Deep clay formations are promising sites for radioactive waste disposal. Clay has properties of particular interest in containing and isolating such waste including low permeability, slow diffusive transport and sealing capacity. While clay is a promising medium for containment, creating the disposal sites by opening large holes can damage the surrounding rock thereby compromising those very properties. In order to assess the long-term safety of such sites, European researchers initiated the ‘Thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks’ (Timodaz) project. The excavation-damaged zone (EDZ) is a region with significant modifications in hydromechanical and geochemical properties (thermo, hydro, mechanical and chemical properties, THMC) creating flow, transport and permeability changes. Scientists sought to characterise the effects of heat-generating radioactive waste, waste that produces a tremendous amount of heat as it decays, on the evolution of the EDZ. Numerical tools were developed to analyse the influence of temperature increase and its distribution on THMC in three different types of clay. The tools complemented experimental work including laboratory and field tests. Timodaz results provided important insight allowing assessment of the thermal damaged zone in relation to safety of clay repositories for nuclear waste. Safer disposal methods could help turn the tide when it comes to nuclear power, giving additional substance to the argument for clean energy.