ALBUQUERQUE, NM – Researchers from Sandia National Laboratories and partner national laboratories will compare theirs Software framework for geological disposal safety assessment at a workshop at the end of April on safety assessment software from international colleagues.
The Sandia-led Geological Disposal Safety Assessment Framework is a computer modeling system developed to answer critical safety assessment questions about future disposal options for spent nuclear fuel deep underground and the system of tunnels, casks and possible concrete-like barriers used to contain the radioactive material far from the surface and contain water sources, said Emily Stein, a Sandia executive who oversees the development of the framework. Work on this framework is supported by the Department of Energy’s Spent Fuel and Waste Science and Technology Campaign.
“The goal of the framework is to provide the DOE with a flexible and intuitive simulation and analysis capability to study various deep geological repository systems,” said Stein. “It needs to be flexible to look at different host rocks and the different processes that can happen in those different rock types. It needs to work with various 3D engineering designs. The DOE also wants it to be fairly easy to explain how various thermal, chemical, hydrological, and mechanical processes have been coupled in the model. There are all these different things going on underground, and the DOE wants to be able to think about the connections between these processes in a somewhat intuitive way.”
Sandia began work on the framework in 2012. Comparing the results from Sandia’s software with the results from international peers’ safety assessment software will increase confidence in the software and models, Stein said. The comparison could also identify areas for improvement.
Importance of a flexible software framework
The US has approx 90,000 tons of spent nuclear fuel – rods of uranium no longer used to generate electricity in nuclear power plants – being stored in nuclear power plants across the country, and that number will continue to rise. While nuclear delivers more than half of the carbon-free stream A permanent solution to spent nuclear fuel is needed in the US.
There is a broad international scientific consensus that a geological repository is the safest and most secure method of permanently disposing of this spent nuclear fuel, Stein said. Sandia’s framework was developed to assess the safety of future geological deposits and to assist in the licensing effort. It can model everything from what happens to the spent nuclear fuel rods and the canisters that contain them, to the rock that is just inches from the canisters, or rock that is miles away.
“All of us in the international nuclear community are in the same boat – we need a safe place for our waste,” said Paul Mariner, a Sandia senior engineer on the project. “The framework is a system of codes that we can use to create an overall systems model of a potential repository for a safety assessment. What really matters is a probabilistic risk assessment, which is a way to carefully and methodically answer any questions from regulators and citizens about a repository by considering the probabilities of various events that could lead to the release of radioactive material.”
The geological disposal safety assessment framework is based on two core Sandia software packages: PFLOTRAN and Dakota. PLOTRAN is open-source software, maintained and developed at Sandia and several other national labs, that models how chemicals react and how liquids and gases flow underground, Stein said. “This is an important set of tools because the most important thing we are dealing with in a deep geological repository is the movement of radioactive atoms out of the repository.”
Dakota is Sandia software specializing in uncertainty sampling and sensitivity analysis, ie determining which input values have the greatest impact on the final results. Since no one can predict the future, Dakota assures the researchers that the results can be trusted, even if, for example, their estimate of the likelihood of a major earthquake at that location in the next million years is uncertain. Laura Swiler, a Sandia computer scientist, was instrumental in ensuring the team had the sensitivity analysis tools needed for the security assessment framework, Stein said.
“Calculations to quantify the uncertainty help us to better characterize the repository system and also help us to assess the importance of certain inputs and processes affecting the final results,” said Mariner.
Stein added that the framework is massively parallel and runs on supercomputers, which is important for modeling what happens in a repository over a million years.
Recently, a Sandia team led by Michael Nole made some significant improvements to PFLOTRAN to speed up calculations of certain challenging processes and to improve how dry soil behaves like a sponge, Stein said.
International competition strengthens trust
At the end of April, 10 different teams of repository scientists from around the world, including teams from Canada, Germany and Taiwan, will compare the safety assessment models they have developed for two different reference cases. These reference cases are hypothetical repositories in two different rock types that would be relevant to each team. A reference case is a repository in a salt formation. The other reference case concerns a repository in crystalline rock. Each case contains a number of key questions that took about two years to fully define, along with the details of the hypothetical repository.
Comparing the results of Sandia’s software with the results of other teams’ software will increase confidence in the software and models, Stein said. In particular, teams will compare the results of their models to see if there is consensus among multiple teams. Then the models that produced different results are examined to find out why their results were different. This allows repository scientists from all over the world to learn from each other and improve their models. This comparison is part of an international collaboration called Development of coupled models and their validation against experimentsor DECOVALEX.
“The DECOVALEX initiative provides an important framework for repository science experts from around the world to test and improve simulation models important for assessing the safety of geological disposal,” said Jens Birkholzer, chair of the initiative and principal scientist at Lawrence Berkeley National Laboratories. “Seventeen partner organizations, 50 modeling teams and more than 100 simulation experts are currently involved in this global effort. Through a collaborative model comparison, DECOVALEX shows that long-term predictions of complex underground processes related to geological disposal can be achieved with a high degree of reliability.”
Specifically for the two DECOVALEX reference cases, the Sandia team extended the framework to model how salt mined from a salt-based repository and then replaced around waste containers can self-heal back into solid, impermeable blocks over millennia, it said Stone. They also refined their model of how water can flow around cracked crystalline rock and potentially move radioactive atoms within a geological repository. These two improvements are likely to be useful in answering safety assessment questions about future spent nuclear fuel disposal sites in the United States.
In the future, Stein and her team hope to add more machine learning algorithms to the framework to speed up the modeling of some complex processes without sacrificing accuracy or transparency, she said.
“This kind of comparison, where we all start with the same two problem descriptions and develop numerical models around them, is an important way to instill confidence in safety assessment models and the tools we use for the simulations,” says Stein. “We cannot compare this large million-year simulation with a real experiment of the same scale. It is a very important exercise.”
Other DOE national labs involved in the development of the framework and the DECOVALEX initiative include the Argonne, Idaho, Lawrence Berkeley, Lawrence Livermore, Los Alamos, Oak Ridge, and Pacific Northwest National Laboratories.
Sandia National Laboratories is a multipurpose laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the US Department of Energy’s National Nuclear Security Administration. Sandia Labs has significant research and development responsibilities in the areas of nuclear deterrence, global security, defense, energy technologies and economic competitiveness, with principal facilities in Albuquerque, New Mexico and Livermore, California.
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