which is overseen by the Nuclear Decommissioning Authority.
The watchdog’s report said: “Sellafield has demonstrated that it can remove safely the most hazardous waste but is not progressing quickly enough to meet its plans.”
The silo is one of several old structures at Sellafield that contain highly radioactive materials and currently pose an “intolerable” risk, the NAO said.
It was built in the 1960s – and then extended three times – to house discarded pieces of cladding that previously encased fuel for Britain’s 11 Magnox nuclear power stations.
Inside the silo, 22 concrete-lined containers – each about 15 metres deep and partially underground – are filled with water to keep the pieces of cladding submerged. This is because the cladding is largely made from magnesium, which ignites and burns easily.
However, over time the water and the radioactive metal have mingled and become a highly toxic soup known as liquor.
British officials first detected a leak from the facility in the 1970s, by monitoring the rate at which water levels dropped within the containers.
That leak initially sealed itself, likely because something plugged the hole. But it then started again five years ago when work began to prepare the silo to be cleaned.
A spokesman for Sellafield Ltd stressed that it was currently not considered a threat to workers or the public and that the radioactive elements of the liquid were being soaked up by clay in the ground beneath the facility.
He added: “We use a network of monitoring wells around the Sellafield site and off-site to determine whether radioactivity is migrating via groundwater networks.
“This gives us a high degree of certainty that the liquor is not migrating and remains bound up under the building.”
Read More: Nuclear waste plant ‘leaking 2,100 litres of contaminated water a day’
