The Sellafield cleanup could overspend by as much as £913 million.

That’s according to a new report from the National Audit Office (NAO), which suggests although some programmes to reduce risk and hazard in legacy ponds and silos have been successful, the Nuclear Decommissioning Authority (NDA) is still facing continued delays.

In 2015, the NDA’s nine major projects which were in construction were anticipated to cost an additional 60% of their budget at design stage – this has now been reduced to 29% over budget.

The NAO also found evaluating overall performance at Sellafield is difficult due to a range of factors, including projects being cancelled halfway through and other programmes being forecast to reach critical milestones early despite having delivered less work than originally planned.

The NDA has reported the site has achieved £470 million in efficiency savings but has said it does not know where these came from and admits a proportion may not represent genuine reductions in usage.

Amyas Morse, Head of the NAO, said: “The improvements in reducing risk at Sellafield are encouraging but the scale of the challenge is very great and the department could be doing more to support the NDA through better governance and oversight of performance.”

NDA Chief Executive, David Peattie said: “We welcome the report, which reflects the hard work going in to building a more effective NDA, our focus with Sellafield on making and demonstrating better progress in the cleanup and decommissioning work and the positive effect of making Sellafield Ltd a subsidiary of the NDA.

“However, as the NAO rightly highlights, there is still much more to be done, including on how we better evaluate and demonstrate progress.

“The recommendations of this report will inform and support our ongoing work to improve and strengthen the role of the NDA and continue our upwards trajectory of progress at Sellafield and across our other 16 nuclear sites.”