Posiva models groundwater chemistry in Olkiluoto bedrock with supercomputers

Lasse Koskinen and Tiina Lamminmäki

Bedrock and groundwater modelling has been carried out in Olkiluoto on the site for the final disposal of nuclear waste already for four decades. The advancements in information technology now provide opportunities for further studies, i.e., even more precise modelling, allowing an analysis of the evolution of groundwater chemistry as a result of geological and climatic changes.

In March, Posiva launched the project "Reactive migration model for hydrogeochemistry" designed to utilise high-efficiency computation in the analysis of changes occurring in groundwater chemistry over a future timeline of up to a million years. The aim is to use the results of the modelling to specify the degree of filling of the tunnels to be excavated in Olkiluoto bedrock and to extend the leakage limits as a precondition for progress of excavation.

- The starting point of the bedrock studies in Olkiluoto has been to find technical solutions for final disposal which will not be affected by even major global changes. Our aim is to gain through this project an improved understanding of the groundwater chemistry in the bedrock of the final disposal site, explains Posiva's expert Lasse Koskinen.

Cooperation with a Barcelona-based specialist company Amphos21 is a part of the project. This company specialises in the processing of massive data flows as well as in numerical techniques. The expertise of Amphos21 is utilised in the design of final disposal also by SKB, the company responsible for the management of nuclear waste in Sweden.

- Our objective is to study the interactions between water and rock as well as microbiological reactions in the bedrock far into the future. The summary reaction of rock, water and microbes in the bedrock is highly complex, says chemist Tiina Lamminmäki from Posiva.

High computational power is needed due to the length of the modelled time span and the enormous size of some 70 cubic kilometres of the modelled area. The computer will have no shortage of things to compute, as every reaction between rock and water as well as processes transmitted by the microbes constantly change the situation.

- As far as we know, this is the most extensive modelling of groundwater chemistry ever conducted in the world. Posiva is a frontrunner in the final disposal of spent nuclear fuel, and because of this, challenges that nobody has ever had to meet before will be unavoidable, Mr. Koskinen says.

Supercomputing to be introduced next year

The first preliminary discussions went on in the autumn of 2020 and the preliminary study was conducted in the spring of 2021. The three-year project was launched in March 2022.

The actual modelling phase, which is to be launched in about a year's time, will utilise the most efficient supercomputers in the world. Globally, there are several high-efficiency computation platforms that are suited to Posiva's research.

For example, one of the fastest supercomputers in the world, LUMI (snow in Finnish), is about to reach its full potential in the Finnish town of Kajaani this year. The space requirement of the LUMI supercomputer equals about a tennis court and the equipment weighs almost 150,000 kilograms. It has a computational power of more than 1.5 million regular laptops, which is 552 petaflops, or more than 552 million floating point operations per second.

Other possible computation platforms include the MareNostrum computer in Barcelona and the scientific research centre Forschungscentrum Jülich in Germany.

- Before moving into a high-efficiency computation platform, we will be carrying out modelling tests over shorter time spans where lower computational power is sufficient. The computation platform to be used in the project will be selected later, explains Ms. Lamminmäki.

Research may lead to new business opportunities

The scientists emphasise that the project will strengthen Posiva's expertise in research and design of final disposal of spent nuclear fuel, and this will also open up new business opportunities.

- The results of the actual modelling project will naturally only serve final disposal operations in Olkiluoto. Once we understand how groundwater chemistry changes over hundreds of thousands of years, we will also know what kind of repository facilities and engineered barriers are optimal. It is the actual knowledge related to research methods and final disposal methods that may create new business opportunities, suggests Mr. Lamminmäki.

Modelling focuses on three time periods

The modelling to be conducted with high-power computers is divided into three phases. The first phase covers the time period from the last ice age till the present day, or about 10,000 years. The time period modelled at the second phase extends over a little more than a hundred years and includes the construction of the final disposal repository and the actual final disposal operations.

- At the third phase, studies are conducted into changes occurring in groundwater chemistry after the final disposal facility for spent nuclear fuel is sealed in approximately the 2120s. At this phase, the goal is to extend the modelling over such a long period of time that conditions appear to settle in the bedrock and the impact of major climatic phases can be predicted. We hope to gain an understanding of evolution over a period of about a million years, Ms. Lamminmäki says.

Important future milestones include the next ice age and the sea phase when Olkiluoto will be covered with water. The timing of the ice age is tied with the climate change sequence, but according to current views the glaciation stage will occur 50,000–380,000 years from now, depending on the concentration of greenhouse gases in the atmosphere.

- The computations executed with the supercomputer produce a vast amount of data allowing us to analyse the concentration of even individual ions. Numerical data can also be presented as a 3D animation for a visual analysis of the evolution in different parts of the bedrock at different times, Mr. Koskinen elaborates.

Text: Timo Sillanpää
Photo: Tapani Karjanlahti