Researchers from Sandia National Laboratories and partner national laboratories will compare their Geologic Disposal Safety Assessment software framework to the safety assessment software of international peers at a late-April workshop.
Sandia National Laboratories engineers Emily Stein, left, and Paul Mariner, discuss recent results from their Geologic Disposal Safety Assessment software framework for the international collaboration. The software modeled the spread of radioactive materials in a hypothetical spent nuclear fuel repository over 1 million years. (Photo by Rebecca Gustaf) Click on the thumbnail for a high-resolution image.
The Sandia-led Geologic Disposal Safety Assessment framework is a computer modeling system designed to answer critical safety assessment questions about future disposal options for spent nuclear fuel deep underground and the system of tunnels, containers and possible concrete-like barriers used to keep the radioactive material contained far from the surface and water sources, said Emily Stein, a Sandia manager overseeing 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 a flexible and intuitive simulation and analysis capability for investigating different deep geologic repository systems," Stein said. "It has to be flexible to look at different host rocks and the different processes that can occur in those different kinds of rocks. It has to work with different 3D-engineering designs. The DOE also wants it to be fairly straightforward to explain how various thermal, chemical, hydrological and mechanical processes were coupled in the model. There's all this different stuff that goes on underground and the DOE wants to be able to think about the connections between those processes in a way that is somewhat intuitive."
Sandia started working on the framework in 2012. Comparing the results from Sandia's software against the results from the safety assessment software of international peers will build confidence in the software and models, Stein said. The comparison could also highlight areas for improvement.
Importance of a flexible software framework
The U.S. has approximately 90,000 metric tons of spent nuclear fuel - uranium rods no longer used for producing electricity at nuclear power plants - stored at nuclear power plants across the country and this number will keep growing. While nuclear provides more than half of the carbon-free electricity in the U.S., a permanent solution to spent nuclear fuel is needed.
There is broad international scientific consensus that a geologic repository is the safest and most secure method of permanently disposing of this spent nuclear fuel, Stein said. Sandia's framework is designed to assess the safety of future geologic repositories and assist in licensing efforts. It can model everything from what happens to the spent nuclear fuel rods and the canisters that contain them, to the rock mere inches from the canisters or rock 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 lead Sandia engineer on the project. "The framework is a system of codes that we can use to build a total-system model of a potential repository for a safety assessment. It really comes down to probabilistic risk assessment, which is a way to carefully and methodically address all of the questions regulators and citizens have about a repository by accounting for the probabilities of various events that could cause the release of radioactive material."
The Geologic Disposal Safety Assessment framework is built upon two core Sandia software packages: PFLOTRAN and Dakota. PFLOTRAN 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 toolset because the primary thing we're concerned with for a deep geologic repository is the movement of radioactive atoms out of the repository."
Dakota is Sandia software that specializes in uncertainty sampling and sensitivity analysis, in other words determining which input values have the largest impact on the final results. Since no one can predict the future, Dakota assures researchers that the results are trustworthy, even if, for example, their estimate for the likelihood of a big earthquake occurring at the location over the next million years is uncertain. Laura Swiler, a Sandia computer scientist, has been instrumental in ensuring that the team has the sensitivity analysis tools needed for the safety assessment framework, Stein said.
"Uncertainty quantification calculations help us better characterize the repository system and also help us assess the significance of certain input values and processes that affect the final results," Mariner said.
Stein added that the framework is massively parallel and runs on supercomputers, which is important for modeling what happens at 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 improve the representation of how dry soil acts like a sponge, Stein said.
International competition improves confidence
In late April, 10 different teams of repository scientists from across the globe, including teams from Canada, Germany and Taiwan, will compare the safety-assessment models they developed for two different reference cases. These reference cases are hypothetical repositories in two different kinds of rock that would be relevant for every team. One reference case is for a repository in a salt formation. The other reference case is for a repository in crystalline rock. Each case comes with a set of key questions, which took about two years of discussion to fully define along with the details of the hypothetical repository.
Comparing the results of Sandia's software against the results from other teams' software will build confidence in the software and models, Stein said. Specifically, the teams will compare the results of their models to see if there is a consensus reached by multiple teams. Then the models that produced different results will be studied to try to determine why their results were different. This will allow repository scientists from across the globe to learn from one another and improve their models. This comparison is part of an international collaboration called Development of Coupled Models and their Validation against Experiments, or DECOVALEX.
