Effects of long-term climate variation in numerical thermal models under uncertainties – analyses for the site selection process

Computational analysis of host rock integrity is an important tool for assessing the long-term safety of potential final repository sites during site selection process. To ensure the safe operation of a repository and the integrity of the barrier host rock, the temperature increase in the host rock relevant to the containment must be limited. Therefore, knowledge of the current temperature distribution in the subsurface is a prerequisite for designing the final repository.

The focus of this work is on stochastic FE computations for typical geological conditions in sedimentary host rocks in Northern Germany. Using generic models of varying complexity, comparative computational analyses have been carried out to investigate the effect of uncertainties in thermal model parameters and boundary conditions on the transient temperature field. Another focus is set on the thermal effects of permafrost conditions on the temperature field during potential future cold phases, e.g., the penetration depth of permafrost. To evaluate these effects, a temperature-time profile of past climate has been used as the upper boundary condition of the model. This profile was calculated based on a global proxy of a Pliocene–Pleistocene stack of benthic δ¹⁸O isotope records. For the lower boundary condition, a heat flow density has been imposed along the base of the model.

The effect of thermal model parameter uncertainty has been quantified and analysed within three applications considerung different host rocks: [1] 1D-vertical model with parameter variations covering different rock types, [2] 2D-vertical model with claystone in the Lower Cretaceous as host rock and [3] 2D-rotationally-symmetric model of a salt diapir with rock salt as host rock. In [2] and [3] the heat power generated by radioactive waste is accounted for in the simulations via a source term.

For the thermal parameters, which reflect a representative state of knowledge for Northern Germany, uniform distributions are assumed. During the numerical analysis of the generic repository systems [2,3], numerous simulation runs are performed to capture all possible manifestations of the uncertain thermal parameters and their combinations. By statistical evaluation, the impacts of uncertainties on the calculated results can be quantified. These results may also help to assess the robustness of a final repository system.

References

Liu, W., Völkner, E., Minkley, W. & T. Popp (2017): Zusammenstellung der Materialparameter für THM-Modellberechnungen (Ergebnisse aus dem Vorhaben KOSINA)  – Ergebnisbericht; Hannover (BGR).

Lisiecki, L.E. & M.E. Raymo (2007). Plio–Pleistocene climate evolution: trends and transitions in glacial cycle dynamics. Quaternary Science Reviews, 26, (1–2): 56-69. https://doi.org/10.1016/j.quascirev.2006.09.005.

Maßmann, J., Thiedau, J., Bittens, M., Kumar, V., Tran, Tuong Vi, Guevara Morel, C.,Kneuker, T. & S. Schumacher (2022): ANSICHT-II – Methode und Berechnungen zur Integritätsanalyse der geologischen Barriere für ein generisches Endlagersystem im Tongestein – Ergebnisbericht; Hannover (BGR). https://download.bgr.de/zsn/201997/201997_01.pdf.