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For a final disposal of nuclear waste different options are envisaged (https://world-nuclear.org , https://www.iaea.org,…). Waste of low and intermediate activity and containing short lived radionuclides are planned to be stored in surface or near-surface disposal facility. Waste of high activity and/or containing long-lived radionuclides are planned to be disposed of in deep geological repositories.
Cementitious materials are ubiquitous in the current designs of both surface and geological disposals of radioactive waste. In surface disposal, they play the dual role of structural support and waste immobilization matrix and they ensure the safety function of radionuclide retardation. In the concept of geological disposal, their main function is structural reinforcement but they could also, to some extent, contribute to the retention of radionuclides.
Calcium-silicate-hydrate (C-S-H) and calcium-aluminate-silicate-hydrate (C-A-S-H) gels are the most important hydration products of cement (Olmeda et al., 2019; L’Hôpital et al., 2015; Lothenbach et al., 2011; García Lodeiro et al., 2010; Sun et al., 2006) and also the main sorbing phases with respect to radionuclides (Olmeda et al., 2017; Li and Pang, 2014; Atkins and Glasser, 1992; Evans, 2008; Hong and Glasser, 1999). During the disposal period, the C-S-H and C-A-S-H phases will undergo gradual degradation inducing physico-chemical changes notably due to interaction with (underground or surface) water. The change in their composition (different Ca/Si ratio), surface charge and porewater chemistry is expected to modify the extent and the mechanisms of radionuclide retention. In that context, understanding the radionuclide retention mechanisms and linking it to the physico-chemical properties of both C-S-H and C-A-S-H along degradation is essential to assess and predict the role of cementitious materials in retarding the migration of radionuclides in a nuclear waste repository.
Several studies were already undertaken to assess the potential of C-S-H to adsorb radionuclides, also in the course of EU project such as CEBAMA (Grambow et al., 2020). On the contrary only few relevant studies were focused on the sorption properties/capacity of the C-A-S-H phases (Olmeda et al. (2019) and references therein), though some new researches are on-going within the EC project EURAD/CORI. For both C-S-H and C-A-S-H phases, the difficulty of such an assessment is to ensure its representativeness of natural/real conditions. The degradation of C-S-H and C-A-S-H phases leads to their decalcification and dissolution but also to potential carbonation in case of surface disposal. Most of the studies use synthetized C-S-H/C-A-S-H with various Ca/Si and Al/Si ratio to represent the variability of the cements and the different degradation states. It is however not clear how representative these synthetized C-S-H/C-A-S-H are for the natural degradation occurring in contact with (underground)water.
This PhD will be the follow-up of a currently on-going PhD in SCK CEN on the synthesis and degradation routes for C-S-H to evaluate how, at a same Ca/Si ratio, the properties of synthesized C-S-H compare with degraded C-S-H. It will complete and extend the study to get deeper into the understanding of radionuclide sorption on cementitious phases by linking the physico-chemical properties of the main sorbing phases, C-S-H and C-A-S-H to their retention capacity at different degradation states with a special focus on the representativeness of the experiments and the involved mechanisms. A set of radionuclides will be investigated as representative of the following categories: anionic species (ex: Se, Tc, I), actinides/lanthanides (ex: Pu, Am/Eu, Np) and divalent cations (ex: Ni, Sr, Ba/Ra). The objectives of this PhD are as follows:
- to investigate the effect of synthesis and degradation routes on the physico-chemical and radionuclide retention properties of C-A-S-H phases and identifying the route(s) which are considered as the most representative of the real system;
- to identify the sorption mechanisms of relevant radionuclides onto C-S-H/C-A-S-H in blended cement along the degradation path (e.g. leaching and carbonation);
- to develop a conceptual model for radionuclide sorption onto C-S-H and C-A-S-H phases in blended cement along the degradation path (e.g. leaching and carbonation).
The PhD would combine laboratory experiments to quantify the sorption, solid and solution characterization performed on-site or via collaboration (SEM/EDX, XRD, XRF, (LA-)ICP-MS, IC, synchrotron-based techniques…) to identify the mechanisms and geochemical modelling to describe the obtained results and allow further prediction of radionuclide retention on cement.
The PhD student will take part in the research on the high impact topic of nuclear waste disposal. He/she will be integrated in an internationally-recognized team of experts on cement chemistry and on radionuclide sorption and transport in the context of nuclear waste disposal who over the years engaged in several PhD topics (Bruneel, 2021; Phung 2015; Delecaut, 2004) and international (EC) projects (EURAD/CORI, CEBAMA, CATCLAY, FUNMIG, NF-PRO, TRANCOM-Clay/II, Ecoclay 1&2…) related to radionuclide retention and cement chemistry. The student will have the opportunity to combine experimental and modelling work and to access a variety of state-of–the art analytical techniques.
