The distribution of seepage in the proposed repository will be highly variable due in part to variations in the spatial distribution of percolations. The performance of the drip shield and the backfill system may divert the water flux around the waste packages to the invert. Diversion will occur along the drift surface, within the backfill, at the drip shield, and at the Waste Package (WP) surface, even after the drip shield and WP have been breached by corrosion. The purpose and objective of this Analysis and Modeling Report (AMR) are to develop a conceptual model and constitutive properties for bounding the volume and rate of seepage water that flows around the drip shield (CRWMS M&O 1999c). This analysis model is to be compatible with the selected repository conceptual design (Wilkins and Heath, 1999) and will be used to evaluate the performance of the Engineered Barrier System (EBS), and to provide input to the EBS Water Distribution and Removal Model. This model supports the Engineered Barrier System (EBS) postclosure performance assessment for the Site Recommendation (SR). This document characterizes the hydrological constitutive properties of the backfill and invert materials (Section 6.2) and a third material that represents a mixture of the two. These include the Overton Sand which is selected as a backfill (Section 5.2), crushed tuff which is selected as the invert (Section 5.1), and a combined material (Sections 5.9 and 5.10) which has retention and hydraulic conductivity properties intermediate to the selected materials for the backfill and the invert. The properties include the grain size distribution, the dry bulk density and porosity, the moisture retention, the intrinsic permeability, the relative permeability, and the material thermal properties. The van Genuchten relationships with curve fit parameters are used to define the basic retention relationship of moisture potential to volumetric moisture content, and the basic relationship of unsaturated hydraulic conductivity to volumetric moisture content. The van Genuchten curve fit parameters were determined from a least squares fit to the measured Unsaturated Flow Apparatus (UFA) data for Overton Sand backfill and the crushed tuff invert. These constitutive properties are direct inputs to the Non-isothermal Unsaturated saturated Flow and Transport (NUFT) codes and characterize the constitutive properties for these materials within the Engineered Barrier System (EBS). Models are currently being developed for pitting and general corrosion of the drip shield. This AMR develops flow analysis methods (Section 6.2.4) to bound the diversion performance of the drip shield at different stages of degradation by corrosion. Further, this report also develops flow analysis methods for the conceptual model under the assumption of negligible corrosion of the drip shield (Section 5.12) as indicated by design studies (TBV-3808). The conceptual model can be used to bound the diversion performance of the drip shield. In addition, this report provides a simple bounding calculation for assessing flow through apertures within the drip shield (Section 5.4 to 5.8). The bounding calculation uses several assumptions (Sections 5.4 to 5.8). The flow relationship presented in Section 6.2.5 can be used in conjunction with a NUFT analysis of moisture potential around the DS to bound the flow through the drip shield. As an alternative to performing NUFT calculations to assess the partitioning of flow, an analysis approach based upon a closed-form analytical solution at isothermal temperature is developed in Section 6.3.
