The Thorp nuclear reprocessing plant at Sellafield, Cumbria, has recycled its final batch of reactor fuel. But it leaves behind a hugely toxic legacy for future generations to deal with. So how will it be made safe?
Thorp still looks almost new; a giant structure of cavernous halls, deep blue-tinged cooling ponds and giant lifting cranes, imposing in fresh yellow paint.
But now the complex process of decontaminating and dismantling begins.
It is a dangerous job that will take decades to complete and require a great deal of engineering ingenuity and state-of-the-art technology - some of which hasn't even been invented yet.
This is why.
Five sieverts of radiation is considered a lethal dose for humans. Inside the Head End Shear Cave, where nuclear fuel rods were extracted from their casings and cut into pieces before being dissolved in heated nitric acid, the radiation level is 280 sieverts per hour.
We can only peer through leaded glass more than a metre thick at the inside of the steel-lined cell, which gleams under eerie, yellow-tinged lighting.
This is a place only robots can go.
They will begin the first stage of decommissioning - the post-operative clean-out - removing machinery and debris.
Already, a mechanical arm on wheels is being carefully guided through the cave, as operators shout instructions to each other, trying to prevent the machine becoming tangled up in cables.
Once cleared, the cell will be washed repeatedly using water or acids to bring down the level of radioactivity. Ultimately, the plan is for humans to be able to enter the contaminated cave.
"We'll need to look at novel decontamination agents to help us clean out the plant more effectively," says Melanie Brownridge, head of technology at the Nuclear Decommissioning Authority.