null On nuclear waste disposal computationally

Posiva's final disposal solution Onkalo shown in the landscape. Source: Posiva.

On nuclear waste disposal, computationally

CSC and Posiva Ltd have collaborated to study the permafrost-evolution scenario in the future in Olkiluoto area, where the world's first nuclear waste disposal site is currently being mined. CSC offered to Posiva Puhti supercomputer’s computation capacity and Elmer software and gave consulting services to CoSci LLC, a US-based company working for Posiva. Posiva is in the process of applying for a licence for final disposal at the turn of the year and hopes to be able to start disposing of nuclear waste in the 2020s.

Posiva Ltd. is an expert organization in environmental technology, established in 1995 to manage the disposal of its owners' spent nuclear waste. The company is specialized in responsible final disposal solutions to enable the production of nuclear electricity. Company is located in Olkiluoto, Finland, where Posiva has a final disposal facility, called Onkalo ®, which is excavated deep in the bedrock. 

Finland is a pioneer in nuclear waste management. Posiva is about to submit a licence to dispose of high-level nuclear waste in bedrock, being the first organization to start disposing of nuclear waste. Posiva's final disposal solution is the result of decades of multidisciplinary research, requiring extensive expertise. The company works together with numerous Finnish and foreign expert organisations from a multitude of fields, and commissions studies related to nuclear waste management from universities and other institutions of higher education as well as from research institutes and consulting businesses. 

Estimated depth of permafrost between 54 021 and 59 021 AD. Image: Thomas Zwinger, CSC.

Diverse collaboration

Collaboration between CSC and Posiva started in 2015 and has been since really diverse. Collaboration began with a dissertation research by Juha Hartikainen (PhD, Aalto University). The topic of his thesis was “Continuum thermodynamic modelling of porous medium with application to ground freezing” at Aalto University and in order to apply the model to investigations of large-scale repositories, an implementation able to utilize High Performance Computing (HPC) capacities was needed. Hartikainen graduated in 2018. 

In this phase the collaboration was about implementing an existing theory of coupled permafrost with basal hydrology in Elmer, an open source multi-physics simulation software mainly developed by CSC. The development of Elmer started 1995 in collaboration with Finnish Universities, research institutes and industry. The code-base has been maintained and renewed since that time.

Initially, the cooperation focused specifically on the exploitation of Elmer. CSC’s contribution was to adapt Elmer to the abovementioned model and to enable scalable parallel implementations. After completing the model implementation, computing capacity was reserved from Puhti supercomputer. 

Dr. Denis Cohen from CoSCI LLC, in collaboration with Thomas Zwinger from CSC, carried out the simulations of permafrost-evolution scenario at the Olkiluoto nuclear waste repository. In addition, CSC gave consulting services to CoSCI LLC in order to pre-process and conduct those simulations and apply also necessary corrections in the code.

The primary question of the investigation was, to what extend permafrost is able to reach the repository in a timescale from now to several hundred thousand years into the future. This model was set up in three dimensions, covering several tens of kilometres around the repository reaching into a depth of about 10 km into the bedrock.

– Without the existing framework of Elmer, it would have been virtually impossible to implement such a versatile hydro-thermal permafrost model for large parallel computations in such a short time, notes Senior Applications Specialist Thomas Zwinger from CSC.

– This meant a large parallel run with several million of cells in Elmer. To the best of our knowledge, no other software can deliver on a coupled hydro-permafrost simulation on that scale, Thomas Zwinger adds. 

Research and development work have been essential for the final disposal of nuclear waste. Permafrost and its extension to the disposal depth is an area of concern, because at its deepest, permafrost can extend for several hundred metres into the rock, such as in Siberia. 

– The main point in CSC’s and Posiva’s collaboration was to investigate under which circumstances (mainly climatic ones), the repository could be affected by permafrost in the future. Based on these results, however, it appears that under future climate conditions permafrost at Olkiluoto will not extend to the final disposal depth, says Anne Kontula, Head of Posiva's Safety Assessment Programme. 

3D block view at AD 63,223 during maximum permafrost condition of salinity isosurface yc=2.0 g/l, showing groundwater flow vectors along the isosurface (a) and three vertical cross sections (b). Isosurface is colored according to vertical groundwater flux and vectors according to the magnitude. Image: Thomas Zwinger, CSC.

Long-term research collaboration has enabled an operating model in which assessment of the final disposal of nuclear waste could be significantly improved. Posiva's business shows remarkable innovation, where climate and environmental challenges have been turned into opportunities. The intention is that the final disposal of nuclear waste will begin in Finland in Olkiluoto in the 2020s.

Collaboration between CSC and Posiva is very fruitful: Posiva gets reliable and powerful HPC resources with expert support. And CSC is able to contribute with its software development and HPC abilities in cooperation with scientists and engineers from other institutions to a very important solution for the safety of all people in Finland.

– Also worth to be mentioned is, that throughout Posiva, CSC have gained good contact to national and international institutions that in future could be potential cooperators in this field of science and engineering, Zwinger continues.

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Sofia Mänty and Tommi Kutilainen