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This document describes the development and use of the Integrated Flow Code (IFC), a numerical code and related model to be used for the simulation of time-dependent, two-phase flow in the near field and geosphere of a gas-generating nuclear waste repository system located in an initially fully water-saturated claystone (Opalinus Clay) in Switzerland. The development of the code and model was supported by the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettingen, Switzerland. Gas generation (mainly H{sub 2}, but also CH{sub 4} and CO{sub 2}) may affect repository performance by (1) compromising the engineered barriers through excessive pressure build-up, (2) displacing potentially contaminated pore water, (3) releasing radioactive gases (e.g., those containing {sup 14}C and {sup 3}H), (4) changing hydrogeologic properties of the engineered barrier system and the host rock, and (5) altering the groundwater flow field and thus radionuclide migration paths. The IFC aims at providing water and gas flow fields as the basis for the subsequent radionuclide transport simulations, which are performed by the radionuclide transport code (RTC). The IFC, RTC and a waste-dissolution and near-field transport model (STMAN) are part of the Integrated Radionuclide Release Code (IRRC), which integrates all safety-relevant features, events, and processes (FEPs). The IRRC is embedded into a Probabilistic Safety Assessment (PSA) computational tool that (1) evaluates alternative conceptual models, scenarios, and disruptive events, and (2) performs Monte-Carlo sampling to account for parametric uncertainties. The preliminary probabilistic safety assessment concept and the role of the IFC are visualized in Figure 1. The IFC was developed based on Nagra's PSA concept. Specifically, as many phenomena as possible are to be directly simulated using a (simplified) process model, which is at the core of the IRRC model. Uncertainty evaluation (scenario uncertainty, conceptualization uncertainty, parametric uncertainty) is handled by the outer shell of the PSA model; it is not further discussed in this report. Moreover, justifications for the inclusion or exclusion of FEPs as well as for certain simplifying assumptions are available or can be obtained using detailed process models and other supporting information. The IFC is both a numerical code and a model of a repository system. The numerical code is a modification of the multiphase, multicomponent simulator TOUGH2 (Pruess et al., 1999), as implemented within the iTOUGH2 (Finsterle, 2007abc) framework. The code modifications are mainly concerned with the implementation of relevant FEPs as outlined in Nagra (2007a, AN 07-115), as well as removal of processes and features that are not needed within the IFC; the modifications are summarized in Appendix A1. In addition, the IFC includes a model, i.e., a simplified representation of the repository system. Specifically, a computational grid was generated, which includes the emplacement tunnels for spent fuel, high-level wastes, as well as long-lived intermediate-level wastes. Moreover, the model represents engineered barriers (backfill, seals, plugs, etc.), various tunnels and other underground facilities, and includes a simplified representation of the geological structure, i.e., the host rock (including the excavation disturbed zone (EDZ) around the underground openings), confining units, local aquifers, and a highly transmissive zone. The IFC model was designed in close collaboration with Nagra. This report describes all functional requirements of the IFC and how they are implemented in the IFC. The input formats needed to invoke added modeling capabilities are documented. Finally, the IFC model grid is described, and results from a test simulation are presented

iTOUGH2-IFC: An Integrated Flow Code in Support of Nagra's Probabilistic Safety Assessment:--User's Guide and Model Description