Influence of Fracture Heterogeneity on Rock Deformation and Failure (INFORM): A Mechanics-based Multi-scale Framework for Radioactive Waste Disposal

Nuclear power is low-carbon and green energy. It presently provides about 10% of the world's electricity and 20% of the UK's electricity, contributing enormously to global Net Zero emissions. Nuclear power will continue to play an important role in the global transition to a low carbon economy. However, one major disadvantage of nuclear power is that its generation process produces radioactive waste that can remain hazardous for hundreds of thousands of years. Over the past more than 60 years' utilisation of nuclear power in the UK and worldwide, many radioactive wastes have accumulated, most of which are stored temporarily in storage near nuclear power plants. It is vital for us to deal with the waste to protect human health and the environment. A global consensus has been reached in this area, that is to isolate radioactive waste that is incompatible with surface disposal permanently in suitable underground rock formations (i.e., host rocks) by developing a geological disposal facility (GDF). As also set out in the 2014 White Paper, the UK Government is committed to implementing geological disposal, with work on developing this led by Radioactive Waste Management Ltd (RWM).

Developing a GDF relies on a stable rock formation to ensure mechanical stability and barrier function of host rocks. It is therefore essential to understand factors that influence the integrity of rocks. This is challenging partially because of the complexity of rock fractures that are widespread in the Earth upper crust. Although rock mechanical behaviour has a long record of study, attempts to understand the role of fractures on rock deformation still has unresolved issues. For example, natural rock fractures are often dealt with crudely; almost all previous studies of this problem assume rock fractures to be continuous, with zero or very small cohesion that can be neglected. However, it is almost a ubiquitous feature that natural rock fractures in the subsurface are incipient and heterogeneous, with considerable tensile strength and cohesion. This is either due to secondary minerals having recrystallised, bonding fracture surfaces together, or due to rock bridges.

This INFORM project will focus on mineral-filled fractures (i.e., veins) that are frequently seen in the subsurface but often ignored or less researched so far. The aim of INFORM is to increase confidence in the design, construction, and operation of GDFs, by developing a mechanics-based multi-scale framework to understand the influence of fracture heterogeneity on the integrity and deformation behaviour of rocks across scales. The framework will integrate imaging analysis, laboratory experiments, numerical modelling, and field observations, to (1) determine factors contributing to fracture heterogeneity across scales, (2) understand the shear and triaxial deformational behaviour of veined rocks considering natural fracture geometry and heterogeneity, and (3) develop a field-scale model for repository structures considering fracture heterogeneity. Unlike most previous studies, which have focused on the influence of mechanical fractures on rock behaviour, INFORM will for the first time investigate the influence of natural veins, and will consider and implement these observations in the modelling of veined rock behaviour applied to a GDF.

INFORM will "inform" a wide range of audiences with new insights through correlating micro-scale observations and macro-scale deformation of heterogenous veined and fractured rocks. This will be possible with the strong support of our academic and industrial partners (RWM, UK; Jacobs, UK; Northeastern University, China; GFZ, Germany; Stanford University, USA) and the help of our well-designed outreach and publication plans. INFORM will lead to a more accurate and reliable examination of fracture heterogeneity, which will not only directly benefit GDF R&D, but also broader rock engineering applications (e.g., tunnelling, cavern construction).