Missing material points to dangers of nuclear reprocessing, say experts.
The Sellafield plant for reprocessing nuclear fuel in northwest England is unable to account for about 30 kilograms of the plutonium it handled in 2004. That amount would be enough to make seven nuclear bombs.
There is no evidence that this material has been lost from the Sellafield site, which is managed by British Nuclear Fuels. It may not even exist, having been conjured up simply by accounting errors.
But that is not the point, says British nuclear-weapons expert Frank Barnaby, formerly of the Atomic Weapons Research Establishment in Aldermaston, UK. If Sellafield officials were asked to prove that the 'missing' plutonium had not been smuggled off the site, he says, they would not be able to do so.
The discrepancy shows that "you cannot safeguard a reprocessing plant effectively", Barnaby says. "I'm pretty certain security is not stringent enough to defeat a determined group who sets out to steal the radioactive material."
A spokesman for Sellafield says, "From our point of view, no nuclear material is physically missing." Besides, he says, the quantitative accounting of reprocessed material "is just one small element of the security regime". It is "virtually impossible to illicitly divert nuclear material off the site", he adds.
"I don't believe that," Barnaby says.
You can't afford to have that amount of material adrift in the current global climate. Frank Barnaby
Sellafield reprocesses nuclear fuel by extracting plutonium from used fuel rods. The rods can then be used to make 'mixed-oxide' (MOX) fuel for further nuclear-power generation. Most of the material reprocessed at Sellafield comes from Germany, Switzerland, Japan and Sweden.
Keeping track of the plutonium as it passes through the reprocessing system is challenging, because the spent fuel is so radioactive that its plutonium content cannot be measured directly. "You have to rely on the reactor operators' estimates," says Barnaby.
As a result, he says, "some accounting error is inevitable". The nuclear industry agrees, and says that this error is typically around 1%; independent experts have argued that it is more like 3-5%.
The 'missing' 30 kilograms of plutonium falls comfortably within this error margin, given that Barnaby estimates that Sellafield reprocesses around 30 tonnes of the radioactive element each year. But Keith Barnham, a physicist at Imperial College in London who has studied the UK nuclear industry's auditing, says that "it's a lot compared with previous years".
And given that a well-designed nuclear weapon requires only about 4 kilograms of plutonium, Barnaby says, "you can't afford to have that amount of material adrift in the current global climate." Barnham agrees: this sort of uncertainty is "just not good enough", he says.
Barnham and Barnaby say that similar accounting discrepancies are almost inevitable at other reprocessing plants around the world: France, Japan and Russia all run facilities that reprocess spent fuel.
Barnham argues that the auditing is also compromised by the fact that it does not include any plutonium present in the 'canning' material, which is stripped from the spent fuel before reprocessing begins. Five years ago, he pointed out that the UK Ministry of Defence had discovered that it had an extra 300 kilograms of plutonium in its weapons stockpile for which it could not account (see Nature 407, 833-834; 2000).
The shortfall at Sellafield "is a very significant argument for not doing things this way", Barnaby argues. MOX nuclear plants are so expensive, he says, that they need big government subsidies, and only "voodoo economics" can make them seem profitable.
And the only other use for plutonium is in weapons technology, whereas nuclear reactors that use uranium fuel can produce waste that has no military value at all. "We should stop reprocessing," Barnaby says.
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Ball, P. Plutonium books don't balance at UK plant. Nature (2005). https://doi.org/10.1038/news050214-14