Updated Simulations of the FEBEX Full-Scale In-situ Heater Test for High-Level Nuclear Waste Disposal in Crystalline Rock

Abstract:

This work describes modeling analysis on the Full-scale Engineered Barriers Experiment in Crystalline Host Rock (FEBEX) in situ test as part of the SKB Engineered Barrier System Task Force. The FEBEX test was a full-scale test conducted over ~18 years at the Grimsel Rock Laboratory, Switzerland managed by NAGRA. It involved emplacing simulated waste packages, in the form of welded cylindrical heaters, inside a tunnel in crystalline granitic rock and surrounded by a bentonite barrier and cement plug. Sensors emplaced within the bentonite monitored the wetting, heating, and drying out of the bentonite barrier, and the large resulting data set provides an excellent opportunity for validation of multiphysics Thermal-Hydrologic-Mechanical-Chemical (THMC) modeling approaches for underground nuclear waste storage and the performance of engineered bentonite barriers. The current modeling work follows years of modeling studies by many notable modeling teams. These modeling studies generally use two-dimensional axisymmetric meshes, ignoring three-dimensional effects, gravity and asymmetric wetting and dry out of the bentonite engineered barrier. This study investigates these effects with use of the PFLOTRAN code with massively parallel computational methods in modeling FEBEX experimental results.

PFLOTRAN runs were made using high-performance computing facilities at Sandia National Laboratories with up to 160 processors per run. The simulations were run in stages with two heaters operating in Stage 1 (first 5 years) and one heater operating in Stage 2 (18 years) In addition, simulations were conducted with cooling periods after each stage. Simulation results and comparison with experimental data are presented. Reasonable matching of experimental data was obtained.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2020-7378 A.