Calculation of the Thermal Response in a Deep Geological Repository Using the Horizontal Tunnel Placement Concept in Limestone at a Depth of 500 M

Abstract

The Adaptive Phased Management (APM) approach proposed by the Nuclear Waste Management Organization (NWMO) and accepted by the Government of Canada for long term management of Canada;s used nuclear fuel includes eventual containment and isolation of the used nuclear fuel in engineered excavations about 500 m deep in plutonic rock or sedimentary rock. The NWMO is evaluating the horizontal tunnel placement (HTP) method for used fuel containers in a deep geological repository (DGR) in sedimentary rock. The approach is similar to the reference NAGRA conceptual design for the Swiss used-fuel/high-level waste repository in sedimentary rock. Some thermal-only or coupled thermal-mechanical simulations are performed using different programs for the near-field modeling. These simulations evaluate the stability of the rock around the placement tunnel for the HTP method in a DGR located at a depth of 500 m in limestone during the excavation stage and after placement of used nuclear fuel. In the near-field modeling, the thermal boundary conditions are based on a dimensionally infinite horizontal repository. The results from these simulations are only considered to be representative for a finite repository for the first 1,000 years after used fuel placement. In order to correct the near-field modeling results to be representative of a DGR of finite horizontal extent, two far-field models (i.e., infinite and finite horizontal extent) are run for the specific repository conditions and dimensions. The thermal results for the volume very near the container from this near-field modeling are modified to represent the results for a finite repository by subtracting the differences between the results from the far-field models for a finite repository and for an infinite repository. The modified results show that the tunnel wall has a peak temperature of 69.6 °C at 68 years after placement and a second peak temperature of 71.5 °C at 683 years after placement (compared to the original results for second peak temperature 72.5 °C at 1,500 years). Modified temperatures for other selected points in the rock along the horizontal line through the container centre are also obtained for one million years to obtain a better estimate of temperature in the near-field of a finite repository.