2012:18 Technical Note, SR-Site Independent Modelling of Engineered Barrier Evolution and Coupled THMC: Contribution to the Initial Review Phase

This review has focussed mainly on the modelling of the Engineered Barrier System (EBS) evolution, which includes coupled thermal, hydraulic, mechanical and chemical (THMC) processes.  Additionally, the role of the EBS in the wider safety case was reviewed, including its treatment in scenarios and its representation in conceptual models since this provided the motivation for the modelling work that was undertaken by SKB.  The scope of the work described here was to:

  • Review relevant documents concerning SKB’s modelling; and
  • Check one particular modelling area that was judged to be important, based on this review, with a limited set of independent modelling/calculations.

The review covers the early resaturation and swelling / homogenisation of the buffer; the longer-term chemical evolution of the buffer and backfill, corrosion of the copper canister and the chemical and hydrogeological boundary conditions provided by the surrounding host rock.

The reviewers consider that SKB’s modelling of engineered barrier performance generally supports their conclusion that the barriers will perform as required. However, there remain issues that are not addressed and uncertainties that are not explored adequately by SKB’s modelling. 

The thermo-hydro-mechanical modelling of buffer resaturation that is performed by SKB is based on demonstrating a fit to measurements from the in-situ Canister Retrieval Test (CRT) experiment.  The modelling reproduces some of the experimental observations very well, but some key experimental measurements are not considered.  In particular, investigation of the fit to the measured rates of water inflow, which are a critical factor controlling the rate at which the buffer will resaturate, is not given,  Furthermore, the water supply boundary conditions imposed on the CRT are not considered to be representative of those that might be expected in repository conditions.  From the information that it is presented it is therefore not possible to be confident in the ability of SKB’s models to predict realistic rates of inflow, or patterns of resaturation in the buffer, that might be expected under true repository conditions when resaturation arises from flows in small fractures.  Further experimental and modelling work should be performed.