A study on the safety of nuclear waste isolation in claystone under coupled hydro-mechanical-seepage processes: An engineering geology perspective based on a generalized numerical model

Claystone formations, such as France’s Callovo-Oxfordian (COx) claystone, Switzerland’s Opalinus (OPA) claystone and Belgium’s Boom clay, are increasingly regarded as optimal candidates for deep geological disposal of nuclear waste. A critical aspect in evaluating the long-term safety of nuclear waste repositories within these formations is the evolution of fracture networks in the excavation damaged zone (EDZ) surrounding the repository tunnels, particularly regarding their potential role as pathways for fluid migration. Claystones, classified as intermediate geomaterials between soil and hard rock, exhibit unique characteristics. Previous studies on COx and OPA claystone (Wang et al., 2022 and 2024) have informed the development of a generalized model addressing the absence of suitable existing models to describe claystone deformation behavior. This model is applied to analyze hydro-mechanical behaviors, incorporating an additional deformation model ∆ ε added and evolving hydro-mechanical parameters (e.g., saturation degree S r , Young’s modulus E , tensile stress σ t , fracture energy G f , water capacity c, and relative permeability κ R ) as functions of capillary pressure P c . The numerical model effectively replicates the uniaxial and triaxial mechanical damage behavior of COx, OPA, and Boom claystones, accurately simulating the fracture sealing process during resaturation of damaged claystone and providing a comprehensive analysis of EDZ fracture evolution from tunnel excavation to backfilling with bentonite. Excavation-induced fractures that permit fluid seepage represent the primary risk to the containment of nuclear waste in claystone formations. Laboratory and field tests indicate that these fractures self-seal during resaturation, a finding corroborated by numerical simulations. Under stable geological conditions, claystone formations provide exceptional safety for nuclear waste disposal, serving as a reliable host rock. However, the 10,000-year storage span and extreme events like earthquakes may destabilize groundwater systems involving COx, OPA, Boom claystones, and their aquifers. Further research on nuclear waste disposal safety under such extremes is crucial for long-term isolation.