Reactive Transport Modeling of Radionuclide Source-Terms in an Underground Spent Fuel Disposal
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5HDFWLYH7UDQVSRUW0RGHOLQJRI5DGLRQXFOLGH6RXUFH7HUPV LQDQ8QGHUJURXQG6SHQW)XHO'LVSRVDO L. De Windt1, H. Catalette2, J.M. Gras2 Centre for Geological Computer Sciences, Paris School of Mines, rue Saint-Honoré 35, 77305 Fontainebleau, France 2 Electricité de France, Research and Development Division, Les Renardières, 77818 Moret sur Loing, France. 1
$%675$&7 The reactive transport model HYTEC was used to simulate the migration over 100,000 years of cesium, americium and uranium released from spent fuel packages in the near-field components of an underground stiff clay disposal site. A global equilibrium thermodynamic approach including kinetic control of the spent fuel pellets was used with instantaneous release fractions and congruent dissolutions of the rim and the core zones. A failure scenario of the waste package after 10,000 years was considered with magnetite as the main corrosion product. The retention properties of magnetite and the different effects of bentonite and cementitious backfill materials were specifically analysed.
&217(;7$1'2%-(&7,9(6 In the framework of long term studies of high level nuclear wastes in deep geological repositories, a reactive transport model was used to assess the evolution of the radionuclide source-terms in the near-field of a spent fuel disposal. Indeed, under such conditions, a strong coupling between chemical and transport processes is expected. Fully integrated safety calculations codes may fail to assess the variability of transport parameters. The purpose of this paper was to develop a detailed and realistic model of the release of radionuclides from spent fuel packages and their migration in the near-field components. Corrosion products and two types of backfill materials, bentonite and cementitious hydraulic binders including their sorption properties, were introduced in the calculations. The behavior of three elements which have long life isotopes was studied: cesium, americium and uranium. The disposal design and properties as well as the geochemical model features are introduced, followed by the discussion of the interactions between the near-field components and the corresponding geochemical evolutions. The paper ends with the analysis of the radionuclide release and migration.
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',6326$/$1'02'(/)($785(6 'LVSRVDOGHVLJQDQGSURSHUWLHV The studied system is a simplified representation of the near-field of a spent fuel repository in a deep argillaceous formation (as shown in Fig. 1), derived from the various repository designs proposed at present time. The disposal tunnels, 25 m long and 3.3 m in diameter, are composed of four containers of four UOX assemblies each (4.5 m long, 1m in diameter). A container is surrounded by a low alloyed steel canister (13 cm thick) and 1 m thick backfills. Two types of backfill materials are considered in the present study: MX80 bentonite and cement hydraulic binders. The host-rock formation properties are those of stiff clays with an excavated disturbed transition zone (EDZ) of 1.35 m thickness [1]. Diffusion
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