Site-bond modelling of porous quasi-brittle media

Abstract

Geological repository designs employ a multi-barrier approach. The materials, which include wasteforms, backfill and host rock, are typically porous quasi-brittle. Mechanical damage (e.g. nucleation and growth of microcracks) can result in significant changes in permeability. A knowledge of how the permeability is affected is critical to accurate modelling of radionuclide transport. This work proposes a novel 3D lattice-type model for the damage evolution in such materials, referred to as the site-bond model. Its advantages over previous models are that the shape of the lattice cell is physically realistic and that any macroscopic elastic response can be represented, including those of cementitious and geological materials. Damage accumulates as bonds fail upon reaching prescribed failure strengths. These are dictated by a predefined pore size distribution. Concrete is used as a study material. It is demonstrated that the model can predict the macroscopic stressÀstrain response under unconfined tension and compression with emergent non-linearity due to damage evolution. Ongoing work on the prediction of permeability changes with damage is discussed. This is based on the interaction between the model proposed here and a lattice model of the pore space.

Microstructure-informed modelling of damage evolution in cement paste

“…In the site-bond model [20][21][22][23][24], a material volume is represented by an assembly of truncated octahedral cells, each of which has six square and eight regular hexagonal faces, as illustrated in Fig. 6a.…”

“…To overcome such limitations a site-bond model using a bi-regular lattice of truncated octahedral cells for elasticity has been recently proposed by Jivkov and Yates [21] and subsequently used to simulate the micro-crack population and damage evolution in concrete accounting for pore size distribution [20,22,23]. The microstructure of concrete was represented by truncated octahedral cells, which were regarded as the best choice for a regular representation of a solid.…”

Geological disposal of radioactive waste: underpinning science and technology

“…The first major section on wasteforms, containment and criticality begins with an introduction to package evolution and criticality research studies relevant to the UK disposal programme by Cristiano Padovani and co-workers of the NDA (Padovani et al, 2012). There follow papers which describe: waste container durability by Nick Smart (AMEC) and co-workers (Smart et al, 2012); the corrosion and expansion of grouted Magnox waste by James Cronin and Nicholas Collier (NNL) (Cronin and Collier, 2012); the durability of potential plutonium wasteforms under repository conditions by Guido Deissmann (Brenk Systemplanung) and co-workers (Deissmann et al, 2012), the chemical durability of vitrified wasteforms and the effects of pH and solution composition by Stephen Swanton (AMEC) and co-workers (Utton et al, 2012); three-dimensional imaging of inhomogeneous lithologies using X-ray computed tomography in the characterization of drill core from the Borrowdale Volcanic Group by Dirk Engelberg (The University of Manchester) and co-workers (Engelberg et al, 2012); the release of uranium from candidate wasteforms by Nicholas Collier (National Nuclear Laboratory) and co-workers (Collier et al, 2012); the development of criticality safety controls on intermediate-level waste packages by Tim Hicks (Galson Sciences Ltd) and co-workers (Hicks et al, 2012b); an experimental study to evaluate the effect of polymeric encapsulants on the corrosion resistance of intermediate-level waste packages by Robert Winsley (AMEC) and co-workers (Winsley et al, 2012); the site-bond modelling of porous quasi-brittle media by Andrey Jivkov (The University of Manchester) and co-workers (Jivkov et al, 2012); the break-up testing of waste-form materials by Martin Metcalfe (NNL) and co-workers (Metcalfe et al, 2012); and initial studies on the effects of radiation, thermal ageing and aqueous environments on the stability and structure of candidate polymeric encapsulant materials by John Dawson (AMEC) and coworkers (Dawson et al, 2012).…”

Measurement and modelling of reactive transport in geological barriers for nuclear waste containment

“…The proposal has added benefit that the constructed PNM can be paired directly to lattice models of the solid phase developed for analysis of damage evolution via micro-cracking. [19][20][21] This facilitates mechanistic investigations of combined mechanical-thermal-chemical-biological effects on diffusivity, which will be discussed further in the paper.…”