Feasibility study on the immobilization of cesium- and strontium bearing wastes in alkali-activated matrices

In the ASOF project, the effects of partitioning and transmutation on the disposal of spent fuel was investigated. In WP1 of the project, methods were developed to separate a heat emitting Cs and Sr rich solution from a waste solution rich in other fission and activation products and smaller amounts of lanthanides and actinides. The rationale for the separation of the Cs and Sr rich waste stream is that it would allow a smaller disposal surface for the rest fraction. The third work package was dedicated to finding suitable matrices for the immobilization of the Cs and Sr waste stream. One of the potential matrices under investigation are alkali-activated materials, which this report is dedicated to. Alkali-activated materials are increasingly studied as an alternative to conventional cementitious materials as they offer certain advantages on aspects of leaching, carbonation and fire resistance.
The first achievement of this work was the development of reference recipes for alkali-activated materials based on two different precursors: metakaolin and ground granulated blast furnace slag. These reference recipes can be valorized in the future as the starting point for the immobilization of a wide range of solid and liquid waste streams. The use of two different precursors has the added benefit of allowing a large freedom in varying the chemical composition and water content of the matrix, dependent on the requirements for a specific waste stream.
To determine the optimal matrix for the immobilization of the cesium and strontium rich waste stream, a design-of-experiments was executed in which samples with variable waste loading and composition were prepared. After leaching tests samples based on metakaolin proved to offer the best resistance to the combined leaching of cesium and strontium. In these epxeriments, the waste loading was limited according to a scenario in which the heat generated by the waste form is not allowed to influence the host rock in a deep disposal facility. An ongoing doctoral study by Emile Mukiza within the ASOF project investigates higher waste loadings, but this is not discussed further in this report.
Finally, the durability of the immobilized metakaolin based waste form is investigated further. Accelerated carbonation tests revealed that carbonation did not significantly affect the physical properties of the material. Leaching tests in water confirmed that leaching of both cesium and strontium was limited. Accelerated leaching tests in aggressive solutions of ammonium nitrate resulted in higher leaching rates which were still within acceptable limits. These durability tests proved that it is possible to effectively immobilize a cesium and strontium containing liquid waste stream in an alkali-activated material while abiding by heat generation and durability aspects.