A Model for Predicting the Temperature Distribution Around Radioactive Waste Containers in Very Deep Geological Borehole
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A Model for Predicting the Temperature Distribution Around Radioactive Waste Containers in Very Deep Geological Boreholes Karl P. Travis, Neil A. McTaggart, Fergus G. F. Gibb and David Burley Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, U. K. ABSTRACT We present a mathematical model for determining the temperature field around radioactive waste containers in very deep geological boreholes. The model is first used to predict the temperature rise for some simple, but well-established cases with known solutions in order to verify the numerical work. The temperature distribution is then determined for two variants of the deep bore hole concept; a low temperature variant and a high temperature variant. The results from these studies are discussed in terms of their utility in establishing deep borehole disposal as a workable concept. INTRODUCTION Disposal of high level waste (HLW) including spent nuclear fuel and fissile materials in very deep geological boreholes is starting to gain serious attention as a possible safe, long term solution for the management of these wastes [1]. Very deep disposal in the present context refers to large diameter boreholes drilled 4-5 km into the granitic basement of the continental crust. This form of disposal offers many advantages over mined and engineered repositories (usually 300 -800 m in depth). There is, for example, an order of magnitude increase in the geological barrier, providing protection from catastrophic events [1-2]. An additional safety feature arises from the high confining pressures of around 150 MPa at depths of 4.5 km, which ensures that volatile radionuclides, such as I129, remain in a condensed phase. The cost of drilling the boreholes (around US$1.5 million/km [3]) also makes them economically more attractive than the alternative mined repositories; According to a 2002 DOE estimate, Yucca Mountain is projected to cost US$58 billion to build and operate, while a 2005 NIREX study concluded that a HLW repository built and operated in the UK, and based on the Swedish SKB-3 concept, would cost £4.9 billion [4]. Mined repositories have a head start in that they are at an advanced stage of development in several countries. For the case of deep boreholes, predictive computer modelling will have a significant role to play in verifying some of its claims. For an example of a related thermal modelling study, see the SKB report by Marsic et al. [5]. In the very deep borehole concept we are developing, there are two main variants: a low temperature scheme and a high temperature scheme. The latter form relies on the heat output from the waste being of sufficient magnitude to partially melt the surrounding granite, which will then cool and recrystallise to permanently seal the waste into its own granite sarcophagus [6-7]. In both variants it is important to determine the temporal and spatial distribution of the temperature in the borehole and its environment, but the success of the high temperatur
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