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The role of geotechnical engineering for nuclear energy utilisation

ECSMGE Special lecture

Professor Antonio Gens graduated in civil engineering from the Technical University of Madrid in 1972 and followed an MSc in soil mechanics from Imperial College London in 1973 with a PhD in 1982.

Gens was appointed professor of geotechnical engineering at the Technical University of Catalunya in Barcelona in 1988 and in 1999 became head of the geotechnical engineering department.

He has over 20 years experience in geotechnical research, consulting and education. Research has mainly concentrated on numerical analysis, constitutive modelling of soils, study of soil behaviour by laboratory testing and behaviour of partially saturated soils.

He has also consulted widely in projects involving deep excavations, tunnels, diaphragm walls, ground improvement techniques, dams, power stations, foundations, slope stability and site investigations.

An adequate consideration ofthe issues associated with nuclear energy requires examination of the entire nuclear fuel cycle, from uranium mining through nuclear power generation to radioactive waste disposal (Figure 1).

Geotechnical engineering has an important role, sometimes a crucial one, in many phases of the cycle. This shares many features with similar applications in conventional areas ofengineering, but involves additional aspects seldom found elsewhere.

Because ofpublic concerns, construction ofnuclear power plants has reduced dramatically in the past decade; only 33 are under construction, ofwhich 10 are in Russia and eastern Europe and none in western Europe.

Geotechnical activity is therefore limited, but given the importance ofthese facilities, on occasion quite challenging geotechnical problems arise ifthe fundation behaviour does not correspond to that expected. In particular, possible long term differential settlements are critical to facilities' performance, as they may affect safety and emergency systems.

Remediation ofuranium mine sites and associated facilities is another area where geotechnics plays a significant role.

Worldwide production ofuranium mine tailings, where most of the hazardous materials end up, exceeds 20Mt annually. It is therefore necessary that environmental and health risks (including possible radon emission) from these materials are reduced to acceptable levels. Such large volumes generall require that tailings be treated insitu.

It should be noted that many of the most severe problems arise from older mining activities when regulations were not as strict. At a uranium tailings dyke in Andújar, Spain, remedial measures consisted ofslope profe changes to increase the factor of safety and the provision of a sophisticated insulating cover.

However, it is in the area of radioactive waste management (especially high-level waste disposal) where the special characteristics ofgeotechnics applications become more apparent.

This is an area ofintense activity, especially concerning the possible implementation ofa deep geological repository for nuclear waste. Although this does not require special expertise and techniques beyond those ofconventional underground construction, the final objective of the facility imposes additional constraints.

Clear examples are the need to minimise the 'excavation disturbed zone' around the various openings or the preference for non-intrusive reconnaissance methods. The fact that the waste is heat emitting is another obvious departure from more standard conditions.

Assessment ofdeep repository performance involves detailed examination ofmany processes and phenomena affecting the transport ofradionuclides fom the waste canister to the biosphere. Many ofthose processes have very significant geotechnical content and have a number of distinctive features:

lPredictions are required for times well beyond any possible human experience. Rates of radioactive decay are a very important parameter;

lNew variables such as high suction, temperature, and geochemical environment must be incorporated into a new generalised behaviour framework for the understanding ofmulti-phase geotechnical materials lSets ofphenomena are fequently coupled with each other and cannot be studied separately.

Although the degree ofinteraction depends on the problem being examined, it is normally essential to adopt generalised coupled approaches.

lThe range ofgeotechnical scales affecting repository performance is huge; from microstructural features (eg the microfabric ofbentonite barriers) to geological macro scale (eg fracture pattern on a regional scale).

Because it is necessary to go well beyond past experience, resorting to empiricism has very limited value. A significant degree ofuncertainty must be accepted and assessment made on sound physical basis.

This requires research in a number ofareas, combining insitu investigation, laboratory work and modelling. The existence of underground research laboratories allows integration and application ofresearch to conditions similar to the real situation.

Geotechnical engineering is ideally positioned to provide effective responses to nuclear energy's demanding challenges.

References Bossart P, Meier PM, Moeri A, Trick T and Mayor JC (2002). Geological and hydraulic characterisation ofthe excavation disturbed zone in the Opalinus clay ofthe Mont Terri Rock Laboratory. Engineering Geology, 66, 19-38.

Chapman NA and McKinley IG (1987). The geological disposal ofnuclear waste. John Wiley: Chichester.

Svemar C (2001). Äspö HRL-insitu programme and prototype repository. 2nd CLUSTER URL's Seminar, Mol, European Commission, EUR 19954, 57- 67.

Thury M and Bossart P (1999). The Mont Terri rock laboratory, a new international research project in a Mesozoic shale formation in Switzerland. Engineering Geology, 52, 347-359.

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