The process could open up new pathways for radioactivity from the repository to get back to the surface and pollute drinking water, Dr Derek Ockenden warns. This would occur as the energy generated by the nuclear chain reaction turned water in the repository into steam, causing the surrounding rocks to fracture.
UK Nirex, the industry's waste disposal company, is proposing to excavate a huge cavern some 800 metres underground, inland from Sellafield beside the village of Gosforth. Next month, as a step towards that goal, it will apply for permission to dig out a subterranean laboratory at the site to study the rocks and the movement of water.
The dump would take plutonium-contaminated intermediate level wastes, mainly from reprocessing at Sellafield, and is supposed to isolate them for tens of thousands of years.
Dr Ockenden, who lives in Gosforth, worked on plutonium chemistry at Sellafield from 1953 until his retirement four years ago and has since acted as a consultant to the the Atomic Energy Authority. He believes that Nirex has not properly understood how plutonium would behave in the waterlogged repository - a matter directly relevant to his own expertise.
He warns that underground water moving through the repository could carry microscopic particles of plutonium with it which would then concentrate in cracks and rock fissures until a critical mass had accumulated which would trigger a nuclear chain reaction. 'There are hundreds of kilograms of insoluble plutonium oxide in the waste - that's my worry,' he said. 'You only need half a kilogram of plutonium in water to go critical.'
The plutonium could not explode like a nuclear bomb, Dr Ockenden emphasised, but could become a miniature version of a Sizewell pressurised water reactor. 'You could get pretty uncontrollable criticality excursions. There would be a nasty release of radiation, heat, steam, and pressure down
Sir John Knill, chairman of the Government's expert Radioactive Waste Management Advisory Committee, said that it would discuss the issue during its next meeting with UK Nirex on 10 February. Sir John said: 'The risk would be minimised by means of the distribution of different waste forms through the bulk of the repository.' However, other experts pointed out that if, as is the case for Sellafield, water flowed out of the repository area through specific fractures, the risks of plutonium concentration would be greater.
Dr John Holmes, Nirex's director of science, said the company recognised the significance of the issue: 'We have looked for mechanisms to mobilise plutonium and then reconcentrate it in the required quantity and we can't find a problem.' However, he accepted that 'we've got to think hard to ensure that we've covered everything'.
Dr Ockenden emphasised that, after spending a lifetime in the nuclear industry, he is not anti-nuclear nor does he oppose the concept of deep underground disposal of nuclear waste. But he does question the suitability of Nirex's proposed site because it is 'wet' and because there is a head of water from the hills of the Lake District driving flow through the repository area.
Had Nirex gone for a 'dry' site, such as the salt domes which continental countries are exploring, then the possibility of a nuclear chain reaction simply could not arise, he pointed out. But there are no salt domes big enough in Britain, and Nirex has been refused access to another suitable dry site, at Billingham on Teesside.
Nirex will therefore have to rely on computer models to prove that Dr Ockenden's scenario cannot happen. But Dr Ockenden said, 'I do worry about fairly academic guys, a long way from Sellafield, putting assumptions into computer programs and coming up with answers that are not right. I am disturbed by the replies that I have got from Dr Holmes. Originally, he said this was not possible. Then he said they were looking at more complex models. I am not a computer man, but some of their assumptions have been pretty far out.'
Dr Holmes said the company was engaged in sophisticated computer modelling which will continue for several years, until it is satisfied it can put in a full planning application in 1998 or 1999.
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