Principles of geomechanical safety assessment for radioactive waste disposal in salt structures

Today, a large amount of knowledge is available concerning various sites of potential high active waste (HAW) repositories in salt media. Domal Zechstein salt formations have been examined at several sites in Germany. Extensive R&D work was initiated in the former Asse Salt Mine in order to explore the suitability of salt for waste isolation by laboratory tests, theoretical studies and in-situ tests with test results forming a technological base for future repository development.

Resulting from the inhomogeneity of salt structures the demanded safety of a permanent repository for radioactive wastes can be demonstrated only by a specific site analysis in which the entire system, “the geological situation, the repository, and the form and amount of the wastes” and their interrelationships are taken into consideration.

The site analysis has three essential tasks: (1) Assessment of the thermomechanical load capacity of the host rock, so that deposition strategies can be determined for the site; (2) Determination of the safe dimensions of the mine (e.g. stability of the caverns and safety of the operations); and (3) Evaluation of the barriers and the long-term safety analysis for the authorization procedure.

The geotechnical stability analysis is a critical part of the safety assessment. Engineering–geological study of the site, laboratory and in situ-experiments, geomechanical modelling, and numerical static calculations comprise such an analysis.

Within a scenario analysis — according to the multi-barrier principle, the geological setting is checked to be able to contribute significantly to the waste isolation over long periods. The assessment of the integrity of the geological barrier can only be performed by making calculations with geomechanical and hydrogeological models. The proper idealization of the host rock in a computational model is the basis of a realistic calculation of stress distribution and excavation damage effects. The determination of water permeability along discontinuities is necessary in order to evaluate the barrier efficiency of each host rock.

In this paper some important processes for the performance assessment are described, namely creep and fracturing, permeability and infiltration, and halokinesis and subrosion.

For the future, the role and contributions of geoscientific and rock mechanics work within the safety assessment issues (e.g. geomechanical safety indicators) must be identified in greater detail, e.g. considerations of geomechanical natural analogy for calibration of constitutive laws.