1992 Conference Proceedings

Simulation of two-phase carbon-14 transport at Yucca Mountain, Nevada

M.D. White, M.D. Freshley, P.W. Eslinger
Pacific Northwest National Laboratory, Richland, Washington

In Proceedings of Solving Ground Water Problems with Models, Water Well Journal Publishing Company, Dublin, Ohio, 1992.

Abstract

In support of Pacific Northwest National Laboratory's (PNNL's) preliminary total system performance assessment of the proposed high-level nuclear-waste repository at Yucca Mountain, transport of carbon-14 (C¹⁴) in the unsaturated zone was numerically modeled with the Multiphase Subsurface Transport Simulator (MSTS). Total system performance assessments are being conducted to estimate potential cumulative releases and doses from radionuclides being transported through different pathways to the accessible environment from the proposed waste repository. Transport of radionuclides in the gaseous and liquid phases are pathways through which some of the inventory in the proposed repository could reach the accessible environment. Carbon-14 transport in the unsaturated zone at Yucca Mountain was estimated with MSTS by considering two-phase diffusion, advection, phase partitioning, and radioactive decay. Transport results were based on a two-dimensional physical and hydrogeological system that represented an east-west cross section through Yucca Mountain. MSTS solves the nonlinear, partial-differential, conservation equations for water mass, air mass, and thermal energy with an implicit (i.e., forward time-differenced), finite-difference solution scheme. The nonlinearities in the governing partial-differential equations and corresponding finite difference forms are converted to linear form by the Newton-Raphson iteration technique. To compute species transport, MSTS solves a dilute species concentration conservation equation, which assumes that the species may be modeled as a passive scalar with respect to the other governing conservation equations. Liquid- and gas-phase transport within the unsaturated zone were assumed to be driven by surface water recharge and radioactive decay heat generation within the proposed repository. Carbon-14 source rates from failed repository waste canisters were estimated from the source term modeling subtasks associated with PNL's total system performance assessment of the proposed Yucca Mountain repository. Simulation results include estimates of liquid, gas, heat and C¹⁴ transport within the unsaturated zone at Yucca Mountain. Predictions of C¹⁴ distributions surrounding the proposed nuclear waste repository within Yucca Mountain and a brief description of the thermal-hydrogeologic computer code MSTS are presented.