Posiva models the long-term safety of the disposal site for spent nuclear fuel using the LUMI supercomputer

Posiva Oy is the world’s leading final-disposer of spent nuclear fuel and is responsible for geological final disposal in Finland. Posiva’s final disposal solution is the result of decades of multidisciplinary development work, part of which involves underground facilities at ONKALO® in the bedrock of Olkiluoto. Posiva models the evolution of groundwater chemistry in the bedrock for at least 100,000 years as part of ensuring the long-term safety of final disposal. The time span of the final long-term safety assessment is up to one million years. The modeling utilizes, among other things, the LUMI supercomputer.

Posiva Oy and its subsidiary Posiva Solutions are part of the TVO Group. Posiva’s head office, main office, and ONKALO® final disposal facilities are located in Olkiluoto.

Spent fuel is packed in watertight, durable copper canisters for final disposal, which are placed approximately 430 meters deep in the rock. The canisters will remain safe for as long as their contents could pose a significant hazard to living organisms. Posiva employs approximately 90 people, of whom about 40 are involved in activities focusing on long-term safety and site investigation. One of the main tasks of this function is to prepare the safety case for the final disposal site.

– The safety assessment analyses, among other things, how the final disposal solution will withstand earthquakes, future ice ages lasting up to a million years, and the stress caused by continental ice. The safety case prepared in connection with the operating license application is first assessed by the Radiation and Nuclear Safety Authority, which submits its assessment to the Ministry of Economic Affairs and Employment for decision-making on the operating license, explains Tiina Lamminmäki, Chief Chemist at Posiva.

Supercomputer enables comprehensive groundwater chemistry modelling

Posiva has been conducting long-term safety analyses and modelling for a long time. It has collaborated with CSC on climate scenarios and molecular modelling, for example. In addition to the LUMI supercomputer located at CSC’s data center in Kajaani, Posiva has used the JURECA supercomputer in Jülich, Germany, and MareNostrum in Barcelona, Spain.

Modelling is constantly being developed. The ongoing OL-STAR project aims to consider all processes affecting the composition of groundwater that occur in the bedrock – both those related to water transport and various chemical reactions.

A supercomputer enables the combination of different sub-models into a single model. The model is extremely complex: there are many variables, the volume of the area under investigation, Olkiluoto island, is large (approximately 70 km³), and the modelling period is up to 120,000 years. The effects of climatic conditions are assessed beyond the next ice age, when Olkiluoto will once again be under the sea. This requires enormous computing capacity – in other words, a supercomputer.

– The model is first calibrated using changes in groundwater chemistry and flows that have occurred over the past 8,000 years, which have been determined through comprehensive research. Only then can we model the future, Lamminmäki explains.

European supercomputers in use

Lamminmäki’s colleague Lasse Koskinen from Posiva and partner Amphos21 Consulting S.L., a company of the RSK group set in Barcelona, proposed applying for high-performance computing capacity through the EuroHPC JU project call. In Europe, the LUMI, JURECA, and MareNostrum supercomputers are capable of this level of computing. Posiva obtained computing capacity from the LUMI supercomputer and currently also utilizes paid LUMI CPU capacity.

– Of course, we also wanted to use LUMI because of its enormous capacity and the support that is readily available to us. However, Posiva is prepared to use other computers if necessary, and we are currently also using the JURECA supercomputer, Lamminmäki says.

– Getting started with LUMI was straightforward, and we feel our expertise on using it has grown during the past two years. We received support from LUMI’s user support team, LUST, which together with our own IT support overcame all challenges, she notes.

Posiva uses the open source PFLOTRAN modeling program (control volume-based code), developed in the United States, to model groundwater in bedrock. No one has ever done such extensive modelling before, so the challenges are recognized.

– Our partner Amphos21 is constantly working on optimisation to ensure that the software works well in LUMI. We are also in constant contact with the PFLOTRAN software developers. We have also had to modify our own working methods, as our modelling is constantly evolving, Lamminmäki explains.

LUMI attracts international attention

The numerical computing work is managed in Barcelona, so partner Amphos21 has learned a lot and gained experience from the way LUMI handles computing projects. The JURECA supercomputer in Jülich also has capacity reserved for Posiva’s project that can be quickly put into use. However, due to the competitive edge the CSC high performance computing infrastructure offers, Posiva is looking forward to continuing to use LUMI.

– LUMI’s enormous computing capacity has been extremely useful to us. We know that it is still more efficient than other supercomputers we have used – among the top 10 of the most powerful supercomputers of the world. There is international interest in the use of LUMI, and we have been able to present our modeling at the international CMWR2024 conference held in Arizona, for example, says Lamminmäki.

– Posiva Solutions sells its expertise and modeling skills internationally. LUMI has opened up new opportunities for this, and increased interest in the company among high-performance computing and nuclear waste management communities, she continues.

Focus on sustainable and long-term development

– Sustainability is at the heart of Posiva’s carbon-free energy strategy, and the basis for all our work. All our operations are considered from a sustainability perspective, so it is extremely important for us. Our tools and product development must also operate on its terms, says Lamminmäki.

Posiva’s modelling is not optional – it’s essential. As the work continues, so does the need for high-performance computing. Posiva’s primary vision is to continue using LUMI. The company is also applying for resources via EuroHPC JU calls to secure consistent availability of HPC resources, even if the LUMI resources are not guaranteed.

– Our model also needs to be further calibrated from the current 8,000 years to 100,000 years and beyond, up to 120,000 years. This means that there will be computational needs well into the future, Lamminmäki sums up.

Author: Tiina Leiponen, CSC – IT Center for Science

Images: Posiva Oy