)tc- W( Application Of'geother/vt4 to Assess Groundwater Transients in a High Level Radioactive Waste Repository

The GEOTHER/VT4 computer code is an improved version of the GEOTHER computer code. The GEOTHER/VT4 code was applied to a two-dimensional simulation of a waste package in a high-level waste repository to predict the thermal-hydraulic environment where steam formation may occur. The thermal-hydraulic environment is important for assessing waste package container corrosion and packing material swelling behaviors, and evaluation of the near-field geochemical conditions. The waste package was assumed to be situated in the Cohassett flow of the Hanford Site, located in Washington State. This analysis provides the first assessment of the duration and size of a two-phase environment for the waste package. 1.0 INTRODUCTION The GEOTHER/VT4 code has been developed at the Pacific Northwest Laboratory (PNL) in support of Basalt Waste Isolation Project (BWIP) effort. This code is based on the GEOTHER code (Faust and Mercer 1983) but has been improved to make it suitable for the high-level radioactive waste repository applications. The objective of this work is to assess thermal-hydraulic conditions resulting from nuclear heating effects in the high-level radioactive waste package and its surroundings where a twophase (steam and water) condition may occur. The waste package is an engineered barrier used to isolate high-level radioactive waste from the accessible-environment. The current BWIP waste package design (see Fig. 1) consists of the radioactive waste, a container, and packing material. The groundwater and thermal conditions provide decision-making information for waste package container corrosion and packing material swelling testing program, and evaluation of the nearfield geochemical conditions. 2.0 THE GEOTHER/VT4 CODE The GEOTHER/VT4 code is a three-dimensional, two-phase groundwater simulation code. The code is based on a general mathematical model of a waste repository system that describes the three-dimensional flow of groundwater (vapor and liquid water) and heat transport from nuclear waste in a porous medium. The code solves the mass, momentum, and energy balance equations for vapor and liquid water and a heat conduction equation for porous/nonporous materials (e.g., host rock and waste container). Using Darcy's Law in place of the momentum equations, the conservation equations are reduced to three nonlinear partial differential equations in terms of the dependent variables, pressure, enthalpy, and rock/ solid temperature.