But a Public Accounts Committee report on the clean-up warns that the UK’s most hazardous nuclear site remains dangerously behind schedule, with the total clean-up bill now heading for a staggering £136bn.

The PAC said the new Programme and Project Partners (PPP) model introduced by Sellafield is showing early signs of success, with four out of five major projects approved since 2018 now on track to meet original time and budget expectations.

Two of those schemes have been awarded back-to-back ‘Green’ ratings by the Infrastructure and Projects Authority, meaning “successful delivery appears highly likely”.

The PPP model sees Sellafield work closely with a select group of long-term contractors over a 20-year period, with incentives built in to boost planning, develop the workforce and eliminate duplication.

MPs said there was now “tangible evidence” that project management was improving and the overall direction of delivery was heading the right way.

But the committee said the wider programme remains deeply troubled.

• The now-paused Replacement Analytical Project wasted £127m and ran five years late

• Four major schemes have run over budget by a combined £1.15bn since 2018

• Radioactive water continues to leak from the Magnox Swarf Storage Silo, the UK’s most dangerous building

• Delays to the planned large-scale radioactive waste Geological Disposal Facility mean that new surface temporary waste buildings costing up to £760m each may be needed at the site.

MPs are demanding tougher oversight from government, clearer medium-term targets, and urgent action to address asset deterioration before safety is further compromised.

Sir Geoffrey Clifton-Brown MP, PAC chair, said that the new delivery model was promising, but it was arriving against a backdrop of failure.

“Every day at Sellafield is a race against time to complete works before buildings reach the end of their life.

“Our report contains too many signs that this is a race that Sellafield risks losing. It is of vital importance that the Government grasp the daily urgency of the work taking place at Sellafield, and shed any sense of a far-off date of completion for which no-one currently living is responsible.”

Sellafield Ltd is now rolling out lessons from the PPP approach to medium-sized projects, while the Nuclear Decommissioning Authority has launched a cross-site leadership group to share best practice.

The committee also raised red flags over workplace culture, with 16 NDA gagging agreements signed in the last three years and over £370,000 paid in employment-related claims in 2023-24.

Sellafield 100-year clean-up plan