The role of superplasticizers and their degradation products on radionuclide mobility
- Autores: David Manuel García Cobos
- Directores de la Tesis: Mireia Grivé Solé (dir. tes.), Lara Duro Pérez (codir. tes.), Joan de Pablo Ribas (tut. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Xavier Gaona Martínez (presid.), Vicenç Martí (secret.), Mónica Felipe Sotelo (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería de Procesos Químicos por la Universidad Politécnica de Catalunya
- Materias:
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- Resumen
The most widely accepted option for the long-term management of radioactive waste materials considers their disposal in deep underground facilities (examples are Finland, Belgium, France, Sweden, etc.). Such installations should ensure the safety of the waste disposal and minimize possible future external waste perturbation. Concrete and concrete-based materials are widespread in those facilities. Superplasticizers (SPs) are organic polymers used to improve several concrete properties such as workability. The hyperalkaline conditions developed in cementitious environments, can cause chemical transformations of these polymeric materials (degradation, aging, etc.) with the subsequent production of new organic compounds with different chemical properties. Several organic compounds have a high capacity to form stable complexes with some radionuclides of interest from the point of view of radiological and toxicological doses. Therefore, the understanding on the nature and strength of the interactions between radionuclides and organic admixtures present in the concrete formulations (and their degradation products) is of the outmost importance to conduct adequate assessments of the future performance of the disposal facility.
Overall the work presented in this thesis tries to discern the effect that polycarboxylic ether-based (PCE) SPs present in the concrete admixtures used in the deep disposal repository may have on radionuclide mobility. To this aim, the stability of PCE towards temperature, radiolysis and hydrolysis degradation processes has been also studied. This provides a test of the integrity of SPs under the expected repository conditions.
This work presents a state-of-the-art of the degradation process of SPs and a thermodynamic study on the effect that model compounds, considered as proxy SPs degradation products, have on the chemistry of several radionuclides (Ni, Eu, U). Short-chain organic compounds such as acetate, phthalate, oxalate, phenol, urea, etc., have been confirmed as possible SPs degradation products. Our results indicate that the complexation capacity of the proxy SPs degradation products considered (i.e. acetate, phthalate, phenol and urea) towards Ni Eu and U is almost negligible under alkaline conditions, while relatively important in the near-neutral pH range. In parallel, the experimental degradation of two PCE SPs, one commercial and one synthetic, has been investigated in this work. Our results indicate that the studied SPs remain unaltered when exposed to hydrolysis processes. On another hand, high temperatures or radiation doses cause important changes in the SPs structure, although the main chemical groups remain unaltered.
The effect of a PCE SPs, Glenium®27, on nickel hydroxide solubility has been experimentally evaluated. The results indicate that when Glenium®27 is added to water and then mixed with cement, this polymeric material is stabilized (e.g. adsorbed into the cement phases) and not released back to the aqueous solution, with negligible effects on the mobility of nickel. On the contrary, when added directly into synthetic cement porewater samples at high dosages the solubility of nickel hydroxide increases in more than two orders of magnitude. Thermodynamic calculations indicate that the effect of such component on Ni is comparable to the effect exerted by other organic compounds (i.e. isosaccharinate, gluconate, oxalate), thus pointing towards these organics as good surrogates for understanding SPs complexation capacity.
