Study on Canister sinking : Analysis of consolidation, creep and stress variations

Sammanfattning

The copper canister, which contains spent fuel bundles enclosed in clad tubes, is surrounded by a bentonite buffer. This buffer is a critical component of the KBS-3 concept for nuclear waste storage, protecting the canister from shear forces and limiting the transport of corrosive species. MX-80 Wyoming bentonite is a potential buffer material due to its low hydraulic conductivity and ability to limit radionuclide transport in case of canister failure. In the KBS-3V method, the canister and buffer are installed vertically into a deposition hole 500 meters underground. This study investigates the compressive behavior and long-term creep of the buffer material to ensure it maintains structural integrity under pressure. The analysis uses existing material models in COMSOL Multiphysics. A soil consolidation study was conducted using the Modified Cam-Clay model for fully saturated soils. This study calculated initial conditions such as stress, pressure, displacement, and bulk modulus, which was converted into an E-modulus for use in a linear elastic creep analysis. The second stage involved creep analysis using the Singh-Mitchell creep law for natural clays, evaluating buffer behavior over 100,000 years. The consolidation study showed equal deviatoric and volumetric strains, with no plastic strain, indicating void reduction and increased material density. The most significant displacement occurs during void dissipation. In the extreme case with a swelling pressure of 80 kPa, vertical displacement without creep reached approximately 4.3 mm. When accounting for creep, total vertical displacement increased to about 7.2 mm. Results indicate that long-term creep does not cause significant additional displacement due to initial consolidation effects. Sensitivity studies further suggest the buffer is relatively unaffected by reductions in swelling pressure.