Encapsulation of Mg–Zr alloy in metakaolin-based geopolymer

Investigations were carried out to propose a suitable material for the encapsulation of Mg–Zr alloy wastes issued from fuel cladding of the first generation nuclear reactors. Stability over time, good mechanical properties and low gas production are the main requirements that embedding matrices must comply with in order to be suitable for long run storage. One of the main issues encapsulating Mg–Zr alloy in mineral binder is the hydrogen production related to Mg–Zr alloys corrosion and water radiolysis process. In this context, metakaolin geopolymers offer an interesting outlook: corrosion densities of Mg–Zr alloys are significantly lower than in Portland cement. This work firstly presents the hydrogen production of Mg–Zr alloy embedded in geopolymers prepared from different the activation solution (NaOH or KOH). The effect of addition of fluorine on the magnesium corrosion in geopolymer has been investigated too. The results point out that sodium geopolymer is a suitable binder for Mg–Zr alloy encapsulation with respect to magnesium corrosion resistance. Furthermore the presence of fluorine reduces significantly the hydrogen release. Then, the impact of fluorine on the geopolymer network formation was studied by rheological, calorimetric and 19F NMR measurements. No direct effect resulting from the addition of fluorine has been shown on the geopolymer binder. Secondly, the formulation of the encapsulation matrix has been adjusted to fulfil the expected physical and mechanical properties. Observations, dimensional evolutions and compressive strengths demonstrated that addition of sand to the geopolymer binder is efficient to meet the storage criteria. Consequently, a matrix formulation compatible with Mg–Zr alloy encapsulation has been proposed. Finally, irradiation tests have been carried out to assess the hydrogen radiolytic yield of the matrix under exposure to γ radiation.