Work

High-Performance Computing for subsurface flow and transport modelling of Olkiluoto

Year: 2026

Type: conference-paper

Abstract: The long-term safety assessment of ONKALO®, the world’s first deep geological repository for nuclear waste at Olkiluoto (Finland), requires predicting the hydro-geochemical evolution of a fractured crystalline bedrock over thousands of years. This is done by using a comprehensive/holistic hydro-geochemical reactive transport modelling methodology to add new concepts (e.g., cation exchange, cement leachates) and improve other processes in existing models (e.g., anion exclusion, matrix diffusion, surface hydrology). A significant bottleneck is the Nernst-Planck (NP) formulation used to represent multi-component diffusion processes like anion exclusion. In particular, the NP approach is numerically stiff in large-scale models with approximately 50 million degrees of freedom. This paper presents a cation exchange (CE) formulation equivalent to the NP equations that enables anion exclusion description and reduces significantly the computational cost of large-scale 3D conservative and reactive transport simulations. It also includes an analysis of the scalability of PFLOTRAN on the supercomputer LUMI for production simulations, and the study of the palaeo-hydro-geochemical evolution of the Olkiluoto site using both NP and CE approaches. The results demonstrate that a CE model can be tuned to match the NP formulation, providing a practical tool for further modelling the hydro-geochemical evolution of Olkiluoto.

Source: Procedia Computer Science

DOI: 10.1016/j.procs.2026.08.018