Modeling of the T S D E Heater Test to Investigate Crushed Salt Reconsolidation and Rock Salt Creep for the Underground Disposal of High-Level Nuclear Waste

Abstract:

Rock salt is a potential medium for the underground disposal of nuclear waste because it has several assets, in particular its water and gas tightness in the undisturbed state, its ability to heal induced fractures and its high thermal conductivity as compared to other shallow-crustal rocks. In addition, the run-of-mine, granular salt, may be used to backfill the mined open spaces.

We present simulation results associated with coupled thermal, hydraulic and mechanical processes in the TSDE (Thermal Simulation for Drift Emplacement) experiment, conducted in the Asse salt mine in Germany [1]. During this unique test, conceived to simulate reference repository conditions for spent nuclear fuel, a significant amount of data (temperature, stress changes and displacements, among others) was measured at 20 cross-sections, distributed in two drifts in which a total of six electrical heaters were emplaced. The drifts were subsequently backfilled with crushed salt. This test has been modeled in three-dimensions, using two sequential simulators for flow (mass and heat) and geomechanics, TOUGH-FLAC and FLAC-TOUGH [2]. These simulators have recently been updated to accommodate large strains and time-dependent rheology. The numerical predictions obtained by the two simulators are compared within the framework of an international benchmark exercise, and also with experimental data. Subsequently, a re-calibration of some parameters has been performed. Modeling coupled processes in saliniferous media for nuclear waste disposal is a novel approach, and in this study it has led to the determination of some creep parameters that are very difficult to assess at the laboratory-scale because they require extremely low strain rates. Moreover, the results from the benchmark are very satisfactory and validate the capabilities of the two simulators used to study coupled thermal, mechanical and hydraulic (multi-component, multi-phase) processes relative to the underground disposal of high-level nuclear waste in rock salt.

References:

[1] Bechthold et al., 1999. BAMBUS-I Project. Euratom, Report EUR19124-EN.

[2] Blanco Martín et al., 2014. Comparison of two sequential simulators to investigate thermal-hydraulic-mechanical processes related to nuclear waste isolation in saliniferous formations. In preparation.