The minimum diploma level of the candidate needs to be
- Master of sciences
- Master of sciences in engineering
The candidate needs to have a background in
- Chemistry
Estimated duration
4 years
References
Atkins, M. and Glasser, F.P. (1992) Application of Portland Cement-Based Materials to Radioactive Waste Immobilization. Waste Management 12, 105-131.
Bruneel, Y. (2021) Sorption of radionuclides on glauconite from the Neogene. PhD thesis, KUL, Belgium
Delecaut, G. (2004) The geochemical behaviour of uranium in the Boom clay. PhD thesis, UCL, Belgium
Evans, N.D.M. (2008) Binding Mechanisms of Radionuclides to Cement. Cement and Concrete Research 38, 543-553.
García Lodeiro, I., Fernández-Jimenez, A., Palomo, A. and Macphee, D.E. (2010) Effect on Fresh C-S-H Gels of the Simultaneous Addition of Alkali and Aluminium. Cement and Concrete Research 40, 27-32.
Grambow, B., López-García, M., Olmeda, J., Grivé, M., Marty, N.C.M., Grangeon, S., Claret, F., Lange, S., Deissmann, G., Klinkenberg, M., Bosbach, D., Bucur, C., Florea, I., Dobrin, R., Isaacs, M., Read, D., Kittnerová, J., Drtinová, B., Vopálka, D., Cevirim-Papaioannou, N., Ait-Mouheb, N., Gaona, X., Altmaier, M., Nedyalkova, L., Lothenbach, B., Tits, J., Landesman, C., Rasamimanana, S. and Ribet, S. (2020) Retention and Diffusion of Radioactive and Toxic Species on Cementitious Systems: Main Outcome of the Cebama Project. Applied Geochemistry 112, 104480.
Hong, S.-Y. and Glasser, F.P. (1999) Alkali Binding in Cement Pastes. Cement and Concrete Research 29, 1893-1903.
L’Hôpital, E., Lothenbach, B., Le Saout, G., Kulik, D. and Scrivener, K. (2015) Incorporation of Aluminium in Calcium-Silicate-Hydrates. Cement and Concrete Research 75, 91-103.
Li, K. and Pang, X. (2014) Sorption of Radionuclides by Cement-Based Barrier Materials. Cement and Concrete Research 65, 52-57.
Lothenbach, B., Scrivener, K. and Hooton, R.D. (2011) Supplementary Cementitious Materials. Cement and Concrete Research 41, 1244-1256.
Olmeda, J., Henocq, P., Giffaut, E. and Grivé, M. (2017) Modelling of Chemical Degradation of Blended Cement-Based Materials by Leaching Cycles with Callovo-Oxfordian Porewater. Physics and Chemistry of the Earth, Parts A/B/C 99, 110-120.
Olmeda, J., Missana, T., Grandia, F., Grivé, M., García-Gutiérrez, M., Mingarro, M., Alonso, U., Colàs, E., Henocq, P., Munier, I. and Robinet, J.C. (2019) Radium Retention by Blended Cement Pastes and Pure Phases (C-S-H and C-a-S-H Gels): Experimental Assessment and Modelling Exercises. Applied Geochemistry 105, 45-54.
Papadokostaki, K.G. and Savidou, A. (2009) Study of Leaching Mechanisms of Caesium Ions Incorporated in Ordinary Portland Cement. Journal of Hazardous Materials 171, 1024-1031.
Phung, Q.T, Effects of carbonation and calcium leaching on microstructure and transport properties of cement pastes. PhD thesis, UCGent, Belgium
Pointeau, I., Landesman, C., Giffaut, E. and Reiller, P. (2004) Reproducibility of the Uptake of U(Vi) onto Degraded Cement Pastes and Calcium Silicate Hydrate Phases, Radiochimica Acta, p. 645.
Sun, G.K., Young, J.F. and Kirkpatrick, R.J. (2006) The Role of Al in C–S–H: Nmr, Xrd, and Compositional Results for Precipitated Samples. Cement and Concrete Research 36, 18-29.
Tits, J., Iijima, K., Wieland, E. and Kamei, G. (2006) The Uptake of Radium by Calcium Silicate Hydrates and Hardened Cement Paste. Radiochimica Acta 94, 637-643.
