SKB Task Force GWFTS: Pragmatic Validation Using Predictive Modeling Exercises

SKB and several other nuclear waste management organizations have established the international SKB Task Force on Modeling of Groundwater Flow and Transport of Solutes (TF GWFTS) to support and interpret field experiments. Work is organized in collaborative modeling tasks. Objectives of the task force are to develop, test and improve tools for conceptual understanding and simulating groundwater flow and transport of solutes in fractured rocks. The ongoing Task 10 focuses on validation of such models. A pragmatic approach is chosen where predictive modeling is an important tool, which is built on:

Pragmatic validation consistent with the IAEA definition of "fit for purpose" validation considering the limited spatial and temporal scale of available characterization data.

Use of multiple conceptual and numerical models to quantify uncertainties/sensitivities due to underlying concepts, parameters and assumptions.

Confidence building considering model conditioning, calibration and rejection. Conditioning and calibration data and methods may be particularly important where site-specific predictions are based on limited/sparse data.

Sensitivity and uncertainty assessment of key parameters.

Progressive validation as additional data is collected, guided by the results of previous data limitations and model studies.

Robust model audit to identify and evaluate assumptions and limitations to ensure transparency.

Prediction-outcome exercises to evaluate whether a model is an adequate representation of the real system being modelled by comparing model predictions with in-situ and laboratory observations.

In the first validation exercises, data from a rock block extracted from the Flivik quarry in Sweden are used. The objectives of these exercises are:

Development of concepts and models for flow and transport at the single fracture scale.

Consideration of the importance of hydro-mechanical coupling (normal loading only) on flow and transport.

Development of modelling approaches for prediction of:

Flow and transport in single fractures.

Upscaled fracture properties from borehole to deposition hole scale.

Building a starting point for pragmatic validation, i.e., a prediction-outcome exercise.