Radiation effects on clay mineral properties
Résumé
The influence of radiation on clay mineral properties remains relatively poorly documented, despite its importance for the safety assessment of high level nuclear waste repositories (HLNWR) where clays are components of the engineered barrier or the host rock in many countries. Various radiation-induced point defects have been observed in kaolinite, dickite, montmorillonite and illite. In kaolinite, the defects stable at the scale of geological periods have been used for geochemical applications or dating. Nevertheless, the potential influence of point defects on clay properties has not been demonstrated. Ballistic effects of heavy ions (alpha recoil nuclei) are able to amorphize the structure of clay minerals, as itwas shown for smectite either irradiated with ion beam or doped with alpha emitters, leading to consistent amorphization doses. Several estimations show that amorphization of smectite from the engineered barrier would occur after 1000 years of exposure arising from waste leakage. Besides, amorphization by ionizing radiation in a HLNWR is unlikely because required dose is unrealistically too high. Consequences of amorphization on physico-chemical properties such as swelling, CEC, solubility are potentially very important but have not been described for clay minerals. Ionizing radiations can reduce structural Fe3+ in montmorillonite and may thus potentially modify its layer charge and CEC. The influence of irradiation on physico-chemical properties such as CEC is revealed only at high irradiation doses, larger than several MGy, and remains relatively limited. Nevertheless, contradictory results still exist about the effect of irradiation on redox and surface reactivity. These parameters together with their consequences on properties, obtained on model specimen used in laboratory or test experiments, must be considered in the performance assessment of the engineered barrier of a nuclear waste disposal. This paper reviews the various aspects of irradiation effects on clay minerals with emphasis on physicochemical properties, after a presentation of the methodology of experimental irradiation.