£20m 'Swiss army knife' will harvest nuclear material
18 July 2011
It’s been dubbed the most sophisticated Swiss army knife ever built. A 16-piece tool designed to reach deep inside one of Britain’s earliest atomic experiments and harvest the nuclear material that once promised to revolutionise how the nation generated its electricity.
Measuring 40ft in length, each of its 16 different tool-bits has been designed to withstand the harsh operating conditions inside the Dounreay Fast Reactor. The reactor shut down in 1977 after almost 20 years of experiments and is now being decommissioned, allowing energy bosses to reap the last of the plutonium and uranium from its unique “breeder” zone.
A custom-built retrieval arm will spend three years inside the reactor vessel, carefully cutting free 977 metal rods standing vertically in a hexagonal rack around the near-empty core.
Each rod will be cut free from its mounting and transferred to a waiting basket, ready to lifted through the roof of the reactor and returned to the outside world after 50 years. French engineers designed and built the tool needed to do the job safely at a cost of £20 million. It has now been moved into position above the reactor, ready to descend into the darkness of the reactor vessel below and begin harvesting the valuable metal. “The reactor was a one-off design and so is the tool we need to take out the breeder rods,” said Alex Potts, the engineer in charge of the project at Dounreay Site Restoration Ltd. “It’s too toxic in there for anyone to do the job manually – the radiation levels are still very high and the residual traces of liquid metal coolant add to the hazard – so we need a tool capable of doing the job by remote control. It’s a pretty sophisticated version of a Swiss army knife the team came up with.”
The solution came from an alliance of British and French companies hired to help with decommissioning the reactor. Framatome, the French nuclear outfit, designed and built the retrieval arm. Each detachable tool-bit cost £100,000, weighs 37-93kg and covers the range of equipment needed to retrieve the metal rods – grabs, manipulators, milling and cutting.
Up to three tool-bits will be in use at any one time and can be replaced by another three carried in a special tool-box without needing to remove the tool itself from the depths of reactor. The rest of the tool-bits will be stored above the reactor, ready to be swapped during service and maintenance breaks. Special radiation-proof cameras and spotlights will guide operators working around the clock in a control room 20 feet above in the hall of the containment sphere.
The operation is expected to take three years to remove the 977 rods from the breeder zone around the core and the single remaining fuel pin stuck in the core.
Some 2000 rods surrounded the core when the reactor operated between 1959 and 1977. Over half were removed after it shutdown but 977 were left in place. Some had become jammed and a shortage of storage space at the site delayed their removal. Now, with the site being razed to the ground, the rods need to be removed before the rest of the reactor can be dismantled safely. Its decommissioning is being carried out by DSRL for the Nuclear Decommissioning Authority, which owns Dounreay.
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