In October 1957 a fierce fire raged in a British nuclear reactor while increasingly desperate measures were taken to put it out.
Through the night, employees used scaffolding poles to push flaming uranium fuel cans through and out of the reactor at Windscale on the Cumbrian coast.
Flooding it with carbon dioxide gas did not help, so tonnes of water were poured on, despite fears it could cause a hydrogen explosion. Eventually it was put out by turning off the fans which blew cooling air through it.
While Windscale’s Pile Number 1 burnt, a cocktail of radioactive particles poured from the 400 foot tall chimney built to expel the cooling air.
They overwhelmed the cloth baffle filters, installed on the chimney top as an afterthought during construction, and were carried on the wind, reaching London and being detected on the continent. For six weeks, milk production had to be banned in 200 square miles near the plant because grass was contaminated with radioactive iodine.
It was the world’s second worst reactor accident, surpassed only by Chernobyl.
As the 50th anniversary – October 9-11 – approached, owner UKAEA showed off the progress it has made in decommissioning the reactor and its long redundant but undamaged sister, Windscale Pile No 2, and discussed its future plans. Yet the really serious clean-up work has yet to begin.
That intensively regulated and cautious task will take 20 years or more. No one quite knows what it will cost – one senior manager suggested £500 million.
The two reactors were thrown up at the end of the 1940s to enable Britain to make its own plutonium for nuclear weapons, freeing the UK from its dependency on the US.
The graphite reactors, much larger than any built in Britain since, were surrounded by a ‘bio-shield’ of steel and concrete walls several metres thick creating a structure the size of a block of flats housed in steel-clad sheds as big as cathedrals.
More than 3,000 narrow fuel channels ran horizontally through each. Foot-long cylindrical cans of uranium fuel, encased in aluminium, would be shoved into the channels at the front of the reactor then pushed in deeper by operatives wielding poles.
Eventually the transformed fuel would fall out of the reactor’s rear end into skips in a water-filled concrete gully, to be hauled away, cooled down in a pond, cut up and processed.
During operation, when the control rods were withdrawn and the reactors went critical, a hurricane of cooling air was forced through and up the chimney to control the temperature.
The reactors began behaving unpredictably a couple of years after starting work in 1950. While the chain reaction ran, ‘Wigner energy’ built up in graphite. This latent energy builds up as graphite atoms are displaced by neutrons in the crystal lattice. It can suddenly be released causing potentially dangerous heating.
UKAEA found the energy could be released, in a semi-controlled way, by deliberately heating the graphite above normal operating temperatures. This was done by restarting the chain reaction without the usual copious blast of cooling air. But the operators never worked out a reliable way of releasing the increasingly difficult build-ups of energy. And so, on October 8 1957, an attempted release caused overheating which ignited fuel canisters.
In the 50 years since, UKAEA has concentrated on sealing off the reactor block, cleaning up the radioactively contaminated concrete air and water ducts which lead into it, and researching a safe, practicable way of dismantling the main structure.
Only this summer has UKAEA been able to take a close look deep inside. Remote cameras revealed the expected ugly mess – quantities of ash, molten aluminium and uranium, and fuel cans in various states of damage.
UKAEA had thought that opening the reactor risked restarting a chain reaction and an explosion. It contemplated pumping the reactor full of inert gas then dismantling it with robots - a difficult and expensive project. But, after further analysis, it believes these risks are not real and has managed to convince its paymaster, the Nuclear Decommissioning Authority, and regulators the Nuclear Installations Inspectorate and the Environment Agency.
So now UKAEA, in partnership with two giant engineering companies, AMEC and CH2M Hill, is developing the latest version of a step-by-step lifetime plan to return both piles, next to BNFL’s huge Sellafield site, to brownfield land. Tens of thousands of tonnes of waste will be produced, mostly non-radioactive concrete and metal which can be treated as ordinary demolition waste.
The first phase is to remove all the fuel and most of the debris from within Pile 1. To do this, a semi-robotic machine has been designed to reach a long arm up the fuel channels. Different tools can be added to the end to twist, grab, tap, scrape and scoop the material it finds. Based on mining technology, it is being developed in a warehouse where a replica of part the reactor has been built.
A radioactive waste plant will have to be built beside the reactor for the clean-up. Here the fuel, classed as intermediate-level nuclear waste, will be encased in resin within sealed drums.
Dick Sexton, senior Windscale piles manager, said: “There’s real progress. We have a robust scheme and strategy, an advanced working prototype of the fuel retrieval machine and we’re testing out the process for conditioning the radioactive waste.”
Mr Sexton of Colorado-based CH2M Hill plans to remain in Cumbria for several more years. “It’s a cool project, cleaning up after the world’s second worst reactor accident,” he said.