Diffusion measurement of caesium in compacted bentonite

Abstract

Caesium diffusion in compacted bentonite beds was studied to determine the transport mechanism. Apparent diffusion coefficients determined at various temperatures indicated a surface diffusion mechanism when compared with values estimated under pore diffusion control assumption. However, temperature dependence of the diffusion coefficients showed a tendency similar to that of pore diffusion. Chemical-transport calculations showed that the cation-exchange reactions possibly described the above disagreement in the mechanism.

Sorption and diffusion behaviours of strontium in sodium-type bentonite bed

“…Cation-exchange reactions were calculated to reproduce the data in acidic pH region using the constants for other exchangeable cations than Sr and CEC for the bentonite [10]. Sodium, which gave the highest concentration among typical exchangeable cations in the ultrafiltrate (Na: 1.7 × 10 -2 , K: 2.3 × 10 -4 , Ca; 5.3 × 10 -5 , and Mg: 1.1 ×10 -4 mol L -1 ), was examined as the competing cation in the calculations.…”

“…4. Apparent diffusion coefficient (D a ) and other related parameters like effective diffusion coefficient (D e ), free water diffusion coefficient (D v ), porosity of the bed (ε), bulk density (ρ), calculated D a (D a calc ), and surface diffusion coefficient (D s ) were obtained in the same manner as in the previous study [10]. Analysis of the diffusion data is summarized in Table 4.…”

“…Diffusion experiments were carried out using "In-diffusion" technique for loosely compacted bentonite with packing density of 1 g cm -3 at temperatures of 25 and 70 °C with the same apparatus used in the previous study [10], in which the diffusion cell was improved as illustrated in Fig. 1.…”

“…The same subsequent sample treatment preceding the radioactivity measurements as that in Ref. [10] was used after the diffusion experiments to measure Sr distribution in the bentonite bed samples.…”

“…Their sorption on bentonite has been investigated considering sorption isotherms and a cation-exchange reaction mechanism [3][4][5][6]. Also model parameters of bentonite and smectite for both cationexchange and the surface complexation reactions have been carried out and used in modelling studies of radionuclide migration behaviour [7][8][9][10].…”