Cemented waste deterioration by sulphate attack: chemistry and effect on structural and mechanical properties

Promoter

De Belie Nele, (Universiteit Gent (UGent)), nele.debelie@ugent.be

SCK•CEN Mentor

Valcke Elie, evalcke@sckcen.be, +32 (0)14 33 31 33

Expert group

R&D Waste Packages

SCK•CEN Co-mentor

Seetharam Suresh , sseethar@sckcen.be , +32 (0)14 33 32 08

NIRAS/ONDRAF Mentor

Seif Ben Hadj Hassine

Short project description

For decades, the nuclear industry has used cement-based materials to immobilise low- and intermediate-level short-lived and long-lived radioactive waste (precipitation sludge, concentrates, resins, … ) originating from the operation of nuclear power plants and from the chemical reprocessing of spent fuel (IAEA, 2013). Whereas because of the simplicity of the technique one could expect that this practice would result in a stable waste form that is well compatible with the final disposal site, there are many examples that show that this is not always the case, e.g. the gel-formation attributed to alkali-silica-reaction (ASR) in the concrete operational waste of the nuclear power plant of Doel (ONDRAF/NIRAS, 2014).

In fact, there are several possible processes that result in the deterioration of cement-based materials: Ca2+ leaching (decreasing the Ca/Si ratio in the CSH), carbonation (generating CaCO3), sulphate attack (formation of expanding secondary minerals such as ettringite, gypsum and thaumasite), and the alkali-silica reaction (ASR; formation of an alkali-silicate gel). Many of these reactions result in the formation of expanding materials and/or other processes that cause volumetric deformation and fissures by internal stresses (Neville, 1996; Brunetaud et al., 2008; Pabalan et al., 2009; Wang, 2009; Bouzabata et al., 2012; Phung, 2013). Besides these processes, the mixing of large quantities of salt (e.g. NaNO3, Na2SO4, Na3BO3, … ) with cement, mortar or concrete additionally results in complications in terms of curing time and mechanical stability of the waste (e.g. IAEA, 2013), and results in the formation of other types of (Na-bearing) minerals.

Objectives

With this PhD we want to improve our mechanistic and phenomenological understanding of the chemistry of delayed ettringite formation (DEF) and of its effect on the mineralogical, structural and mechanical properties of the waste.

To reach these objectives, the study would be divided in four parts.

  1. Literature survey and identification of the degradation mechanisms of DEF and of the existing standard tests for detection of the susceptibility to DEF of a cement-based mixture
  2. Adaptation of the existing SCK•CEN test cells (used for the study of bituminised waste) to measure the expansion (in constant stress conditions) or pressure (in constant volume conditions) for a degrading cemented waste form

  3. Conduction of tests to study the chemistry of (only) the delayed ettringite formation process and the effects on mineralogical, structural and mechanical properties of the waste. Envisaged characterisation techniques are XRD, SEM-EDX, thin layer petrography, SIMS, MAS-NMR, BET, MIP, … Other analysis techniques may appear to be necessary as the project proceeds, and some will have to be subcontracted to or to be performed in collaboration with non-SCK•CEN groups (CEA, PSI, … ). The characterisation techniques should be applicable as much as possible to radioactive samples.

  4. Conduction of tests in the newly developed test cells (under part 2) to measure the expansion or the pressure; at regular times: analyses as described under part 3.

References

  • Bouzabata, H., Multon, S., Sellier, A., Houari, H. (2012) Effects of restraint on expansion due to delayed ettringite formation. Cement and Concrete Research 42(7), 1024-1031.

  • Brunetaud, X., Divet, L., Damidot, D. (2008) Impact of unrestrained Delayed Ettringite Formation-induced expansion on concrete mechanical properties. Cement and Concrete Research 38(11), 1343-1348.

  • IAEA (2013) The behaviours of cementitious materials in long-term storage and disposal of radioactive waste. IAEA-Tecdoc 1701. IAEA, Vienna, Austria.

  • Neville, A.M. (1996) Properties of concrete. New York City, New York: John Wiley & Sons.

  • ONDRAF/NIRAS (2014) Actieplan voor veilig beheer van vaten met gelvorming. http://www.niras.be/sites/default/files/Persdossier_actieplan_20140926_0.pdf.

  • Pabalan, R.T., Glasser, F.P., Pickett, D.A., Walter, G.R., Biswas, S., Juckett, M.R., Sabido, L.M., and Myers, J.L. (2009) Review of literature and assessment of factors relevant to performance of grouted systems for radioactive waste disposal. Report CNWRA 2009-001 (Contract NRC NRC-02-07-006). Center for Nuclear Waste Regulatory Analyses, San Antonio, Texas, United States of America.

  • Phung, Q.T., Maes, N., De Schutter, G., Jacques, D., and Ye, G. (2013) A methodology to study carbonation of cement paste and its effect on permeability. In '4th International Conference on Accelerated Carbonation for Environmental and Materials Engineering (ACEME-2013)', 459-463.

  • Wang, L. (2009) Near-field chemistry of a HLW/SF repository in Boom Clay. Scoping calculations relevant to the supercontainer design. SCK•CEN-ER-17, Mol, Belgium.

 

The minimum diploma level of the candidate needs to be

Master of sciences in engineering

The candidate needs to have a background in

Bio-engineering , Physics , Mathematics , Chemistry , Mechanica
Before applying, please consult the guidelines for application for PhD.