Review of SKB's Work on Coupled THM Processes Within SR-Can. External review contribution in support of SKI's and SSI's review of SR-Can

In this report, we scrutinize the work by the Swedish Nuclear Fuel and Waste Management Company (SKB) related to coupled thermal, hydrological and mechanical (THM) processes within the SR-Can project. SR-Can is SKB's preliminary assessment of long-term safety for a KBS-3 nuclear waste repository, and is a preparation stage for the SR-Site assessment, the report that will be used in SKB's application for a final repository. We scrutinize SKB's work related to THM processes through review and detailed analysis, using an independent modeling tool. The modeling tool is applied to analyze coupled THM processes at the two candidate sites, Forsmark and Laxemar, using data defined in SKB's site description models for respective sites. In this report, we first provide a brief overview of SKB's work related to analysis of the evolution of coupled THM processes as presented in SRCan, as well as supporting documents. In this overview we also identify issues and assumptions that we then analyze using our modeling tool. The overview and subsequent independent model analysis addresses issues related to near-field behavior, such as buffer resaturation and the evolution of the excavation-disturbed zone, as well as far-field behavior, such as stress induced changes in hydrologic properties. Based on the review and modeling conducted in this report, we conclude by identifying a number of areas of weaknesses, where we believe further work and clarifications are needed. Some of the most important ones are summarized below: 1) We found that SKB's calculation of peak temperature might not have been conducted for the most conservative case of extreme drying of the buffer under dry rock conditions and an unexpectedly high thermal diffusion coefficient. Our alternative analysis indicates that temperatures close to 100 might be achieved under unfavorable (and perhaps unexpected) conditions in which the buffer is dried to below 20% near the canister. We believe SKB should conduct further analyses to show that such extreme drying of the buffer to below 20% could not occur, or that such drying would not result in a peak temperature higher than 100 deg C. 2) We found that SKB's estimates for the time of full resaturation of the buffer might be underestimated, because the analysis is based on models assuming nearby water feeding conditions. Moreover, SKB's analysis does not consider the potential impact and uncertainties regarding water-retention properties of the rock mass and the potential impact of ventilation-induced drying during the operational phase is not addressed. SKB's estimated time to full resaturation is valid for an assumed distance to water feeding boundary of 12 m and for one single assumed retention curve of the rock. We believe SKB should provide additional analyses to show that the assumed distance to the water-feeding boundary is reasonable and conduct additional sensitivity analyses on water-retention properties and ventilation effects. 3) We found that SKB's reliance on the backfill as an important source for water supply to resaturate the buffer, in the case of extremely dry rock conditions, may be unjustified. If a bentonite-rock mixture (30/70) is used, the buffer may be resaturated by water supply from the backfill, but then the saturation in the backfill would decrease, preventing it from swelling and thereby keeping it from fulfilling an important safety function indicator criterion. If Friedland Clay is used as backfill, its capillary suction at emplacement would be higher than that of the buffer, and therefore water would be sucked from the buffer into the backfill, effectively keeping the buffer dry. We believe SKB should conduct further studies or reconsider the backfill design, to assure buffer resaturation from the backfill in the case of extremely dry rock conditions. 4) We found that SKB's geomechanical analysis of the potential for rock-mass failure correctly identifies a high potential for spalling failure around the deposition holes at both Laxemar and Forsmark. However, a strong potential for tensile failure in the rock wall of tunnels and its consequence forming a continuous damaged zone along the tunnels is not identified. Moreover, SR-Can does not address the possibility of long-term time-dependent degradation of rock-strength parameters. SKB's assumption that the long-term strength is equal to the relatively short-term strength observed in in situ experiments might not be sufficiently conservative. We believe SKB needs to address the issue of time-dependence in the mechanical parameters as a part of their safety assessment. 5) We found that SKB correctly identifies possible stress-induced changes in permeability near excavations, as well as thermal-mechanically induced change in the far-field permeability. However, SKB analysis does not consider the possibility of large-scale shear reactivation in the far field. Many fractures at the site might already critically stressed for shear. During the thermal period, shear stresses around the repository will increase. We believe that SKB needs to evaluate potential permeability changes due to such shear reactivation and their importance for radionuclide transport. Modeling results developed by the SKB and in this report involve application of complex coupled-processes modeling. An independent analysis using a different model simulator than SKB, is necessary for an in-depth check of SKB's results, to identify issues that might have been overlooked, to test assumptions, and to evaluate how sensitive their results are to such assumptions. The results presented in this report are related to SR-Can, but should also be considered by the SKB when defining their work scope on coupled THM processes for the upcoming SR-Site assessment. Thus, further site-specific analyses on these important aspects for the performance assessment of the future Swedish deep geological disposal of spent nuclear fuel should be conducted