There are two such vaults at the heart of British Nuclear Fuels' new thermal oxide reprocessing plant (Thorp) at Sellafield. For the moment, the plutonium elevator, like the rest of Thorp, is undergoing preliminary tests before operations commence in earnest at the end of the year.
Then, the vaults will be sealed by order of the European Commission, without whose permission no one can to enter. Within a decade, the vaults will contain nearly 60 tons (60,960kg) of plutonium - enough to make 10,000 atomic bombs of the type that destroyed Nagasaki.
By the turn of the century, British Nuclear Fuels' Sellafield plant will be storing more than half the world's stocks of plutonium. The entire world stockpile of plutonium will be around 180 tons (182,880kg) by then, and Sellafield will have nearly 100 tons (101,600kg) of it.
In a separate plant at Sellafield, BNFL has already separated out nearly 40 tons (40,640kg) of plutonium from the uranium and radioactive waste contained in spent fuel discharged from the first generation of Britain's nuclear power stations, the Magnox reactors.
This existing plutonium plant and its associated store - known as 'Finishing Line 5' - ought perhaps to look like Fort Knox, but appears outwardly as a nondescript building with little to distinguish it from hundreds of others on the Sellafield site. It carries out the last stage in the reprocessing of radioactive spent nuclear fuel, converting plutonium nitrate, the dark green, highly acidic liquid which comes out of the reprocessing plant, into plutonium dioxide powder.
Finishing Line 5 packs between 6kg and 7kg (13lb and 15lb)of plutonium at a time into stainless steel cans. It started operating in 1983 and has a maximum throughput of about three tons (3,048kg) a year.
Although plutonium is a silvery grey metal, BNFL produces and stores it in powder form as the chemical compound, plutonium dioxide.
According to Barry Leigh, assistant works manager at Sellafield, each three-litre can of plutonium dioxide powder generates about 60 watts of heat, 'so a plutonium store is hot'. About 15 canisters would produce the same amount of heat as a one-bar electric fire, and there are several thousand in the store. Fans circulate air round the racks containing the canisters, but Mr Leigh calculates that 'we have four days in which to cope with any failure of the ventilation' before the storage vaults would reach dangerous temperatures.
In contrast to the reprocessing plant proper, the plutonium finishing line is not shrouded in thick concrete walls, so it is one of the few places where a visitor can see what is going on. It is also a surprisingly small installation. In one glove box, the plutonium dioxide is poured into a screw- topped aluminium bottle, which is then sealed inside a polythene bag, before being placed inside the stainless steel can and the lid welded on. This operation is conducted inside a room little bigger than the average suburban lounge.
Finishing Line 5 deals with all the plutonium nitrate currently produced as a result of reprocessing operations in Britain. Most of it originates from the civil, power-producing Magnox reactors owned and operated by Nuclear Electric. In addition, it handles shipments of plutonium nitrate from the UK Atomic Energy Authority's fast reactor at Dounreay. But Finishing Line 5 also converts and packages some plutonium on behalf of the Ministry of Defence.
Civil plutonium is subject to inspection by the EC, and by the Vienna-based International Atomic Energy Agency, to verify that it is not being used clandestinely for military purposes. According to Mr Leigh, BNFL staff 'cannot get access to our own plutonium store' because the inspectors have attached seals to doors so that deposits and withdrawals can only be made with their permission. The Ministry of Defence's plutonium goes elsewhere, about which Mr Leigh was tactfully silent.
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