Contribution of bentonite and cementitious material to actinide retention under hyperalkaline conditions and increased ionic strength

Our research aligns to conditions reported by the 'AnSichT' project, which evaluated the feasibility of a repository in German clay formations [1]. According to the developed site model 'NORD', Ca-bentonite and concrete will be used in the geo-engineered barrier as buffer and borehole sealing material as well as for stabilization of disposal tunnels. Pore waters of the North German clay deposits are characterized by high ionic strengths up to 4 M [2, 3]. The contact of such saline formation waters with concrete can result in an enhanced corrosion of concrete which will lead to formation of secondary phases and to the evolution of highly alkaline cement pore waters (10 < pH < 13). The hyperalkaline solutions, in turn, can influence the retention potential of the bentonite buffer as well as of the surrounding clay host rock towards radionuclides.
The U(VI) retention on Ca-bentonite at hyperalkaline conditions 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]. Above a certain pH, depending on the concentration of carbonate in solution, carbonate does not play a role in the aqueous U(VI) speciation anymore due to the predominance of hydrolysis. Two U(VI) surface complexes were identified by site-selective TRLFS and XAS.
The stability of U(VI) and Cm(III) doped calcium (aluminate) silicate hydrate (C-(A-)S-H) phases, as main phases of hardened cement paste, at high ionic strengths conditions was studied applying leaching solutions which simulate the contact with North German claystone formation water [5, 6]. With regard to C-S-H stability and radionuclide release, differences were found in dependence on C/S ratio, composition of leaching solution and kind of radionuclide. The high retention capability of C-S-H gel towards U(VI) and Cm(III) remained constant in NaCl- and Na2SO4-containing solutions with increased ionic strength. In the presence of carbonate, however, U(VI) retention was coupled to the alteration stage of the C-S-H structure as well as to pH evolution of leaching solution. The Cm(III) mobilization from C-S-H gel was very low due to additional Cm(III) incorporation into secondary phases as shown by site-selective TRLFS and XRD.
The results show that both bentonite and cementitious material constitute an important retention barrier for radionuclides under hyperalkaline conditions and increased ionic strength.