HTO and selenate diffusion through compacted Na-, Na–Ca-, and Ca-montmorillonite

Radionuclide transport in smectite clay barrier systems used for nuclear waste disposal is controlled by diffusion, with adsorption significantly retarding transport rates. While a relatively minor component of spent nuclear fuel, ⁷⁹Se is a major driver of the safety case for spent fuel disposal due to its long half-life (3.3×10⁵yr) and its low adsorption to clay (K_D < 10 L/kg), thus a thorough understanding of Se diffusion through clay is critical for understanding the long-term safety of spent fuel disposal systems. Through-diffusion experiments with tritiated water (HTO, conservative tracer) and Se(VI) were conducted with a well-characterized, purified montmorillonite source clay (SWy-2) under a constant ionic strength (0.1 M) and three different electrolyte compositions: Na⁺, Ca²⁺, and a Na⁺ -Ca²⁺ mixture at pH 6.5 in order to probe the effects of electrolyte composition and interlayer cation composition on clay microstructure, Se(VI) aqueous speciation, and ultimately diffusion. Further, the results were modeled using a reactive transport modeling approach to determine values of porosity (ε), D_e (effective diffusion coefficient), and K_D (distribution coefficient for adsorption). HTO diffusive flux was higher in Ca-montmorillonite (D_e=1.68×10^-10m²s^-1) compared to Na-montmorillonite (De=7.83×10^-11m²s^-1). This increase in flux is likely due to a greater degree of clay layer stacking in the presence of Ca²⁺ compared to Na⁺, which leads to larger inter-particle pores. Overall, the Se(VI) flux was much lower than the HTO flux due to anion exclusion, with Se(VI) flux following the order Ca (D_e = 1.03×10^-11m²s^-1) > Na–Ca (D_e = 2.12×10^-12m²s^-1) > Na (D_e = 1.28×10^-12m<. Authors: Fox, Patricia M. [Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)] (ORCID:0000000252641876); Tournassat, Christophe [Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Univ. of Orleans (France); Centre National de la Recherche Scientifique (CNRS) (France)]; Steefel, Carl [Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)]; Nico, Peter S. [Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)]. DOE Contract: AC02-05CH11231. Subjects: 12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Anion exclusion; Engineered barriers; Interlayer cation; Radioactive waste; Selenium diffusion