An Adjoint Sensitivity Model for Radionuclide Transport in Heterogeneous Discrete Fractured Porous Formation in Steady‐State Regime

Abstract

Nuclear waste programs have investigated the feasibility of siting repositories in deep formations to ensure the safe long-term storage of radioactive waste. The ultimate decision to construct and operate a repository is made after many years of site characterization and performance assessment activities. Decision makers rely on models to ascertain the expected performance of the geosystem as a barrier to offsite transport of radionuclides (RNs). Given the reality that the near-field and far-field hydraulic properties which describe the geosystem can never be known with certainty, the propagation of parameter uncertainty to the safety calculations is imperative in order to inform decision makers of the risk of a site under consideration. Nuclear waste isolation programs in the United States (Waste Isolation Pilot Plant (WIPP), https://www.wipp. energy.gov), Canada (Canadian Nuclear Safety Commission, https://nuclearsafety.gc.ca), Europe (France (ANDRA: https://www.andra.fr), Belgium (ONDRAF: https://www.ondraf.be), Switzerland (NAGRA: https://www.nagra.ch), United Kingdom (Radioactive Waste Management: https://rwm.nda.gov.uk)) and the Nordic Countries (Sweden (SKB: https://www.skb.com), Finland (Posiva Oy: https://www.posiva.fi)) have characterized sites in a variety of geologic formations with low permeability host rock, for example, clay, salt, granite, and volcanic tuff. The presence of fractures in a host rock is an important factor for the movement of RNs by diffusion and advection and therefore to the performance of the host rock as a geologic barrier to offsite transport. Moreover, fractures and/or micro and macro cracks can also develop in concrete structures which line waste repositories as a result of interactive physical, chemical, and mechanical processes (Neville, 1995;Page & Page, 2007;Seetharam et al., 2014). The effect of fractures on the release of RNs depends on interactions between fracture characteristics, porous matrix properties, and location of radioactive wastes. Fractures may accelerate the release of RNs from the subsurface waste repositories to the overburden due to their high permeability and porosity compared to those of the surrounding rock formations. On the other hand, exchange between fracture and matrix allows the diffusion of RNs from