Coupled hydro-thermo-mechanical analysis of a deep radioactive waste disposal based on porous media mechanics

Aim of this work is the coupled hydro-thermo-mechanical analysis of a deep radioactive waste disposal using a finite element formulation for thermo-elasto-plastic multiphase geomaterials. The finite element model (COMES-GEO fem code) is based on Porous Media Mechanics where the ACMEG-T thermo-elasto-plastic constitutive model for saturated soils has been implemented. The multiphase material is considered as made of a solid skeleton and open pores, which are filled with water or water and gas. In particular the multiphase media is modelled as a deforming porous continuum where heat, water and gas flow are taken into account]. The gas phase is modelled as an ideal gas composed of dry air and water vapour. Phase changes of water (evaporation-condensation, adsorption-desorption) and heat transfer through conduction and convection, as well as latent heat transfer are considered. Two cases are analysed. Case 1 simulates the normal life condition with the maximum temperature in the canister of 95°C. This computation reveals a temperature bulb surrounding the vertical well initially, that diffuses toward the boundaries and then contracts because of the reduction of the temperature in the well. The volumetric strain distribution is characterized by an initial dilatation near to the well with a consequent compression near the boundaries. The volumetric plastic strain distribution reveals a very small damaged zone surrounding the well. In case 2, a failure condition is simulated, with a maximum temperature of 320°C describing the collapse of the container of the waste. In this case, a huge damaged zone surrounding the well is described (with respect to the normal life conditions) which become partially saturated because of formation of water vapour.