Design and characterization of fluid mortars for the conditioning of tritiated waste

This thesis aims at designing and characterizing fluid mortars for the immobilization of tritiated wastes which will be produced by the ITER experimental fusion facility. The goal is to incorporate a γ-MnO₂/Ag₂CO₃ oxide getter in the cement matrix. This getter will limit the outgassing of tritium from the cemented waste packages. Since the getter loses some of its efficiency when it is in a wet environment, the approach adopted in this work is to design binders that have a high chemical water demand, leading to significant desaturation of the pore network through the hydration reactions. Two binders are therefore proposed. The first one is a magnesium potassium phosphate cement composed of magnesium oxide (MgO) and potassium dihydrogen phosphate (KH₂PO₄) in equimolar proportions. The second one is a calcium sulfoaluminate cement, comprising 80 wt.% ye'elimite-rich clinker and 20 wt.% anhydrite, whose composition has been optimized through thermodynamic and kinetic approaches. Indeed, the results obtained show that the chemical composition of the binder is not the only factor to have an influence on the mineralogical assembly formed during hydration and on the properties of the material obtained. The dissolution rate of the calcium sulfate source also plays an important role: it affects the quantity of bound water to the hydrates and the volume stability of the material in the hardened state.The getter influences the hydration of the two binders in different ways: incorporated at 10 wt.% of the total mass of the material, it tends to accelerate the hydration of the magnesium potassium phosphate cement while delaying the hydration of the calcium sulfoaluminate cement. In both cases, a surface reactivity of MnO₂ is highlighted. A two-site species model is proposed to describe the ionization of the γ-MnO₂ surface as a function of pH. The partial or total destabilization of silver carbonate is also highlighted in relation to the chemical environment imposed by the matrix.The influence of the latter on the trapping of dihydrogen, used as a simulant of tritium, by the γ-MnO₂/Ag₂CO₃ getter is finally investigated. Some materials (pastes or mortars) incorporating 10 wt.% of getter or γ-MnO₂ (used as a reference because it has a similar particle size distribution to that of the getter, but without the trapping properties of H₂, HT or T₂) are designed from the two studied binders. To mimic the progressive release of gaseous tritium by the waste encapsulated in the matrix, two techniques are used:- the encapsulation of metallic aluminum, whose corrosion by the interstitial solution leads to the production of dihydrogen,- the external gamma irradiation of the material, which is accompanied by the production of dihydrogen caused by the radiolysis of water present in the cement matrix. The outgassing of dihydrogen is systematically strongly reduced in the presence of getter. (author)