Measuring diffusion coefficients of dissolved He and Ar in three potential clay host formations: Boom Clay, Callovo-Oxfordian Clay and Opalinus Clay

Abstract

For the long-term management of high-and intermediate-level radioactive waste and/ or spent fuel, many countries prefer disposal in a geological repository. In Belgium, Switzerland and France, argillaceous formations are being explored as potential host formations for this purpose. In this context, knowledge of the diffusion coefficient of He is relevant within two research areas: first, diffusion coefficients are used in safety calculations to evaluate the balance between gas generation (mainly H 2 ) and gas dissipation; and, second, the diffusion coefficients of He and Ar are needed in the diffusion models of natural tracers. Owing to the lack of data on the diffusion coefficients of He and Ar for the different clay host formations, diffusion experiments with dissolved He and Ar were performed on Boom Clay, Opalinus Clay and Callovo-Oxfordian Clay. Samples were confined in a diffusion cell, and diffusion coefficients were measured by using the double throughdiffusion technique. The diffusion coefficients (D p or D pore ) for He in Boom Clay, Callovo-Oxfordian Clay and Opalinus Clay are 12.6 × 10 210 , 4.51 × 10 210 and 7.13 × 10 210 m 2 s 21 , respectively. The diffusion coefficients for Ar in Boom Clay, Callovo-Oxfordian Clay and Opalinus Clay are 18.6 × 10 211 , 4.06 × 10 211 and 3.65 × 10 211 m 2 s 21 , respectively.

The Dependency of Diffusion Coefficients and Geometric Factor on the Size of the Diffusing Molecule: Observations for Different Clay-Based Materials

“…Recently, diffusion coefficients of dissolved gases (He and Ar) have been measured in Boom Clay, Callovo-Oxfordian Clay, and Opalinus Clay and results have been reported by Jacops et al [22] and Jacops et al [23].…”

“…In clay, a relation between the size of the diffusing molecule (expressed by the kinetic diameter) and its diffusion coefficient has been observed [22,23]. The effect of the size of diffusing molecules on its diffusion coefficient in CallovoOxfordian Clay has also been discussed by Dagnelie et al [35].…”

“…As the flanges were provided with a circulation loop, good contact between the water containing the dissolved gas and the clay sample was ensured. More information on the preparation of these samples can be found in Jacops et al [23] and Jacops et al [22].…”

“…Both sides of the test core were connected to a water vessel filled with circa 500 mL oxygen-free synthetic pore water and pressurised with a gas (circa 500 mL gas at 1 MPa). The composition of the pore water used for Boom Clay, and Eigenbilzen Sands, Callovo-Oxfordian Clay, and Opalinus Clay can be found in Jacops et al [22] and for the bentonite samples, degassed demineralised water with 0.05 M NaClO 4 was used. Both vessels were filled with a different gas, but the total pressure was the same.…”

“…Measuring reliable diffusion coefficients of gases is not evident [21] and only a limited amount of data is available in the literature [22]. Recently, diffusion coefficients of dissolved gases (He and Ar) have been measured in Boom Clay, Callovo-Oxfordian Clay, and Opalinus Clay and results have been reported by Jacops et al [22] and Jacops et al [23].…”