World-first project puts supercomputer to work to advance nuclear waste solution modelling for up to a million years

The power of a supercomputer will be harnessed by scientific and strategic environmental consulting company Amphos 21 as part of a comprehensive groundwater chemistry modelling project. The work will significantly advance sustainable solutions to long-term nuclear waste disposal and crucially, demonstrate site safety over a future timeline of up to a million years, factoring in potential natural disasters, submergence and a future ice age.

Spent nuclear fuel deep geological disposal in Finland

Amphos 21, an RSK Group company, will deliver this work for Posiva Oy, a Finland-based world leader in nuclear waste management. Posiva Oy is building the world’s first underground repository for spent nuclear fuel. Deep geological disposal of spent nuclear fuel is, at present, the preferred solution for the final disposal of the most hazardous nuclear waste. The project started in 2022 and is expected to be completed by end of 2026–start of 2027.

Amphos 21 Project Director Orlando Silva said: “The OL-STAR project represents a step-change in how the long-term safety of deep geological repositories is assessed, combining state-of-the-art reactive hydrogeochemical modelling with European High Performance Computing Joint Undertaking (EuroHPC JU) supercomputing to simulate complex subsurface processes across thousands of years.” The project will make use of LUMI, EuroHPC JU’s flagship supercomputer, with computing power described as being equivalent to the combined performance of 1.5 million of the latest laptop computers.

Environmental geoscience for nuclear waste disposal

“By applying our expertise in advanced numerical modelling, nuclear waste management and environmental geoscience, we are delivering one of the most comprehensive simulations ever undertaken for a spent fuel repository. The work not only strengthens confidence in the long-term safety of the ONKALO® facility in Olkiluoto, Finland, but also sets a new international benchmark for science-driven, sustainable solutions to nuclear waste disposal.”

Tiina Lamminmäki, Principal Chemist at Posiva Oy, the principal investigator of the OL-STAR project, said: “Posiva has conducted several safety assessments to demonstrate the longterm safety of the final disposal of spent nuclear fuel, with the evolution of Olkiluoto’s groundwater chemistry being one of the key components. These assessments have included modelling the hydrogeochemical conditions far into the future. In earlier assessments, some simplifications had to be made regarding the processes influencing groundwater chemistry due to limited computational capacity.”

Orlando explained that the OLSTAR project now enables a more realistic modelling of groundwaterchemistry evolution, as the most influential processes can be incorporated thanks to the availability of highperformance computing. The high-efficiency computation offers the capacity to model the hydrogeological and hydrogeochemical evolution of Olkiluoto over a future timeline of up to a million years.

Advanced computer modelling simulates the evolution of timescales

Orlando said: “The present project is motivated by the results obtained during the modelling work developed under a previous EuroHPC grant that gave us access to the LUMI supercomputer, enabling OL-STAR development. Through advanced computer modelling, the OL-STAR project simulates the evolution on timescales spanning over several millennia of groundwater chemistry located in the fractured crystalline bedrock that will store the waste repository known as the ONKALO. The project seeks to consider factors like density-driven flow and microbiological reactions in fractured crystalline rocks coupled with various chemical and environmental events.”

Supercomputing power equal to about 100 years of continuous calculation

He said that the project team had been granted 13.8 million core-hours to carry out many HPC calculations in the LUMI-C partition for a period of 12 months. While using a normal workstation would require, hypothetically, about 100 years of continuous calculation, the granted supercomputing resources will allow the planned work to be completed in less than one year.

“This is probably one of the most ambitious modelling projects of groundwater chemistry to date globally. It integrates all the modelling knowledge developed over the years in a holistic numerical model to simulate the hydrogeological and hydrogeochemical evolution of Olkiluoto in the past, present and future. Furthermore, the groundbreaking nature of this project holds the potential to obtain the scientific-technical interest and recognition of other spent nuclear fuel storage projects globally. Currently, all spent nuclear fuel is stored in interim facilities, vulnerable to natural disasters or national emergencies. The project’s innovative work opens opportunities for collaboration with stakeholders worldwide, contributing to a collective effort in advancing safe and sustainable solutions for nuclear waste disposal.”

Ensuring long-term safety of high-level radioactive nuclear waste

He added that the long-term safety of deep geological repositories for high-level radioactive nuclear waste depends on demonstrating that suitable hydrogeochemical conditions in the host rock can be maintained over timescales lasting thousands of years, and they must also be maintained during future ice age and subsequent submerged and temperate phases of planetary activity.

High computational power is needed to calculate the length of the modelled time span, and the extensive size of about 70 cubic kilometres of the modelled area.

Orlando explained that the Amphos 21 team working on the project has considerable experience in advanced modelling of flow and reactive transport in fractured and porous media, hydrogeology and hydrogeochemistry.

“In addition, the team members are advanced users of supercomputers and the PFLOTRAN code and are continuously contributing to the development of this code. This expertise enabled the project team to develop a comprehensive/holistic hydrogeochemical reactive transport modelling methodology. Supercomputing is the key tool to overcome hurdles arising from model complexity coupled with the fabric of the site’s hydrogeology. This approach allows the adding of new concepts or processes (for example cation exchange, cement leachates) while improving other processes in existing models (such as anion exclusion, matrix diffusion and surface hydrology).”