Electrolyte Diffusion in Compacted Montmorillonite Engineered Barriers

Abstract

The bentonite-based engineered bar rier or packing is a proposed component of several designs conceived to dispose of high-level nuclear waste in geologic reposi tories. Once radionulcides escape the waste package, they must first diffuse through the highly impermeable day-rich barrier before they reach the host repository. To deter mine the effectiveness of the packing as a sorption barrier in the transient release period and as a mass-transfer barrier in the steady release period over the geologic time scales involved in nuclear waste disposal, a fundamental understanding of the diffusion of electrolytes in compacted clays is required. We present, and compare with laboratory data, a model quantifying the diffusion rates of cationic cesium and uncharged tritium in compacted mont morillonite clay. Neutral tritium character izes the geometry (Le., tortuosity) of the particulate gel. After accounting for cation exchange, we find that surface diffusion is the-dominant mechanism of cation tran sport, with an approximate surface diffusion coefficient of 2xl0~6 cm 2 /s for cesium. This value increases slightly with increasing background ionic strength.The implications of this work for the packing as a migration barrier are twofold. During the transient release period, K4 values are of little importance in retarding ion migration. Tbii is because sorption also gives rise to a surface diffusion path, and it is surface diffusion which controls the diffusion rate of highly sorbing cations in compacted montmorillonite. During the steady release period, the presence of sur face diffusion leads to a flux through the packing which is greatly enhanced. In either case, if surface diffusion is neglected, the appropriate diffusion coefficient of ions in compacted packing will be in considerable error relative to current design recommen dations. IntroductionHigh-level nuclear wastes will be stared in underground repositories. To aid the host formation in protecting the bio sphere from the harmful, long-lived radioisotopes contained in the waste pack age, use of an engineered barrier or packing is being considered [6]. This barrier, based upon the day mineral montmorillonite, is envisaged as an impermeable barrier sur rounding the waste canisters which retards the migration of ions in the transient release period due to its high cation exchange capadty [3] and low hydraulic permeability [7]. As time passes and steady-state diffusion through the packing is established, it further serves as a masstransfer resistance retarding the escape of ions from the canister to the fluid percolat ing around the waste package. To deter mine the steady-state plume of radionu clides eminating from the waste repository in the steady-state period and the protec tion an engineered barrier affords the waste disposal scheme in the transient release period, the diffusion rate of ions in com pacted montmorillonite clay must be deter mined.

Caesium and strontium diffusion through sodium montmorillonite at elevated temperature

“…Since the permeabilities of potential geological sites and packing materials are very low ( K Om') (Rolfe & Alymore, 1977;Seitz & Couture, 1983) ground water velocities in the repository are small (< 1 m a - ' Anderson et al, 1982;Neretnieks, 1982. This results in nuclide release that is diffusion-controlled (Jahnke, 1987;Jahnke & Radke, 1987). In this paper we measure diffusion rates of non-radioactive Cs' and Sr' +, two important analogues of long-lived and harmful radioisotopes in high-activity nuclear waste canisters, through Na-montmorillonite, the major component of bentonite.…”

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

“…It has been reported that some cations diffuse through aggregated materials such as bentonite or in fractured media like whole rock at greater rates than those predicted from aqueous phase diffusion alone (Kim et al 1993;Jahnke and Radke 1987;Bradbury et al 1986;Skagius and Neretnieks 1985;Rasmuson and Neretnieks 1983). Surface diffusion has been invoked to explain this phenomenon.…”

Radionuclide Transport Models Under Ambient Conditions

“…Omitting the κ subscript, D is described by the equations Equation 18 accounts for surface diffusion, which can be responsible for significant transport in strongly sorbing media (Moridis 1999;Cook 1989). The surface diffusion flux is given by Jahnke and Radke (1987) as (Bird et al 1960, p. 514), as …”