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The UK's 'most hazardous building' - the story of Sellafield's nuclear disaster
The UK's largest nuclear site – once dubbed the "most hazardous building in the country" – could continue leaking radioactive water into the ground until the 2050s.
That's the damning verdict of a new MPs' report into the ongoing decommissioning of the Sellafield nuclear power plant in Cumbria, which, nearly 70 years ago, was the site of one of the world's worst ever nuclear accidents.
In October 1957, uranium cells inside one of the site's two nuclear reactors caught fire, burning for three days solid.
The fallout remains to this day and, on Wednesday, a report by the Commons public accounts committee criticised the speed of decommissioning the site, warning that officials are in danger of losing a race against time to complete the clean-up safely.
Of particular concern is the Magnox Swarf Storage Silo (MSSS), which was built in the 1960s and consists of 22 vertical underwater compartments, storing around 10,000 tonnes of radioactive waste.
Since 2018, the silo has been leaking enough radioactive water to fill an Olympic swimming pool every three years. The Nuclear Decommissioning Authority (NDA) has described the situation as its "single biggest environmental issue".
The site, initially known as Windscale, was initially built as a royal ordnance factory during the Second World War. After the war, it became the main site intended to produce plutonium for the UK's nuclear weapons programme and became home to Britain's first two nuclear reactors.
During its construction, chief engineer John Cockcroft had insisted that filters were installed at the top of the chimney stacks on the two gas-cooled nuclear reactors, Windscale Pile 1 and 2.
On the morning of 10 October 1957, a planned energy release procedure in Windscale Pile 1 did not go to plan, and unintentionally heated parts of the reactor.
Scientists were expecting the reactor to eventually cool down but then spotted a fire in uranium fuel cells, producing a blaze reaching an estimated 1,300C (2,380F) – causing an incident so bad it was eventually ranked in severity at level 5 out of 7 on the International Nuclear Event Scale.
The severity of events were minimised by the government and nuclear establishment of the time. When the incident started, men wearing radiation suits used scaffolding pipes to try and push the burning fuel rods out of the graphite reactor. High radiation levels meant they could only spend a few hours at the reactor, so they sought more volunteers from a nearby cinema.
The blaze was eventually extinguished when air to the reactor room was restricted, but tons of radioactive materials had already escaped into the atmosphere. Indeed, it was Cockcroft's filter – the pair were collectively known as 'Cockcroft follies' – on Windscale Pile 1's chimney that stopped the disaster from becoming a catastrophe as it limited the amount of radiation released into the air.
The ramifications of the accident were felt for years. It is estimated about 240 cases of cancer were caused by the radioactive leak and all milk produced within 310 square miles (800 square km) of the site was destroyed for a month after the fire.
The disaster proved to be a wake-up call for the nuclear industry. Windscale Pile 1 was permanently shut down, and remains sealed and not yet fully decommissioned while Windscale Pile 2, which was undamaged, was also shut down shortly afterwards.
It also to led to vast operational and technical improvements in nuclear reactor design, technology, licensing and regulation.
Nowadays, Sellafield is one of the largest nuclear sites in Europe, with more than 10,000 people cleaning it behind miles of fencing. The size of a small town, it also has its own infrastructure with facilities including a postal service, an armed police force, shops, canteen and a medical team.
The site's aging buildings plus the highly radioactive nuclear waste and unstable materials stored at the site mean it remains a significant hazard.
Despite being non-operational since 2003, the site has many years of dismantling to go, up to 2125. Indeed, Sellafield still contains hundreds of thousands of tonnes of radioactive waste, including the largest stockpile of civilian plutonium in the world, in excess of 140 tonnes.
Sellafield Ltd, the company clearing the site on behalf of the NDA, has been criticised in the report for missing 'most' of its annual targets, including those at the MSSS.
'The consequence of this underperformance is that the buildings are likely to remain extremely hazardous for longer", the MPs' report said.
The challenge for Sellafield Ltd, and the NDA, is not only cleaning up the site but also maintaining the ageing buildings, which were never designed to be accessed again, and contain the unknown amount of radioactive waste inside.
The site has several high-risk buildings including the Pile Fuel Storage Pond (PFSP), an open-air pond holding irradiated fuel and sludge that was built in the 1950s.
The Pile Fuel Cladding Silo (PFCS) which contains fuel casings and other highly radioactive materials and the MSSS, which holds metallic cladding waste and removed from nuclear fuel rods.
Despite an estimated decommissioning cost of £136bn, according to the report, the leakage on the MSSS is expected to persist until the oldest section of the structure is emptied in the 2050s.
Public accounts committee chairman Sir Geoffrey Clifton-Brown said: 'The sheer scale of the hundred-year timeframe of the decommissioning project makes it hard to grasp the immediacy of safety hazards and cost overruns that delays can have.
'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 Sellafield risks losing.'
The NDA acknowledged that the leak at the MSSS is its 'single biggest environmental issue', saying managing and retrieving waste from the silo was their 'highest priority'.
They said: 'As the report says, the leak in the MSSS is contained and does not pose a risk to the public. Regulators accept that the current plan to tackle the leak is the most effective one.'
The UK has a reported volume of approximately 4.58 million cubic metres of radioactive waste stored in various facilities across the country.
Radioactive waste goes through stages of treatment, packaging, storage and disposal depending on its classification as high level waste (HLW), intermediate level waste (ILW) or low level waste (LLW).
HLW is typically found in liquid form, and is often generated as a by-product during the reprocessing of fuel from nuclear reactors, with current practice to store for at least 50 years before disposal.
ILW consists of mainly steels, graphite, concrete, cement, sand and sludges and is stored until a suitable disposal route becomes available, essentially an underground facility the UK government is planning on building.
LLW is building rubble, soil and steel items that may have come into contact with radioactive material and can sometimes be incinerated or recycled.
In the longer term, the UK government plans to create a geological disposal facility (GDF) to store nuclear waste underground for thousands of years.
The NDA has identified two sites in Cumbria and plans to provide a decision to government on one site after community engagement.
The current planning assumption is that a GDF will be available for intermediate level waste emplacement in the 2050s and high-level waste and spent fuel from 2075.
