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.”
He said: “We have a range of intervention techniques available if the liquor does begin to migrate.
“While the water is radioactive, the vast majority of the radioactivity in the building – 99.5pc – remains held within the solid waste.”
The Office for Nuclear Regulation has also deemed the risk “low”, according to the NAO.
However, some scientists remain undecided on what level of risk it poses and are carrying out “ongoing radiological dose assessments”, the Guardian reported last year.
The issue will be resolved fully when Sellafield Ltd finishes emptying the silo. But since the NAO’s last audit in 2018, the expected completion date for that work has slipped from 2046 to 2059.
The extraction process, carried out in sealed conditions, involves a crane with a teddy-grabber-style claw that reaches into the containers and removes the nuclear waste in batches, dropping them into specially-made steel boxes.
These boxes are then moved to a new, secure storage site at Sellafield.
The work was originally expected to begin in 2021 but it was delayed by a year, with 19 boxes filled in 2022 to 2023 and 23 in 2023 to 2024, the NAO said. Sellafield Ltd aims to fill 546 boxes per year by the mid-2030s.
The company blamed the delays on restrictions imposed during the Covid pandemic, staff shortages and the “deteriorating condition of key buildings and facilities”.