NUCLEAR WASTE DISPOSAL IN CLAYS
Most of the recent experience with thermal effects on soils at elevated temperatures (50-150 ° C) has been gained in the course of development of technologies of deep disposal of High Level Radioactive Waste (HLW) in clay formations (in Belgium, Italy, Spain, UK, France and Switzerland). In the commonly accepted concept of multiple barrier, consisting of waste package, engineered barrier of backfill and buffer, and clay host formation, the former barriers are destined to fail, releasing the radioactive contaminant within 300 yr. Thus, the ultimate containment barrier is the indigenous clay formation itself. Regulations in most countries require proof from the designers of repositories that no radionuclide can reach drinking water before 10,000 years. The time needed for nuclear decay of the most lasting contaminants is about 25, 000ys.
On the other hand, HLW, consisting mainly of spent fuel, emits heat of variable power depending on the delay time with respect to the discharge from reactor, chosen to yield temperatures of 100 to 180 ° C at the contact of the waste package with soil. Three main types of repository designs are considered for clays, at depth between 200-600m: mined tunnel systems with waste emplacement either in spaced drill holes, or directly in tunnels, and borehole systems drilled directly from the surface. No actual repository is to be built before the 2020's.
A principal design factor in these technologies is the temperature developing within the soil mass. Temperature induces thermal expansion in the elastic range and thermal collapse of soils in the plastic behavior range. Through these phenomena and a substantial thermal expansion of pore water, temperature may lead to a significant increase of the pore water pressure during the heating phase, and possibly soil damage during cooling. Pore pressure gradient may substantially increase the natural water flow, enhancing a potential contaminant transport if an early breakdown of the inner engineered barrier occurs. Temperature and pore pressure gradients determine the spacing of tunnels and/or boreholes, which is a principal design parameter. Contaminant transport in clays is assumed to be mainly through advection with water. Therefore, the hydraulic conductivity of the clay mass is another critical factor for the design. The current design methodologies are based on temperature and flow predictions using linear heat diffusion law, and advection with Darcy’s law with constant hydraulic conductivity and possibly with dispersion law and mass decay.
Tomasz Hueckel has been active in this area since early 1980s, pioneering early experiments on thermo-mechanics of clays and developing an early version of thermo-plasticity model for soils.