Effect of polymeric cement additives on the mobility of radionuclides under near-field conditions of a final radwaste repository
Cement or concrete will be used as backfill, liner and building material in a final repository. Besides corrosion products of spent fuel containers, cementitious materials are the first chemical retention barrier for radionuclides in case of their release after water influx. At the same time, cement is exposed to leaching processes and generates highly alkaline solutions (up to pH 13.5). This high pH will also influence the barrier functions of buffers (e.g., bentonites) and host formation, in particular clay rock. Pore waters of clay are characterised by very high electrolyte contents (up to 4 M). Adsorption of actinides / lanthanides in these systems is studied in the joint project GRaZ (“Geochemical Retention of Radionuclides at Cement Alteration Phases”), funded by the German Federal Ministry for Economic Affairs and Energy (BMWi).
As part of this project, the work packages performed by our group are focused on the mobilising potential of polymeric additives contained in cement; so-called superplasticisers, which may be set free by leaching. Their complexing properties could keep radiometals in solution, facilitating their migration. For the example of polycarboxylate ethers as new generation of superplasticisers, conditions of a mobilisation will be identified in systematic investigations of complexation and adsorption in dependence on pH and electrolyte content. The effects of these parameters shall be explained by modelling on the basis of interactions in the constituent subsystems. Taking advantage of the unique analytical benefits of radioactive tracers is the central idea of our experimental work. 14C-labelled polycarboxylate ethers will be synthesised, providing access to very low concentration levels. 152Eu is employed as a tracer analogue of actinides.