At Dungeness B nuclear power station on a remote stretch of the Kent coast in south-east England, workers are making preparations to carefully remove thousands of radioactive fuel elements from its reactors and transfer them to a purpose-built pond for at least 90 days for cooling.

The spent fuel will later be packed into 53-tonne “flasks” fortified with 39cm-thick steel walls before being transported across country by train to Sellafield in Cumbria.

The nuclear facility in north-west England is host to most of the radioactive remnants of Britain’s civil nuclear programme that dates back to the 1950s. These include highly toxic waste that will remain there until a suitable site is found for an underground repository where it will have to be stored for more than 100,000 years to make it safe.

Preparations for the “defuelling” of Dungeness B started with “immediate effect” on June 7 when its majority owner, French state-controlled utility EDF, announced it would close the plant seven years early. It had not been operational since September 2018 as engineers tried to fix problems, including corrosion and cracks in its pipework.

The 1.1GW plant is the first of seven built in the UK between the mid-1960s and late-1980s using advanced gas-cooled reactor (AGR) technology to come out of service. It will kickstart a decommissioning process spanning generations, which sceptics argue strikes at the heart of why no new nuclear plants should be built.

The remaining six AGR plants are due to be retired by the end of this decade at the latest, leaving the more modern Sizewell B plant in Suffolk, which uses pressurised water reactor technology, as the only one operational out of the existing fleet.

“[Decommissioning of] many of these facilities will continue well into the 22nd century,” said Paul Dorfman of University College London’s Energy Institute. “The problem with decommissioning is it always turns out to be more complex than one had imagined.”

Map of UK nuclear power plants

Critics also point out that the decommissioning of Britain’s 17 earliest atomic power sites has been extremely costly. The latest clean-up bill for those sites, which include a generation of nuclear plants known as the “Magnox” stations, is estimated at more than £130bn over 120 years.

Nuclear supporters have seized upon the retirement of the first AGR to highlight that the UK is fast running out of a vital source of low carbon electricity that — unlike the main renewables, wind and solar — is available throughout the year, no matter the weather. The UK has committed to net-zero carbon emission targets by 2050.

Five of the AGR plants are due to be shut down in the next three years, removing more than 5.2GW of the current 8.9GW of nuclear energy capacity, which accounts for just under a fifth of total UK generating capacity.

Only one new plant is under construction, the 3.2GW Hinkley Point C in Somerset, which will be able to provide electricity for 6m homes when it is completed by EDF.

It is due to open in 2026 but has been troubled by spiralling costs and delays. Reports last week of a possible radiation leak at a plant in China, which is based on the same Franco-German reactor technology as that at Hinkley, have heightened concerns among opponents of nuclear power.

Proponents are pushing ministers to make progress in negotiations with EDF about financing a second new station — Sizewell C in Suffolk — which have been ongoing since December.

Tom Greatrex, chief executive of the Nuclear Industry Association, insisted without new nuclear capacity, the electricity system would become more reliant on carbon-intensive, gas-fired power stations — the UK’s single biggest source of electricity — during dark winter days when there is no wind.

“The quid pro quo that you get with [a] system where there is a quite a lot of variable output [from wind and solar] is you get the other extreme of that at other points in the year,” Greatrex said.

Climate activists, such as E3G and Greenpeace, have long argued that the debate over building costly, complex new nuclear plants detracts from investment in cheaper, climate-friendly technologies.

The government said it was “committed to the future of nuclear energy”. It indicated it was looking to make a final decision on “at least one nuclear power station” before the next general election.

The exact arrangements for the decommissioning of Dungeness and the six other AGR plants are subject to negotiation between EDF and the government. It will be financed via a £14.5bn fund set up in 2005.

The French utility is expected to take at least three years to remove all fuel from each site and potentially carry out some early demolition work before handing them over to the UK state-owned Nuclear Decommissioning Authority. EDF declined to comment.

The next stage will probably involve the treatment and removal of waste and demolition of facilities that are no longer needed. Some facilities will be left untouched for 85 years — to allow residual radioactive materials to decay — before demolition.

The NDA is already in charge of decommissioning Britain’s earliest nuclear sites including Sellafield, where the world’s first civil atomic plant Calder Hall was opened in 1956. But it has faced criticism in recent years by MPs, who claim there is a “perpetual lack of knowledge” within the agency about the state of the sites, an accusation it refuted.

Engineers insist decommissioning of the AGRs will be far less complex and costly, given most of the £130bn bill for the earlier sites will be spent on Sellafield. The facility in Cumbria is one of the biggest nuclear sites in the world.

Dawn James, vice-president for nuclear power at Jacobs, the engineering group, said lessons learned from cleaning up the earliest nuclear power facilities would make the next decommissioning wave more efficient.

“[With] the AGRs we have got a lot of learning from that end-of-life process that the Magnox stations went through,” she said.