Britain’s public spending watchdog has launched an investigation into risks and costs at Sellafield, the UK’s biggest nuclear waste dump. The National Audit Office (NAO), which scrutinises the use of public funds, has announced it will examine whether the Cumbria site is managing and prioritising the risks and hazards of the site effectively as well as deploying resources appropriately and continuing to improve its project management. The findings of its investigation are expected to be published this autumn. Sellafield is Europe’s most toxic nuclear site and also one of the UK’s most expensive infrastructure projects, with the NAO estimating it could cost £84bn to maintain the site into the next century. Last year, Nuclear Leaks, a Guardian investigation into activities at Sellafield, revealed problems with cybersecurity, a radioactive leak and a “toxic” workplace culture at the waste dump. Predictions of the ultimate bill for the site, which holds about 85% of the UK’s nuclear waste, vary. It cost £2.5bn to run the site last year, and the government estimates it could ultimately take £263bn to manage the country’s ageing nuclear sites, of which Sellafield accounts for the largest portion. The site employs about 11,000 people and is the world’s largest store of plutonium. It comprises more than 1,000 buildings, many of which were not created with the intention of becoming long-term storage facilities for radioactive material. Sellafield is so expensive that the Office for Budget Responsibility, which monitors threats to the UK government’s finances, has warned that it and other legacy sites pose a “material source of fiscal risk” to the country. The NAO previously examined activities at Sellafield in 2018. It found some aspects of project management had improved but that more needed to be done to get a grip on vast costs and risks. Amyas Morse, the head of the NAO at that time, found that the Nuclear Decommissioning Authority (NDA), which is tasked with management of Sellafield, needed to improve its explanation of its progress so that parliament could hold it to account. This challenge was underlined when the Guardian uncovered how a worsening leak from a huge silo of radioactive waste at Sellafield could pose a risk to the public. The leak, from one of the “highest nuclear hazards in the UK” – a decaying building known as the Magnox swarf storage silo – is expected to continue for at least a further 30 years. This could have “potentially significant consequences” if it gathers pace, risking the contamination of groundwater, according to an official document. This was just one of a catalogue of safety risks arising from ageing infrastructure at the site. A document sent to members of the Sellafield board in November 2022, and seen by the Guardian, raised widespread concerns about a degradation of safety across the site, warning of the “cumulative risk” from failings ranging from nuclear safety to asbestos and fire standards. Responding to the issues late last year, a Sellafield spokesperson said: “The nature of our site means that until we complete our mission, our highest hazard facilities will always pose a risk.” Sellafield is owned by the NDA, a quango sponsored and funded by the Department for Energy Security and Net Zero that is tasked with cleaning 17 sites across the UK. The NDA said it had a “responsibility to deliver for the public, including on value for money”. “We welcome this continued scrutiny and look forward to working with the NAO,” a spokesperson said.