BLT-EC (Breach, Leach Transport, and Equilibrium Chemistry), a Finite-Element Model for Assessing the Release of Radionuclides from Low-Level Waste Disposal Units: Background, Theory and Model Description

摘要

Performance assessment models typically account for sorption and dissolution-precipitation by using an empirical distribution coefficient (Kd) that combines the effects of all chemical reactions between solid and aqueous phases. There is an increasing awareness that performance assessments based solely on empirically based Kd models may be incomplete, particularly for applications involving radionuclides having sorption and solubility properties that are sensitive to variations in the in situ chemical environment. To accommodate such variations and to assess impact on radionuclide mobility, one must model radionuclide release, transport, and chemical processes in a coupled fashion. This modeling was incorporated into the two-dimensional, finite-element, computer code BLT-EC which can predict container degradation, waste-form leaching, and advective-dispersive, multispecies, solute transport. BLT-EC accounts for retardation directly by modeling the chemical processes of complexation, sorption, dissolution-precipitation, ion-exchange, and oxidation-reduction reactions. Herein the authors: (1) describe in detail physical and chemical processes that control the release and migration of radionuclides from shallow land LLW disposal facilities; (2) formulate the mathematical models that represent these processes; (3) outline how these models are incorporated and implemented in BLT-EC; and (4) demonstrate application of BLT-EC on sample problems.