ABSTRACT:

The proposed nuclear waste repository at Yucca Mountain, Nevada currently includes a minimum of 50 years forced ventilation inside of emplacement drifts prior to repository closure. To regulate the heat generated from emplaced waste packages, the ventilation during the pre-closure period should be continuous. Off-normal thermal scenarios that consider temporary shutdown of the pre-closure ventilation are investigated to determine the impacts of ventilation shutdown on the thermal-hydrologic-mechanical behaviors of the emplacement drifts. In-drift heat transfer processes including radiation, convection, and conduction are studied. The analysis provides a ventilation heat removal ratio that varies on the drift location and the ventilation duration. The heat removal ratio is transferred and utilized in the NUFT thermal-hydrology software. The NUFT software is used to investigate the thermal-hydrologic impacts on the repository rock mass for the off-normal thermal scenarios with various shutdown durations at various pre-closure times. The predicted rock mass temperature evaluated from the thermalhydrologic analysis is applied for the thermal-mechanical analysis of the off-normal thermal scenarios. The results show that degradation and rockfall of the emplacement drifts due to the off-normal thermal scenarios will be minimal, and it is concluded that the impacts of off-normal thermal scenarios on the stability of the emplacement drifts will be insignificant.

INTRODUCTION

A geologic high-level nuclear waste repository was proposed by the U.S. Department of Energy at Yucca Mountain, Nevada. The proposed nuclear waste repository, located in mined geologic formations approximately 300 m deep, is subject to heat generated from emplaced nuclear waste packages. In order to regulate the heat from the waste packages, the proposed nuclear waste repository currently plans a minimum of 50 years forced ventilation in the emplacement drifts prior to repository closure. The intended ventilation should be continuous throughout the entire pre-closure period.

The normal thermal scenario (i.e., a minimum of 50 years continuous ventilation during the pre-closure period) has been studied in various reports [1, 2, 3]. The studies include thermal, thermal-hydrologic, and thermal-mechanical behaviors and performance of the emplacement drifts due to the continuous ventilation during the pre-closure period. Off-normal thermal scenarios are investigated to assess the impacts of a temporary loss or shutdown of the ventilation on the thermal-hydrologicmechanical behaviors of the repository emplacement drifts. The off-normal thermal scenarios consider the temporary shutdown of the pre-closure ventilation (e.g., 1-day, 1-week, or 1- month) at various pre-closure times (e.g., 2-year, 5- year, and 10-year). The maximum duration of ventilation shutdown is assumed to be 1 month, since the loss of forced ventilation for as long as 30 days would be very unlikely [4]. Details of the thermal-hydrologic-mechanical study of the preclosure off-normal thermal scenario are presented in Scoping Analysis on Sensitivity and Uncertainty of Emplacement Drift Stability [5].

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