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Plutonium's journey to the centre of the Earth

Plutonium can be disposed of more safely by very deep disposal, some scientists believe. Emily Baldwin reports.

Geoscientist Online 15 October 2007

Excess plutonium (Pu) not destined for burning as a mixed oxide fuel or in reactors is both a long-term waste management problem and a potential security threat.

Much of the world’s Pu inventory of around 1800 tonnes exists in the form of spent fuel from which the Pu has not, or may never be, separated. Any unused Pu must be disposed of safely, but this can be a difficult and costly procedure. One of the primary concerns, particularly for mined repositories, is that groundwater will eventually gain access to the waste and leach out radionuclides, transferring them back to the biosphere before decay has rendered them harmless.

In a new study, Fergus Gibb, Kathleen Taylor (University of Sheffield) and Boris Burakov (V.G. Khlopin Radium Institute, St Petersburg) have suggested disposing the Pu in deep boreholes, situated up to 6km deep, where there is less dynamic hydrogeological activity, reducing the chances of radionuclides re-entering the biosphere. The great depth of burial also provides greater security against terrorist and accidental intervention. Fully cased and cemented boreholes 0.27m and larger are routinely sunk to over 6km in the geothermal energy industry at costs of around $8 million and commercial drilling rigs with this capability are currently available, making this a viable option for Pu disposal.

The authors propose using granite encapsulation, which eliminates the chances of aqueous leaching. The key to the scheme is the prior encapsulation of the Pu-bearing waste form in rock identical to the granitic host rock of the borehole. This could be achieved by disseminating small pieces of the Pu in crushed granite which is then partially melted and completely recrystallised under conditions of suitably slow cooling. Until recently it was thought that this could only occur by extremely slow natural cooling over hundreds or thousands of years but recent work by the authors has shown that this can be done in just a few months. Crucial to the proposed scheme is that during encapsulation the Pu does not dissolve in or react with the silicate melt and that there is no diffusion of Pu out of the waste form. A series of experiments were performed to investigate this and showed that no dissolution occurred, and sharp contacts remained between the materials even after a prolonged time period.

Following the manufacture of granite cylinders, they would be disposed of by insertion into fully cased boreholes sunk into granite to a depth of around 6km. Over time, intra-rock fluids such as dense saline brines present in the host granite will seep slowly into the borehole and invade spaces around the granite cylinders. The fluids would have equilibrated with the granite so will also be in thermodynamic equilibrium with the granite cases and will have no tendency for reaction or mineralogical alteration of the cylinders that might allow fluids access to the Pu bearing waste forms. A 6km deep borehole with waste cylinders deployed over the lowermost 2km would dispose of eight tonnes of Pu and keep it isolated from the immediate environment until the physical destruction of the continental crust by geological processes over a time period of millions - if not billions - of years.

Ref: Fergus G.F. Gibb, Kathleen J. Taylor and Boris E. Burakov 2007: Deep Borehole Disposal of Plutonium. Conference paper presented at the Materials Research Society’s Symposium on the Scientific Basis for Nuclear Waste Management, Sheffield, September 2007. Journal of Nuclear Materials, Proceedings of the Materials Research Society, both in press. f.gibb"at"