Earthquake-Induced Shear Displacements and Transmissivity Changes in 3D Fracture Systems: Implications for Long-Term Safety Assessment of Nuclear Waste Repositories

Abstract

ABSTRACT: During the long service period of nuclear waste repositories, large earthquakes may occur around the repository site and coseismically trigger the shear displacement of secondary fractures intersected with waste canisters. The induced irreversible shear of natural fractures may further cause significant dilation and transmissivity enhancement, favoring the transport of leaked radionuclides into the groundwater system. Thus, it is of central importance to quantify the shear displacement and transmissivity variation in fracture systems during potential earthquake scenarios. Based on the detailed site characterization data of a planned repository at Forsmark, Sweden, we develop a three-dimensional seismo-mechanical model based on the 3Dimensional Distinct Element Code (3DEC). We consider explicitly a seismogenic fault zone and its surrounding fracture networks associated with a power-law length scaling and a Fisher orientation distribution. An earthquake with a magnitude of Mw = 5.62 caused by the rupture of the primary fault is modeled by simulating its transient rupture propagating radially outwards from a predefined hypocenter at a specified rupture speed, with the rupture dynamics controlled by a strength weakening law. We explore different plausible scenarios of fracture networks and model their shear displacements and transmissivity changes in response to the earthquake. Our simulation results have important implications for the long-term safety assessment of planned nuclear waste repositories in crystalline rocks. 1. INTRODUCTION The long-term safety assessment of nuclear waste repositories for up to one million years has been an important topic of research for many countries (e.g. US, UK, Germany, Sweden, Switzerland, China, etc.). For example, during the past decades, extensive research has been undertaken by the Swedish Nuclear Fuel and Waste Management Company (SKB) on the long-term security of a planned underground radioactive waste repository at Forsmark. Geological, geomechanical, seismological and hydrogeological characteristics of the repository site have been investigated based on drilled data, experimental tests and numerical simulations (Fälth and Hökmark 2006; Fox et al. 2007; Hökmark et al. 2010). The concept for spent nuclear fuel disposal applied by SKB is the KBS-3 system involving three protective barriers: the inner barrier is a copper canister with cast iron insert, the middle one is a bentonite clay buffer, and the outer one is the crystalline bedrock (Andersson et al. 2013). During the lifetime of up to one million years, the repository is likely to undergo one or two postglacial earthquake(s) with a moment magnitude ≥5, potentially threatening the integrity of the canisters by inducing shear slip of secondary fractures. If the shear displacement of a fracture that intersects with the canister exceeds 50 mm, a shear failure of the canister may happen (Falth et al. 2010). The earthquake-induced stress loading and unloading on fractures as well as the resulting dilation process could lead to a strong variation of fracture apertures and significant transmissivity changes in the fracture network (Olsson and Barton 2001; Lei et al. 2014; Lei et al. 2016). Then, the radionuclides escaping from canisters may enter the groundwater system through fractures associated with earthquake-induced increased transmissivity.