POSIVA Report 2016-15



Analyses of the Stability of a KBS-3H Deposition Drift at the Olkiluoto Site during Excavation, Thermal Loading and Glacial Loading


Suikkanen, J., Lönnqvist, M., Hökmark, H.



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This report describes a detailed study on the rock mechanics aspects of the KBS-3H repository concept for the upcoming Safety Case analysis. Within this study, the stability of the horizontal deposition drift, associated with the KBS-3H concept, is analysed under different stress conditions occurring during the repository lifespan. The influence of these changes in stress conditions is analysed during three different phases of the repository lifespan: the construction phase, the thermal phase and the glacial phase.

The modelling approach, starting from a repository-scale thermal model and advancing towards the final point of a detailed fracture mechanics study of an individual deposition drift is described in the report.

From the repository-scale modelling of the thermal phase, it is found that in some parts of the repository there is a possibility that the rock could be pre-heated and pre-stressed at the time of construction or deposition because of the heat generated by canisters deposited in nearby drifts several years previously. 

From the modelling of the drift construction, it is found that damage can occur in spatial locations or at positions with localised high magnitudes of unfavourably oriented in situ stress components. The rock damage zone will be extended during the thermal phase, when the tangential stresses at the floor and roof are increased by 70–75 MPa. The detailed fracture mechanics models show that swelling pressure of bentonite has a major impact on the extent of damage occurring during the thermal phase and during the subsequent glacial phase.

It is found that when modelling the glacial period by linear elastic models, the stresses on the roof and floor of the deposition drift are lower during the glacial period than during the thermal phase. During the glacial phase, the stability of intersecting fractures reduced, therefore increasing the aperture of rock damage zone due to the increase in pore pressure during the glacial phase.

The main conclusion is that damage can be expected during all phases of the KBS-3H drift lifespan: the construction, thermal and glacial phases. The rock damage is concentrated on the roof and floor of the drift, where tangential stresses are highest. During the construction phase, minor damage can be expected in fractured locations. However, the rock damage zone on the floor and roof of the deposition drift is most profound after 30–50 years from the start of final deposition of the spent nuclear fuel. The damage, when forming, tends to be fracturing orientated in a tangential direction of the drift perimeter.


KBS-3H, Rock Mechanics, Fracture Mechanics, Safety Case.


Analyses of the Stability of a KBS-3H Deposition Drift at the Olkiluoto Site during Excavation, Thermal Loading and Glacial Loading (pdf) (19.2 MB)


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