A good understanding of the processes and mechanisms controlling the radionuclides transport in the various system components of a disposal system is necessary to assess its long-term safety. In Belgium, geological disposal in a poorly indurated clay is recommended for the long-term management of the high-level and/or long-lived radioactive waste whereas surface disposal is planned for the low-level and short-lived radioactive waste.
Sorption is one of the key processes controlling the release of radionuclides from the disposal system (i.e. radioactive decay during the residence time). Boom Clay, as main barrier in the considered geological disposal concept, exhibits considerable sorption. But also glauconite, present in some of the geological formations surrounding the Boom Clay (e.g., Diest, Kasterlee…) may be regarded as a possible sorption sink in the disposal concept. Moreover, this mineral is presently considered as one of the potential complementary sorption sinks to be added in the sandy embankment of the surface disposal facility for low and intermediate short lived radioactive wastes.
Glauconite is an iron potassium phyllosilicate ((K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2) and can exist in different stadiums, related mainly to the age of the sediment. In each stadium, glauconite will have different physical and chemical properties, and hence a different sorption behaviour for the radionuclides. Young or embryonal glauconite has the structure of an iron-rich smectite with high CEC (cation exchange capacity), low mineral density and low K2O content. More evolved glauconite resembles more like an illite with low CEC, higher mineral density and higher K2O content. Next to differences in maturity, it can also differ in degree of oxidation and origin (Mallants et al., 2003).
Therefore, this study will require a complete characterization of the glauconite to be used (e.g., structural formula, structure, basic sorption properties…). A preliminary study (Maes et al., 2007) with basic sorption tests indicated that the glauconite sands might indeed sorb some critical radionuclides as Ra, Sn and U, but not Cl, I and Se. The radionuclides to be investigated should encompass a monavalent cation (e.g. Cs), a divalent cation (e.g. Ba/Ra), an actinide (e.g. Tc) and/or a fission product.
Maes, N. & Van Ravestyn, L. (2007). Mol 1: Investigation of the spatial variability of the Boom Clay with respect to the migration properties for strong sorbing and critical radionuclides. Technical note concerning the batch sorption experiments on glauconite sands. RP.WD.023 (DS2.72) SCK•CEN-R- 4431 Restricted contract report.
Mallants, D., Labat, S., Gedeon, M. (2003) Bijkomende sitekarakterisatie voor de nucleaire zone Mol-Dessel: bepaling van de hydrogeologische parameters. Restricted contract report SCK•CEN-R-3703.