Modeling multiphysics of traveling wave reactor spent fuel disposal in deep crystalline host rock

Abstract

This study is the first investigation of a method to dispose of the spent fuel of the Travelling Wave Reactor (TWR), an innovative nuclear reactor design. Because significantly higher heat is produced in the central region of the rods than in conventional spent fuel, TWR spent fuel presents new challenges. This work studies the disposal of TWR high-linear-power-spent fuel in deep boreholes in crystalline host rock. The boreholes are 5 km deep, separated horizontally by 200 in, and the spent fuel is enclosed in metallic canisters placed vertically in the deposition boreholes in the bottom 2 km. Nuclear regulators require analysis of the repository's performance for one million years. Other than human intrusion, groundwater transport is the only important mechanism for escape of radioactive material from the repository. Heat decay, combined with the natural geothermal flux, causes groundwater to flow, compromising radioactive containment. The numerical model used to study this problem must accurately predict the thermal field and induced fluid flow at different time and length scales, with strong coupling of all physics. Given these requirements, the numerical simulations of the coupled thermo-hydraulic behavior of a nuclear waste repository are computationally very expensive. To perform the repository simulations, we modified an open-source, finite element-based, fully implicit, fully coupled hydrothermal C++ code, FALCON, based on the MOOSE framework (Multiphysics Object Oriented Simulation Environment). Our simulations show that a first local maximum temperature in the rock near the central borehole of the array occurs within 30 years of disposal (76°C), and an extended period of elevated temperatures with a larger absolute maximum (96°C) begins at 5,000 years. Neither supercritical conditions nor boiling are reached. Thermally driven fluid flow leads to particles from the waste breaking through at the surface at about 150,000 years. A comparison with nuclear waste from conventional Pressurized Water Reactors (PWR) shows that TWR spent fuel produces lower temperatures than PWR. spent fuel for the first 3,200 years. After this time, TWR temperatures surpass PWR results. The flow characteristics for PWR and TWR are similar. The breakthrough time can be extended by increasing the spacing between the boreholes.