Deep Borehole Disposal Research: What have we learned from numerical modeling and what can we learn?
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Deep Borehole Disposal Research: What have we learned from numerical modeling and what can we learn? Karl P Travis and Fergus G F Gibb Immobilisation Science Laboratory, Department of Materials Science & Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom. ABSTRACT Geological disposal of HLW and spent nuclear fuel (SNF) in very deep boreholes is a concept whose time has come. The alternative – disposal in a mined, engineered repository is beset with difficulties not least of which are the constraints placed upon the engineered barriers by the high thermal loading. The deep borehole concept offers a potentially safer, faster and more costeffective solution. Despite this, international interest has been slow to materialize, largely due to perceived problems with retrievability and uncertainty about the ability to drill accurate vertical holes with diameters greater than 0.5 m to a depth of 4-5 km. The closure of Yucca Mountain and the subsequent recommendations of the Blue Ribbon Commission have lead to a renewed interest in deep borehole disposal (DBD) and the US DoE has commissioned Sandia National Labs, working with industrial and academic partners (including the University of Sheffield), to undertake a program of R&D leading to a demonstration borehole being drilled somewhere in the continental USA by 2016. In this paper, we focus on some of the key safety and engineering features of DBD including methods of sealing the boreholes, sealing and support matrices for the waste packages. Numerical modeling has, and continues to play, a significant role in expanding and validating the DBD concept. We report on progress in the use of modeling in the above contexts, paying particular attention to constraints on the engineering materials resulting from high heat loading. INTRODUCTION The deep borehole disposal (DBD) concept involves the drilling of a vertical hole to a depth of 4-5 km into the granitic basement of the continental crust. The hole is then lined with steel casing and the lower 1-2 km filled with waste packages together with a sealing and support matrix (e.g. a cementitious grout). Throughout the disposal zone (DZ) the casing is perforated for various reasons including weight reduction. The borehole is then backfilled and sealed above the disposal zone. Hole diameters can vary from 8.5 to 24 inches, but this is largely dependent on current drilling envelopes and the type of waste being considered[1]. 1 . Although DBD is a true multi-barrier concept, its chief advantage over a repository is an order of magnitude greater geological barrier. At depths of 4-5 km, lateral movement of groundwater through igneous rock such as granite is generally limited due to the very low hydraulic conductivities usually found at such depths while density stratification has frequently resulted in highly saline groundwaters being isolated from the near surface groundwaters for millennia. Apart from safety, speed of implementation is another major advantage offered by DBD over a repository; a single bor
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