Multiple Continuum Approach to Modeling Radionuclide Transport in Fractured Networks

Matrix diffusion coupled with sorption is currently considered one of the most important radionuclide retardation mechanisms in crystalline rock (SKBF 1983). Traditional discrete fracture models implementing matrix diffusion can be computationally expensive and/or only applicable to simplified radionuclide transport problems. Upscaling to a continuum model can reduce computational burden, but models based on only a primary continuum neglect fracture-matrix interaction. PFLOTRAN, a subsurface flow and reactive transport code, simulates a secondary continuum (matrix) coupled to the primary continuum (fracture) modeled as a disconnected one-dimensional domain using a method known as the Dual Continuum Disconnected Matrix (DCDM) model (Lichtner, 2000). This model is implemented as part of the multiple continuum transport capability in PFLOTRAN. Since secondary continua are isolated from one another, the formulation for the secondary continuum equations is embarrassingly parallel, making PFLOTRAN's multiple continuum model ideal for complex full-scale crystalline repositories using high performance computing.

This work presents several benchmarks to compare the multiple continuum model to analytical solutions and a large-scale test problem consisting of deterministic and stochastic fractures in a one cubic km domain modelling advection and diffusion of a conservative tracer with diffusion of the tracer into the rock matrix. The tracer was modeled using two different methods in sequence: first, with dfnTrans particle tracking software (Hyman et al. 2015) (a DFN representation), and second, with the multiple continuum model in PFLOTRAN. To simulate transport in PFLOTRAN, the fractures were upscaled via a Python script mapDFN (Stein et al., 2017). We find that the DCDM representation of the upscaled fracture network produces results comparable to the DFN and analytical solutions where available, verifying this method. The multiple continuum (DCDM) model was then applied to a fractured domain considering radionuclide isotope sorption, partitioning, decay, and ingrowth via the Used Fuel Disposition (UFD) Decay Process Model in PFLOTRAN.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-9390 A