Description

The performance assessment (PA) task of DECOVALEX-2027 involves comparison of the models and methods used in post-closure performance assessment of deep geologic repositories. The task will center around building upon two generic disposal concepts for commercial spent nuclear fuel (SNF) developed in DECOVALEX-2023(D2023): a reference case describing a generic repository in a fractured crystalline host rock and a generic repository in domal salt formation. Although a direct comparison cannot be made between simulations of a crystalline repository and simulations of a salt repository, it is expected that foundational aspects will be transferable between concepts, for instance, methods of coupling process models, propagating uncertainty, or conducting sensitivity analysis.

The primary objectives of the performance assessment (PA) task are to build confidence in the models, methods, and software used for PA of deep geologic repositories, to bring to the fore additional research and development needed to improve PA methodologies, and to cultivate awareness of international PA practices among participating countries and teams. The objectives will be accomplished through expanding the analyses of the D2023 results completed by the participating teams in their PA frameworks, including: (1) benchmark and process models (e.g., diffusion and salt creep); (2) deterministic simulation(s) of the entire PA model for defined reference scenario(s); (3) probabilistic simulations of the entire PA model; and (4) uncertainty quantification (UQ) and sensitivity analysis (SA) methods/results for probabilistic simulations of defined reference scenario(s).

Reference Cases

Generic repository in fractured crystalline host rock

The DECOVALEX-2023 crystalline reference case uses the KBS-3V emplacement concept developed for the Swedish and Finnish repository programs (Pettersson and Lönnerberg 2008). The KBS-3V concept is developed for a repository mined at a depth of approximately 500 m in sparsely fractured crystalline rock. Copper canisters, each containing a nominal inventory of 4 pressurized water reactor (PWR) assemblies, are emplaced within rings of compacted bentonite in vertical deposition holes beneath the floor of a deposition tunnel, and tunnels are backfilled. Processes of interest include radionuclide transport in fractured rock, influence of the stress field on fracture transmissivity, and coupling between fracture flow, bentonite erosion, and canister corrosion.

Generic repository in salt host rock

The DECOVALEX-2027 reference case will continue to develop and analyze results from D2023 reference case for a generic repository in a domal salt formation. Interested teams will participate in the comparison of forward models of increasing complexity, which seeks to understand the uncertainty introduced by modeling choices (model dimensionality, model fidelity, alternate models, methods of coupling) and uncertain model inputs. Model inputs of interest include characterization of the natural and engineered barriers, key Features, Events, and Processes (FEPs), conceptual models and parameterization, and choose individual process models for benchmarking and comparison (e.g., salt creep, crushed salt reconsolidation, diffusive transport, thermal conduction). Reference cases previously published in the United States (bedded salt), Germany (bedded and domal salt), and the Netherlands (domal salt) provided the basis for the D2023 and updated D2027 salt reference cases, including Sevougian et al. (2016), Bollingerfehr et al. (2018), and Prij et al. (1989).

Approach

The following outline structure is proposed for the salt reference case:

• Step 0S: Develop numerical models from the D2023 Task Specification. Returning teams update/improve D2023 model if needed. Benchmark cases for diffusion and salt creep.
• Step 1S: Define parameter uncertainty and create workflow for sample generation, propagation, and analysis.
• Step 2S: Define creep closure and diffusion models, propagate the “model form” uncertainty through the simulation, and perform analysis. Parameter sensitivity analysis, using traditional methods and variance-based methods which require surrogate model construction.
• Step 3S: Generate samples for creep closure and diffusion models, propagate the “model form” uncertainty through the simulation, and perform analysis. Generate samples for creep closure and diffusion models, propagate the “model form” uncertainty through the simulation, and perform analysis. Definition of constitutive model types (e.g. different creep closure models) will be specified and parameterized for sensitivity analysis (SA). Perform SA for various model form choices.
• Step 4S: Combine closure model and diffusion model uncertainty with parameter uncertainty, perform combined analysis. Combine model form uncertainty with parameter uncertainty, perform combined sensitivity analysis.

The following outline structure is proposed for the crystalline reference case:

• Step 0C: Develop numerical models from the D2023 Task Specification. Returning teams update/improve D2023 model if needed.
• Step 1C: Establish and simulate the D2027 reference case and benchmarks. Consider benchmark cases for stress effects on fracture flow and one for well capture.
• Step 2C: Enhance the reference case. Consider adding effects of stresses, a new canister failure scenario, and addition of a well.
• Step 3C: Establish a set of uncertain inputs and UA methods and propagate them in the reference case.
• Step 4C: Establish a set of SA methods and metrics and use them to analyze reference case results.

Participating Groups

Crystalline reference case:

• United States Department of Energy (DOE – USA)
• Canadian Nuclear Safety Commission (CNSC - Canada)
• Nuclear Waste Management Organization (NWMO – Canada)
• Gesellschaft für Anlagen- und Reaktorsicherheit (GRS – Germany)
• Federal Institute for Geosciences and Natural Resources (BGR – Germany)
• Korea Atomic Energy Research Institute (KAERI – Republic of Korea)
• Taiwan Power Company (TPC – Taiwan)
• Swedish Radiation Safety Authority (SSM – Sweden)

Salt reference case:

• United States Department of Energy (DOE – USA)
• Gesellschaft für Anlagen- und Reaktorsicherheit (GRS – Germany) with the Federal Company for Radioactive Waste Disposal (BGE – Germany)
• Centrale Organisatie Voor Radioactief Afval (COVRA – Netherlands)
• Federal Office for the Safety of Nuclear Waste Management (BASE – Germany)
• Quintessa (United Kingdom)

Further Information

For further information, please contact the task leaders:

• Tara LaForce and Rick Jayne for salt.
• Paul Mariner and Rosie Leone for crystalline.

References

1. Bollingerfehr, W., Bertrams, N., Buhmann, D., Eickemeier, R., Fahland, S., Filbert, W., Hammer, J., Kindlein, J., Knauth, M., and Wenting, L., 2018. Concept developments for a generic repository for heat-generating waste in bedded salt formations in Germany. Synthesis Report (No. BGE TEC 2018-13). BGE TECHNOLOGY GmbH
2. Pettersson, S. and B. Lönnerberg 2008, 16-18 June 2008. Final Repository for Spent Nuclear Fuel in Granite - The KBS-3V Concept in Sweden and Finland. Paper presented at the International Conference Underground Disposal Unit Design & Emplacement Processes for a Deep Geological Repository, Prague.
3. Prij, J., van Dalen, A., Englund-Borowiec, G., Glasbergen, P., de Haas, J.B.M., Jong, C.T.J., de Jong, E.J., Köster, H.W., Nijhoff-Pan, I., Roodbergen, H.A., Slagter, W., van Weers, A.W., and Znastra, D.A., 1989. Safety Evaluation of Disposal Concepts in Rock Salt. Final Report (IL 369; OPLA 89-08)
4. Sevougian, S. D., E. R. Stein, M. B. Gross, G. E. Hammond, J. M. Frederick, and P. E. Mariner 2016. Status of Progress Made Toward Safety Analysis and Technical Site Evalutations for DOE Managed HLW and SNF. SAND2016-11232R. Sandia National Laboratories, Albuquerque, NM.