38permeability. The generally good agreement between simulated and measured THM data shows that adopted continuum model approaches are adequate for simulating relevant coupled THM processes at the DST. Moreover, TM-induced rock-mass deformations were reasonably well predicted using elastic models, although some individual displacements appeared to be better captured using an elasto-plastic model. It is concluded that fracture closure/opening caused by change in normal stress across fractures is the dominant mechanism for TM-induced changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. This indicates that TM-induced changes in intrinsic permeability at the DST, which are within one order of magnitude,, tend to be reversible.

This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings: (a) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (b) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermalmechanically-induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal-mechanically-induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermalmechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts.

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Small amounts of dust will be deposited on the surfaces of waste packages in drifts at Yucca Mountain during the operational and the preclosure ventilation periods. Salts present in the dust will deliquesce as the waste packages cool and relative humidity in the drifts increases. In this paper, we evaluate the potential for brines formed by dust deliquescence to initiate and sustain localized corrosion that results in failure of the waste package outer barrier and early failure of the waste package. These arguments have been used to show that dust deliquescence-induced localized or crevice corrosion of the waste package outer barrier (Alloy 22) is of low consequence with respect to repository performance.Measured atmospheric and underground dust compositions are the basis of thermodynamic modeling and experimental studies to evaluate the likelihood of brine formation and persistence, the volume of brines that may form, and the relative corrosivity of the initial deliquescent brines and of brines modified by processes on the waste package surface. In addition, we evaluate several mechanisms that could inhibit or stifle localized corrosion should it initiate.The dust compositions considered include both tunnel dust samples from Yucca Mountain, National Airfall Deposition Program rainout data, and collected windblown dust samples. Also considered is sublimation of ammonium salts, a process that could affect dust composition prior to deliquescence. Ammonium chlorides, nitrates, and even sulfates sublimate readily into ammonia and acid gasses, and will be lost from the surface of the waste package prior to deliquescence.Arguments are developed using a logic-tree approach, based upon that presented in Apted et al. (2005, JOM), evaluating the potential importance of localized corrosion by high-temperature deliquescent brines. We expand upon the approach used in those documents, considering a wider range of dust and brine compositions, conditions, and arguments. In order for dust deliquescence to cause failure of the waste package, each of the following propositions must be affirmative: 1) Can multiple-salt deliquescent brines form at elevated temperature? Yes. Some important salt phases in the dust (e.g., ammonium phases) will sublimate prior to deliquescence. However, other salts in the dust are stable, and form eutectic assemblages that can deliquesce at temperatures much higher than the individual salt components. For most single salt phases (nitrates, chlorides, and carbonates) boiling points at one atmosphere are limited to temperatures below 120°C-boiling points of saturated salt solutions represent the maximum temperature of deliquescence at a given pressure. Multi-salt mixtures always boil at higher temperatures than the individual salt components. The boiling points for important salt assemblages predicted to occur on the waste package surface have been investigated experimentally. The two-salt mixture NaCl+ KN03 boils at a maximum temperature of 134°C and the three-salt mixture, NaCl + KN03 + NaN03, at over ...

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