Molecular dynamics simulation of thermal, hydraulic, and mechanical properties of bentonite clay at 298 to 373 K

Nanoscale Prediction of the Thermal, Mechanical, and Transport Properties of Hydrated Clay on 106- and 1015-Fold Larger Length and Time Scales

“…The range of dry densities examined here covers the full range of interest in HLRW storage (on the order of 1300 to 1700 kg·m –3 ) . Our range of dry densities extends to lower values as the preparation of the initial simulated configuration (through sequential insertion of oriented, randomly placed smectite particles) achieved dry densities only up to 781 kg·m –3 as discussed in our previous paper . The range of temperatures examined here covers the range considered in most repository designs (298 to 373 K).…”

“…The heat capacity of pure liquid water (4157 to 4183 J·kg –1 K –1 ) is three to four times larger than that of dry Na-montmorillonite (851 to 889 J·kg –1 K –1 ), , resulting in the observed increase in the heat capacity of bentonite with χ w . The temperature dependence of C P , while not presented in Figure , is not negligible: our previous simulations demonstrate that it increases by 9.7 ± 0.9 J·kg –1 K –1 per K as temperature increases from 298 K to 373 K at ρ d = 781 kg m –3 . The temperature dependence of heat capacity as a function of dry density is subjected to limited variations as shown in Figure A3 in the Supporting Information.…”

“…Bentonite is predominantly composed of montmorillonite clay, a 2:1 layer-type clay mineral in the smectite family. 130 The systems simulated in this work were generated as described in Zheng et al 23 Briefly, the initial phyllosilicate structure was generated based on the pyrophyllite unit cell structure of Bickmore et al 131 Isomorphic substitutions of octahedral cations (Al 3+ ) by lower valence cations (Mg 2+ ) were distributed randomly but not allowed to share bridging oxygen atoms. 132 Clay particles were then charge balanced by placing Na + ions 1 nm above or below the isomorphic substitution sites.…”

“…146 Other details of our simulation methodology are presented in Zheng at al. 23 and in Underwood and Bourg. 42 The initial configuration was solvated using 187,131 water molecules, resulting in a system containing 766,701 atoms.…”

“…As noted above, recent years have seen the emergence of alternative repository designs whereby temperature within the clay barrier may reach up to 473 K. , The properties of compacted smectite at these higher temperatures will be examined in a future study. Our simulations build on approaches tested and validated in our recent examination of Na-smectite at low dry density, but we now progressively dehydrate the system to high dry densities to simulate clay dehydration processes observed in bentonite buffers. Our data analyses quantify a series of material properties of hydrated Na-smectite including thermal properties (heat capacity, thermal conductivity, thermal expansion coefficient), chemomechanical properties (suction), and mass transport properties (hydraulic conductivity, water and Na self-diffusivity).…”