Crack healing in rocksalt via diffusion in adsorbed aqueous films: Microphysical modelling versus experiments M.E. Houben ⇑ , A. ten Hove 1 , C.J. Peach, C.J. Spiers HPT Laboratory, Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands article info Article history: Available online 13 October 2012 Keywords: Crack healing/sealing EDZ Geological storage Radioactive waste Damage evolution abstract Microcracks within the excavation damaged or disturbed zone (EDZ) in a salt-based radioactive waste repository (or an energy storage facility) can heal/seal by mechanical closure driven by compaction creep, by surface-energy-driven processes like diffusive mass transfer, and by recrystallization. It follows that permeability evolution in the excavation damaged zone around a backfilled or plugged cavity will in the short term be dominated by mechanical closure of the cracks, while in the longer term diffusive mass transfer effects are expected to become more important. This paper describes a contribution to assessing the integrity of radioactive waste repositories sited in rocksalt formations by developing a microphysical model for single crack healing in rocksalt. More specifically, single crack healing models for cracks con- taining a thin adsorbed water film are developed. These microphysical models are compared with single crack healing experiments, which conclusively demonstrate diffusion controlled healing. Calibration of unknown model parameters, related to crack surface diffusivity, against the experimental data enable crack healing rates under repository conditions to be estimated. The results show that after the stress re-equilibration that follows repository sealing, crack disconnection can be expected on a timescale of a few years at laboratory humidity levels. However, much longer times are needed under very dry con- ditions where adsorbed aqueous films are very thin. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Despite the disaster caused at the Fukushima nuclear plant by the tsunami that struck the Tohoku region of Japan on 11 March 2011, the rapidly growing need to reduce anthropogenic emissions of CO 2 to the atmosphere is presently driving renewed interest in nuclear energy and therefore in geological disposal or storage of radioactive waste. Against this background, rocksalt is still very much on the agenda as a suitable host rock for radioactive waste repositories (see Fig. 1), because of its low permeability and its po- tential for fracture healing due to its ductile rheological properties (Langer, 1993, 1999; Silberschmidt and Silberschmidt, 2000; Arson et al., 2012). After the construction of a mined repository in a rock- salt formation, the salt surrounding the openings will be mechan- ically, hydraulically and geochemically altered (Cai and Kaiser, 2005), forming a narrow zone of damaged, permeable rocksalt known as the excavation damaged or disturbed zone (EDZ) (Tsang et al., 2005). Following disposal of the radioactive waste, the repos- itory rooms, boreholes, drifts and shafts will be backfilled with crushed salt removed during construction. The backfill will, on the one hand, support the converging walls of the excavated cavi- ties, limiting or ultimately reversing the mechanical damage occur- ring in the walls. On the other hand, it will serve as a ductile, compacting fill that will help to isolate the disposed wastes from the biosphere. As well as understanding the behaviour of the backfill, it is important that the extent, the transport properties and the evolu- tion of the EDZ surrounding the various mined openings can be predicted. These are determined by the creep behaviour of the salt and backfill material and by the initiation, growth and healing of cracks and fractures in the EDZ, on the macro and micro-scale, that accompany stress redistribution during convergence of the mined openings (Tsang et al., 2005; Zhu and Bruhns, 2008). In the last 30 years, much research has been done on the mechanical behav- iour of rocksalt and of backfill as well as on damage development (Senseny et al., 1992; Peach and Spiers, 1996; Hunsche and Hampel, 1999; Lux et al., 2000; Silberschmidt and Silberschmidt, 2000; Schulze et al., 2001; Hou, 2003; Cinar et al., 2006; Alkan et al., 2007; Liang et al., 2007; Günther and Salzer, 2007; Lux, 2009). This has shown that when the deviatoric stress in the con- verging walls of a cavity or borehole reaches a sufficiently high va- lue compared to the mean stress, i.e. passes through the so-called dilatancy boundary in stress space (Hunsche, 1998), microcracks develop, mainly along grain boundaries but also within grains, 1474-7065/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pce.2012.10.001 ⇑ Corresponding author. Present address: RWTH-Aachen University, Lochnerst- rasse 4-20, 52062 Aachen, Germany. Tel.: +49 (0)241 80 98439; fax: +49 (0)241 80 92358. E-mail address: m.houben@ged.rwth-aachen.de (M.E. Houben). 1 Present address: Petrotechnical Data Systems B.V., Rijswijk, The Netherlands. Physics and Chemistry of the Earth 64 (2013) 95–104 Contents lists available at SciVerse ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce