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Metamorphism could offer radioactive waste solution

MAN-MADE METAMORPHISM could solve the problem of disposing of high level radioactive waste, according to research by the University of Sheffield.

Work by geologist Dr Fergus Gibb at the Department of Earth Sciences suggests that if specific concentrations and volumes of waste are stored in a specially designed container placed deep in the earth's crust, peak decay temperatures of up to 900 degrees C should be generated.

If maintained for long enough, this should induce a zone of partial melting in the rock immediately surrounding the canister. Once temperature and melt volume have peaked, the rock will cool and recrystallise, sealing the waste in solid rock 'sarcophagus'.

Research, A new scheme for the very deep geological disposal of high-level radioactive waste , was published in the January issue of the Journal of the Geological Society and was originally funded by British Nuclear Fuels.

Waste would have to be placed at depths of between 4km and 5km, in massive granite or granitic basement rock, with at least 0.5km of similar rock below the bottom of the large diameter borehole.

With drilling costs estimated at about £1M/km, it is likely that a number of canisters would be stored, placed in a single borehole separated by rock 'spacers', in an array of multiple boreholes or in splayed boreholes from a single well-head.

Once placed, the boreholes would be backfilled with crushed granite and sealed.

Surrounding the waste's granite tomb would be a wider, thermally metamorphosed zone where sub-solidus recrystallisation and annealing would seal any fractures in the rock (pre-existing or caused by thermal expansion during heating). This geological barricade would be 'substantial, and for some time selfregenerating and self-sealing, long-lived, multiple barrier to the escape of radionuclides into any hydrological system which might exist at such depths.'

Even if leakage did occur, Gibb believes that any water in the rock would not have moved for millions of years and would not return to the surface, as long as the borehole was adequately backfilled and sealed. Depth of containment also reduces the risk of damage by earthquakes.

Gibb points out that the scheme is only some way past the concept stage and more research is needed to establish full scientific feasibility. Further work is needed on the recrystallisation, metamorphism and fracturefilling processes in granitic rocks, modelling of heat flow and hydrogeology and development of suitable containers and borehole sealing techniques, he says.

But he believes the method is a 'potentially safer and environmentally robust solution to the problem of high level waste'. It is suitable for any form of solid high level waste, he says, as long as it is capable of providing the necessary heat.And because rock is much more uniform at these depths the number of potential sites increases.

'The scheme can be extended to accommodate almost any scale of disposal programme and lends itself particularly well to the idea of a small number of international disposal sites, 'he says.

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