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ROCK AROUND THE CLOCK

David Holmes and Richard Shaw report on the progress of research into disposal of the UK's radioactive waste.

In 1997 the Secretary of State for the Environment accepted the fi ndings of an earlier public inquiry and rejected nuclear waste consultant Nirex's application for an underground rock characterisation facility near Sellafield.

This halted a process, started in 1986, to locate an underground repository to dispose of certain radioactive wastes. The era of secrecy and decisions by 'experts in smoke-filled rooms' was over.

Government began an open and transparent process to review radioactive waste management in the UK, called Managing radioactive waste safely, and more recently made Nirex independent of the nuclear industry.

The government established the Committee on Radioactive Waste Management (CoRWM) in 2003 to examine all the options. This committee is seeking expert opinion on four possible options: deep geological disposal, phased geological disposal, shallow disposal of shortlived waste and long-term interim storage on or just below the surface.

All these rely on the local geology, to some extent, to provide stability and containment should material leak from the engineered facility designed to isolate the waste. At a depth of several hundred metres or more geology can isolate waste in a stable environment for the 100,000 years that it remains potentially harmful to the biosphere. It can provide long fl ow paths for slow-moving groundwater.

CoRWM is looking at high-level waste as well as the low and intermediate level waste considered by Nirex in 1986. Spent nuclear fuel and separated uranium and plutonium may also be included. As a consequence, the rock surrounding any buried facility must be able to cope with both heat and gas generation. Not all rocks are suitable, but many are.

Recent thinking has highlighted the importance of being able to recover the waste for 300 years or more after emplacement in any facility. Retrievability, as this is called, requires that engineered cavities underground remain stable for long periods. Any underground facility would include tunnels, vaults and silos for storage and access routes and the rock must be able to accommodate these.

The dimensions of these facilities can be varied depending on rock properties. However, there are constraints determined by previously agreed waste package sizes that set lower limits on cavity dimensions.

Not all rocks can maintain larger cavities for long enough.

Maintaining accessible cavities will result to some extent in the rocks around any facility draining of groundwater, reduced hydraulic pressures and producing flow towards the openings. Over several hundred years this could disturb the natural balance of oxidising and reducing waters and water/rock interactions with the potential for mineralogical changes in pores and the linings of fractures.

This would be in addition to any disturbance of the rock-mass, produced during construction and as the facility 'ages'. Near-fi eld engineering (grouting, rock bolting, and lining) may be needed to maintain rock integrity. All these factors are under active research in facilities located in many different rock types around the world. Such research indicates that any problems can be solved, but the practicality of such solutions needs to be confi rmed in the UK geological setting.

The geochemistry of rocks and the way it interacts with groundwater can retard the migration of any material that escapes from a surface or underground facility.

Some rocks have a greater capacity to retard than others. Much more is known about the processes involved than was the case 10 years ago.

Climate change and extreme events such as fl ooding, glacial and periglacial action and tsunamis are better understood. Sea level change may require that access to any future facilities is away from the coast or at a reasonable elevation.

Glaciation could recur in the UK over the next 100,000 years. Looking back over a similar period, the climate has been temperate, as now, for perhaps only 15% of the time; the rest has been cold or very cold.

Surface facilities are more vulnerable to surface events but at depth the geology mitigates any impact.

In the last decade, although the geology has changed very little, understanding of the processes of groundwater movement and water/rock interaction have greatly improved. Our understanding of the three-dimensional geology has increased.

There is now much greater computing power to model threedimensional geology and geological processes and to present the information in a form the human mind can better appreciate.

Considerable experience and information is also available from studies around the world, including such countries as Finland, Sweden, France, USA, Japan and Switzerland. This encompasses work in underground and surface research facilities associated with many geological settings.

International research is proving that various rock types such as hard crystalline, clays, mudrocks and evaporites are suitable hosts for repositories. Geotechnical engineering and geo science studies in general have advanced through activities in such facilities so that there is both a good understanding and demonstration that these rock types are capable of assuring long-term containment of radioactive wastes.

CoRWM reports in summer 2006 to recommend an option (or options) for the continued safe management of radio ctive wastes. The government will then decide how to implement these recommendations.

David Holmes is director of environment and hazards and Richard Shaw is principal geologist at the British Geological Survey.

Large scale gas injection test 420m below the surface in a gallery of the -sp÷ Hard Rock Laboratory in Sweden (Photo J Harrington, BGS, ®NERC 2005. )

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