Bentonite and concrete: Efficient barrier materials for actinide retention under hyperalkaline conditions at increased ionic strengths and in presence of organics

The safe disposal of radioactive waste from operation and decommissioning of nuclear power plants in geological repositories requires the application of multiple barriers to isolate the waste from the biosphere. Bentonite and cementitious materials are foreseen as buffer and borehole sealing material or for stabilization purposes. Pore waters of the North German clay deposits are characterized by high ionic strengths up to 4 M [1,2]. The contact of such saline formation waters with concrete will result in an enhanced corrosion of concrete and to the evolution of highly alkaline cement pore waters (10 < pH < 13), which in turn, can react with the bentonite buffer as well as with the clay host rock, changing their retention potential towards radionuclides. Moreover, the role of organics (as admixtures in cement-based materials or waste constituents [3]) on actinide retention has to be studied.
The U(VI) retention on Ca-bentonite in mixed electrolyte solutions (‘diluted Gipshut solution’, I = 2.6 M) was found to be very effective at pH>10, even in the presence of carbonate and despite the prevalence of anionic aqueous uranyl species [4]. By means of luminescence and X-ray absorption spectroscopy, the U(VI) speciation could be clarified. A substantial contribution of calcium (aluminum) silicate hydrates (C-(A-)S-H), formed as secondary phases in the presence of Ca due to partial dissolution of alumosilicates at hyperalkaline conditions, to the retention of anionic actinide species in clayey systems was shown [5]. Citrate and 2-phosphonobutane-1,2,4,-tricarboxylate (PBTC) were found to reduce U(VI) retention only when present at high concentrations.
The U(VI) and Eu(III)/Cm(III) retention by C-A-S-H, formed due to Al-rich additives in cement formulations, was studied applying samples with Ca/Si molar ratios of 0.8, 1.2 and 1.6, representing different alteration stages of concrete, and with increasing Al/Si molar ratios of 0, 0.06 and 0.18 in each series. Furthermore, the impact of temperature (25°C, 100°C, 200°C) on both the C-A-S-H structure and the actinide retention mechanism was studied. Solid-state 27Al and 29Si NMR spectroscopy along with XRD revealed that enhanced temperatures increase the crystallinity of the material with the appearance of neoformed crystalline phases. Surface-sorbed, interlayer-sorbed or incorporated actinide species were detected by luminescence spectroscopy. Actinide mobilization due to high ionic strengths or presence of organics (gluconate, PBTC or nitrilotriacetate (NTA)) was very low [6,7].
The results show that both bentonite and cementitious material constitute an important retention barrier for actinides under hyperalkaline conditions at increased ionic strengths and in presence of organics.