An approach to the assessment of high-level radioactive waste containment - II: radionuclide releases from an engineered barrier system

This paper is the second in a series describing the models used in the Engineered Barrier System Performance Assessment Code (EBSPAC) to represent processes that govern the failure of waste packages (WPs) and the release of radionuclides from the engineered barrier system (BBS). These models are specifically adapted to the US Department of Energy (DOE) WP design, adopted in 1996, for the proposed high-level radioactive waste (HLW) repository at Yucca Mountain (YM). The design consists of a double-wall overpack composed of two concentric containers of different metallic materials in a horizontal drift emplacement. EBSPAC was developed to determmistically evaluate the performance of the engineered barriers and to be used as the source term module in the Center for Nuclear Waste Regulatory Analyses (CNWRA)/Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) code. EBSPAC has two distinct parts. The part dealing with the radionuclide release subsequent to WP failure is the focus of this paper in which various models (i.e. dry-air oxidation and aqueous dissolution of spent fuel (SF), gaseous and aqueous release of radionuclides) are presented, whereas modeling of the WP failure is described in a companion paper. An example problem is presented to illustrate computational results obtained with the code analyzing the influence of several critical input parameters for the source term related to the repository and EBS designs and resulting environmental conditions. The source term calculations are confined to the radionuclides being released just outside of the WP. Both gaseous and aqueous release calculations are performed using models in which radionuclide decay, in-growth of daughter products in the chains, degradation process of SF, temporal variation of inventory in the WP, and spatial variations in the properties of the surrounding material are included. The degree of complexity varies from model to model as necessary simplifications are made, while ensuring conservatism.