Ice ages and nuclear waste isolation

The greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets. The next major glaci al retreat is expected at ca 70 ka, well within the lifetime of high grade nuclear waste, but it is not yet clear how long man's greenhouse effect may delay it. This contribution discusses the potential problems posed to European waste isolation sites during erosion by ice and over-pressurizing of meltwater an d gasses in a lithosphere flexed by major ice sheets. These depend on the t arget rocks and the location of the site with respect to the ice-streams an d margins of future ice sheets of particular size. No sites are planned under the centres of future ice sheets in Europe where end-glacial earthquakes can be expected to reactivate major faults, nor wh ere ice can be expected to deepen and lengthen fjords along the Atlantic co ast. Sites in the Alps may be vulnerable to radical changes in the patterns of glacial troughs. The stability and geohydrology of sites in coastal are as beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or, in enclosed basins like the Mediterranean, ever lower. T he greatest problems are likely in lowland regions exposed by the rapid ret reat of thick ice fronts where large lakes on or under thick warm-based ice are dammed by more distal cold-based ice. Groundwater in subhorizontal fra ctures dilated by glacial unloading may reach over-pressures capable of hyd raulically lifting megablocks of bedrock with fracture permeability and-or the ice damming them so that less permeable substrates are susceptible to i ncisions eroded to depths of ca 360 at locations controlled mainly by ice t opography, kinematics and history. (C) 1999 Elsevier Science B.V. All right s reserved.