POSIVA Report 2012-47



Thermo-Hydro-Mechanical Modelling of Buffer, Synthesis Report


Toprak, E., Mokni, N., Olivella S. & Pintado, X.



Page count:



ISBN 978-951-652-229-9


This study addresses analyses of coupled thermo-hydro-mechanical (THM) processes in a scheme considered for the spent nuclear fuel repository in Olkiluoto (Finland). The finite element code CODE_BRIGHT is used to perform modelling calculations. The objective of the THM modelling was to study some fundamental design parameters.

The time required to reach full saturation, the maximum temperature reached in the canister, the deformations in the buffer–backfill interface, the stress–deformation balance between the buffer and the backfill, the swelling pressure developed and the homogenization process development are critical variables.

Because of the complexity of the THM processes developed, only a single deposition hole has been modelled with realistic boundary conditions which take into account the entire repository. A thermal calculation has been performed to adopt appropriate boundary conditions for a reduced domain.

The modelling has been done under axisymmetric conditions. As a material model for the buffer bentonite and backfill soil, the Barcelona Basic Model (BBM) has been used. Simulation of laboratory tests conducted at B+Tech under supervision of Posiva has been carried out in order to determine the fundamental mechanical parameters for modelling the behaviour of MX-80 bentonite using the BBM model.

The modelling process of the buffer–backfill interface is an essential part of tunnel backfill design. The calculations will aim to determine deformations in this intersection, the behaviour of which is important for the buffer swelling.

The homogenization process is a key issue as well. Porosity evolution during the saturation process is evaluated in order to check if the final saturated density accomplishes the homogenization requirements.

This report also describes the effect of the existence of an air-filled gap located between the canister and the bentonite block rings in thermo-hydro-mechanical behaviour of the future spent nuclear fuel repository in Olkiluoto. The presence of the 10 mm air gap has an influence on the thermal, hydraulic and mechanical response of the buffer. The closure of the gap is controlled by the swelling deformation developed as the bentonite buffer saturates. Under unsaturated conditions, the buffer will not transfer heat efficiently, and that may disturb heat dissipation and lead to somewhat higher canister temperature. This is more evident when the hydration of the buffer takes place more slowly because the gap remains open during heating. The kinetics of water supply is affected by the hydraulic conductivity of the different elements and in particular by the hydraulic conductivity of the host rock formation.


Repository, spent fuel, thermal modelling, hydraulic modelling, mechanical modelling, thermo-hydro-mechanical.


Thermo-Hydro-Mechanical Modelling of Buffer, Synthesis Report (pdf) (4.7 MB)


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