Radionuclide geochemistry and Migration

One of the main aspects in evaluating the safety of a potential radioactive waste repository in a geological formation, is to understand the geochemical and physical processes that influence the mobility of the radionuclides in the geochemical environment imposed by the host rock (in Belgium the Boom clay is studied as a reference host rock) and to quantify these processes. This information is needed to make reliable predictions of the long-term safety.

Once radionuclides become released from the waste packages and the surrounding engineered barriers they can migrate by diffusion and advection through the host rock towards the biosphere. This transport occurs via the pore solution of the host rock and is function of the quantity of the dissolved radionuclides (RN) in the solution phase (= the mobile part). But when radionuclides come in contact with a solid matrix (for instance the clay host rock), they will redistribute between the solid and the aqueous phases.

The equilibrium distribution is determined essentially by two processes:

  • Recipitation-dissolution
  • Sorption-desorption

These processes depend on the water/solid composition and pore water properties such as pH, Eh, pCO2, presence of complexing agents (carbonates, humic substances) as they determine the radionuclide chemical speciation and hence the equilibrium distribution.

Main research activities

Our research typically involves experimental work in surface laboratory and in the HADES underground research facility in combination with geochemical and transport modelling:

  • Characterisation of the geochemical environment and physical properties (= starting basis): water/solid composition, pH, Eh, pCO2, porosity (size, volume, morphology)
  • Studies on the radionuclide speciation (current focus on: U/Th, Am/Eu, Se, Tc)
    • Identification and quantification of precipitation/dissolution reactions: RN solubility limits, redox transitions, RN speciation
    • Identification and quantification of complexation reactions with naturally present complexants (carbonates, humic substances): complexation constants, RN speciation
  • Studies on the radionuclide uptake (current focus on: U/Th, Am/Eu, Se, Tc, Sr, Cs)
    • Identification and quantification of sorption/desorption processes onto the host rock and purified minerals: sorption isotherms and sorption edges, determination of distribution coefficients, derivation of surface complexation constants
  • Studies on the radionuclide transport (HTO, I, Cs, Sr, HCO3-, U, Am, Tc, Se, Np, Pu, Humic Substances complexed with RN, ….)
    • Direct determination of RN transport parameters (lab) through host rock and engineered barrier materials (bentonites, concrete): diffusion coefficients, porosity, retardation factor
    • Demonstration of diffusive driven transport and up-scaling issues by long-term&large scale migration experiments in HADES underground research laboratory
    • Colloidal transport

This research is undertaken as part of the Waste Disposal programme of NIRAS/ONDRAF and parts are also conducted in collaboration with other research groups as part of EC projects (EC4FP Trancom-Clay, EC5FP Trancom-II, EC6FP FUNMIG, EC6FP NF-PRO).

Publications & Open reports

P. De Cannière, H. Moors, P. Lolivier, P. De Preter, M. Put, "Laboratory and In-Situ migration experiments in the Boom Clay". Nuclear Science and Technology, Final Report EUR16927, EC, Luxemburg (1996)

A. Dierckx, M. Put, P. De Cannière, L. Wang, N. Maes, M. Aertsens, A. Maes, J. Vancluysen, W. Verdickt, R. Gielen, M. Christiaens, P. Warwick, A. Hall, J van der Lee, "Transport of radionuclides due to complexation with organic matter in clay formations (Trancom-Clay)". Nuclear Science and Technology, Final Report EUR19135, EC, Luxemburg (2000)

N. Maes, L. Wang, G. Delécault, T. Beauwens, M. Van Geet, M. Put, E. Weetjens, J. Marivoet, J. Van der Lee, P. Warwick, A. Hall, G. Walker, A. Maes, C. Bruggeman, D. Bennett, T. Hicks, J. Higgo, D. Galson, "Migration Case Study : Transport of radionuclides in a Reducing Clay Sediment (TRANCOM 2)" Contract No FIKW-CT-2000-00008. EUR-21022, 2004 or SCK•CEN-BLG-988.

N. Maes, "Uranium retention and migration behaviour in Boom clay – status 2004", NIROND-TR report 2007-10 Full Draft report Rev. 1, 2007.

ContactMaes Norbert