Coupled permafrost-groundwater simulation applied to a spent fuel nuclear waste repository

<p><span lang="en-GB">The Olk</span><span lang="en-GB">i</span><span lang="en-GB">luoto spent nuclear fuel repository in Eurajoki, Finland, is the first one on the planet that will go operational in </span><span lang="en-GB">foreseeable</span><span lang="en-GB"> future. The long-term safety of this repository with respect to future ice-age conditions and the consequently occurring permafrost and altered groundwater flow conditions needs to be evaluated. To this end, a </span><span lang="en-GB">Darcy </span><span lang="en-GB">model for saturated aquifer groundwater flow combined with a heat transfer module accounting for phase change (i.e. freezing) as well as a solute and a bedrock deformation model have been implemented in the multi-physics Finite Element Method code Elmer. </span><span lang="en-GB">The set of equations is based on continuum thermo-mechanic principles. </span><span lang="en-GB">The application of this newly developed model to Olkiluoto aims to simulate </span><span lang="en-GB">the evolution of permafrost thickness,&#160;talik&#160;development, and groundwater flow and salinity changes at </span><span lang="en-GB">and around the repository</span><span lang="en-GB"> during the next 120,000 years. </span><span lang="en-GB">This is achieved by solving the aforementioned model components in a coupled way in three dimensions on the mesh that discretizes a rectangular block of </span><span lang="en-GB">8.8 km by 6.8 km, </span><span lang="en-GB">stretching from the surface of Olkiluoto</span><span lang="en-GB"> down to a depth of 10 </span><span lang="en-GB">km, </span><span lang="en-GB">where a geothermal heat flux is applied</span><span lang="en-GB">. The horizontal resolution of 30 m by 30 m </span><span lang="en-GB">in combination with</span><span lang="en-GB"> &#8211; imposed by the thickness of different temporarily varying soil and rock layers </span><span lang="en-GB">imported from high resolution data </span><span lang="en-GB">- vertical resolutions of down to 10 cm </span><span lang="en-GB">result in a mesh containing 5 million nodes/elements </span><span lang="en-GB">on which the system of equations is</span><span lang="en-GB"> solved using CSC&#8217;s HPC cluster mahti. The h</span><span lang="en-GB">igh </span><span lang="en-GB">spacial gradients</span><span lang="en-GB"> in permeability (</span><span lang="en-GB">e.g. </span><span lang="en-GB">from soil to granitic bedrock) </span><span lang="en-GB">impose numerical challenges for the s</span><span lang="en-GB">imulations </span><span lang="en-GB">that </span><span lang="en-GB">are </span><span lang="en-GB">forced</span><span lang="en-GB"> by RCP 4.5 climate scenario. </span><span lang="en-GB">The </span><span lang="en-GB">investigated time-span</span><span lang="en-GB"> contains</span><span lang="en-GB"> cold periods between AD 47,000 and AD 110,000. </span><span lang="en-GB">Surface conditions are provided using freezing/thawing n-factors based on monthly temperature variations and wetness index defining varying conditions of vegetation. Our scenario run is</span><span lang="en-GB"> able to </span><span lang="en-GB">project</span><span lang="en-GB"> permafrost development at high spatial resolution </span><span lang="en-GB">and shows clear impact</span><span lang="en-GB"> of permeable soil layers and faults in the bedrock that focus groundwater flow and solute transport.</span></p>