2021:19 Importance Sampling Time-Domain Random Walk (ISTIDRAW) Version 1.0 User Guide A Code for Radionuclide Transport in Pathway Networks

SSM perspective

Background

To support regulatory review of post-closure safety assessments of geological disposal systems it is of importance to possess regulatory independent modelling capacity. Typically, SSM’s independent modelling includesperforming simple scoping calculations, utilizing the implementer’s own models with other equation solvers, or defining alternative conceptual models.

In order to model radionuclide transport in safety assessments, the Swedish Nuclear Fuel & Waste Management Co. (SKB), has used timedomain particle tracking (TDRW) methods, algorithms that e.g. include the flexibility to account for variability and uncertainty in transport parameters. SSM has, to develop expertise in TDRW methods and to possess independent modelling tools in this respect, tasked Southwest Research Institute with developing a radionuclide transport model based on TDRW concepts in MATLAB®. The resulting code is titled ISTIDRAW (Importance Sampling Time Domain Random Walk). The following report includes a description of the model as well as a user guide for the developed computational tool.

Results

Southwest Research Institute has developed a code for radionuclide transport in pathway networks, Importance-Sampling Time-Domain Random Walk (ISTIDRAW) Version 1.0. The code offers an independent modelling tool for SSM to perform radionuclide transport analyses, to check radionuclide transport computations performed by SKB, to evaluate limitations and nonstated assumptions of numerical methods, to explore the importance of alternative release scenarios with e.g. different representations of variability and uncertainty, and develop an understanding of the strengths and limitations of TDRW methods.

Relevance

The outcome of the project rendered a computational exploratory tool that SSM can use for independent modelling in future reviews of safety assessments in e.g. license applications for geological disposal facilities for radioactive waste.

Need for further research

The set of algorithms derived in the present project to sample trajectories in channel networks will, within the framework of a recently initiated research project (SSM2020-2460), be developed in order to e.g. further simplify the radionuclide transport pathways. The sampling algorithms will be interfaced with channel networks developed by researchers at Uppsala University (SSM2019-1201), aiming to further assess the long-term implications of channel networks in safety assessments.