Modelling Deep Borehole Disposal of Higher Burn-up Spent Nuclear Fuels
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Modelling Deep Borehole Disposal of Higher Burn-up Spent Nuclear Fuels Karl P. Travis1, Fergus G. F. Gibb1 and Kevin W. Hesketh2 1 Department of Materials Science & Engineering, University of Sheffield, Sheffield S1 3JD, UK. 2 National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, WA3 6AE, UK. ABSTRACT Higher burn-up (> 50 GWd/t) spent nuclear fuels (SNF) present problems for long-term management and disposal in mined repositories, principally because of their higher heat output. Here we present results from heat flow modeling of an alternative scheme for disposing of SNF deep borehole disposal (DBD). We focus on how temperatures on the outer surface of the containers evolve, affect the melting and re-solidification of the high density support matrix (HDSM) and their consequences for the feasibility of this disposal concept. We conclude that not only is DBD a viable option for higher burn-up SNF, but it could be a practical disposal route for a range of combinations of SNF ages and number of fuel pins per container. INTRODUCTION The reactors deployed in new nuclear builds over the next two or three decades are most likely to be Generation III light water reactors (LWR). Irrespective of whether they use uranium dioxide or mixed oxide (MOX) fuels, they will seek to extract more energy from the fuel than their predecessors through higher burn-ups, i.e. 55 GWd/t or greater. From a used nuclear fuel (UNF) disposal management perspective this creates problems, especially those arising from the higher radioactivity and heat outputs. One disposal concept that is relatively insensitive to radiation and temperature and so could overcome these problems is deep borehole disposal (DBD). Deep borehole disposal [1] places more emphasis on the geological barrier and less on the engineered barriers. Its basic principle is illustrated in figure 1 and can be summarized as follows. Large diameter boreholes at least 4 km deep are sunk into a suitable host rock, usually the granitic basement of the continental crust, and waste packages are deployed in the lower reaches of the hole. With a geological barrier an order of magnitude greater than most mined repositories, DBD takes advantage of the very low bulk hydraulic conductivities (< 10-11 m/s) that can be found at such depths even in fractured rocks. In addition to improved safety, costeffectiveness, security, flexibility and environmental impact [1], DBD offers important advantages over mined repositories for disposal of SNF and other HLW. Among these are tolerance of a wide range of compositions and heat outputs, relative ease of siting, shorter implementation timescales and the option of dispersed disposals DBD is also earthquake proof. In this paper we introduce a new variant designed to dispose of higher burn-up spent UO2 and MOX. First we discuss the physical model and give details of our improved and refined computational model. We then present the results of modeling “experiments” to explore the large parameter space including: the number of fuel pins per
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