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The Neogene-Paleogene glauconite sands of Belgium cover the Boom Clay deposits that are candidate host for radioactive waste disposal. It is unclear if the highly permeable sand formations may act as an additional barrier for radiocesium ( 137 Cs) or could be added as a complementary sorption sink in a surface disposal concept. Glauconite is an Fe-rich phyllosilicate that is mainly present as 250-125 µm sized pellets in sand, it is unknown to what extent and how fast these pellets may bind 137 Cs. Pelletized clays embedded in sand may have poorly accessible high affinity sites for 137 Cs. The 137 Cs sorption on 11 different glauconite sands was measured in batch in a background solution of 0.1 M CaCl2 and 0.5 mM KCl. The log transformed 137 Cs distribution coefficient Kd (L kg -1 ) after 30 days reaction ranged 3.4-4.3, surprisingly close to the Kd of the Boom Clay (3.5). Isolated glauconite fractions exhibited similar 137 Cs sorption potentials (log Kd 4.1-4.3) as the reference Illite duPuy (4.4). The small Kd variation among the Neogene-Paleogene sands was explained by its glauconite content (r=0.82). The 137 Cs sorption kinetics (1-57 days) of milled pellets (<2 µm) confirmed slower reaction with intact pellets than with milled samples. Additionally, the Kd values of milled samples (57 days) sorption are 1.1-1.5 fold larger than the corresponding intact pellets, suggesting that not all Cs binding sites are accessible in intact pellets. Strongly weathered pellets exhibited cracks (visible with SEM). In these pellets the Kd was similar for milled and intact pellets suggesting that cracks increase the accessibility of the inner sorption sites. After 8.5 months the Kd values were 1.6-1.8-fold above corresponding 1 month data and these long-term reactions were more pronounced as total sand K content was larger. An adsorption-desorption experiment illustrated that 137 Cs sorption is not fully reversible.

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The NEA Thermochemical Database (TDB) Project (www.oecd-nea.org/dbtdb/) provides a database of chemical thermodynamic values treating the most significant elements related to nuclear waste management. The work carried out since the initiation of TDB in 1984 has resulted in the publication of thirteen major reviews and a large set of selected values that have become an international reference in the field, as they are characterized for their accuracy, consistency and high quality. Herein, we describe the basis, scientific principles and organization of the TDB project, together with its evolution from its inception to the present organization as a joint undertaking under Article 5(b) of the Statute of the OECD Nuclear Energy Agency (NEA).

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In situ migration experiments using different radiotracers have been performed in the HADES Underground Research Facility (URF), built at a depth of 225 m in the Boom Clay formation below the SCK–CEN nuclear site at Mol (Belgium). Small-scale experiments, mimicking laboratory experiments, were carried out with strongly retarded tracers (strontium, caesium, europium, americium and technetium). Contrary to europium, americium and technetium which are subjected to colloid mediated transport, the transport of strontium and caesium can be described by the classic diffusion retardation formalism. For these last two tracers, the transport parameters derived from the in situ experiments can be compared with the laboratory-derived values. For both tracers, the apparent diffusion coefficients measured in the in situ experiments agree well with the laboratory-derived values.In the large-scale experiments (of the order of metres) performed in the URF, non-retarded or slightly retarded tracers (HTO, iodide and H14CO3–) were used. The migration behaviour of these tracers was predicted based on models applied in performance assessment calculations (classic diffusion retardation) using migration parameter values measured in laboratory experiments. These blind predictions of large-scale experiments agree well in general with the experimental measurements. Fitting the experimental in situ data leads to apparent diffusion coefficients close to those determined by the laboratory experiments. The iodide and H14CO3– data were fitted with a simple analytical expression, and the HTO data were additionally fitted numerically with COMSOL multiphysics, leading to about the same optimal values.