Finland’s Posiva Oy has launched a project for modelling the groundwater chemistry in the bedrock of the site for the final disposal of used nuclear fuel far into the future using one of the most efficient computers in the world. Posiva, which is responsible for the final disposal of used nuclear fuel, is jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj. “This is probably the most demanding modelling of groundwater chemistry to date, anywhere in the world. The goal of the project is to verify the balancing of long-term safety with economic sustainability. This will open the way to industrial final disposal far into the future,” Posiva noted.

Bedrock and groundwater modelling has been carried out in Olkiluoto on the site for the final disposal of nuclear waste for four decades. The advancements in information technology now provide opportunities for 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.

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. Amphos21 expertise is sldo utilised in the design of final disposal by SKB, which is responsible for the management of nuclear waste in Sweden.

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.

Preliminary discussions took place in autumn 2020 and the preliminary study was conducted in spring 2021 before the three-year project was launched in March. The modelling phase, which will be launched in about a year, will utilise the most efficient supercomputers in the world. Globally, there are several high-efficiency computation platforms that are suitable, Posiva noted.

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 possibilities 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, explained Posiva chemist Tiina Lamminmäki. “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.” 

The modelling 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 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 used nuclear fuel is sealed in approximately the 2120s,” said Lamminmäki. “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.”

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 linked to climate change, but according to current opinion the glaciation stage will occur 50,000–380,000 years from now, depending on the concentration of greenhouse gases in the atmosphere.