The Assessment of the Long-Term Evolution of the Spent Nuclear Fuel Matrix by Kinetic, Thermodynamic and Spectroscopic S
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THE ASSESSMENT OF THE LONG-TERM EVOLUTION OF THE SPENT NUCLEAR FUEL MATRIX BY KINETIC, THERMODYNAMIC AND SPECTROSCOPIC STUDIES OF URANIUM MINERALS. JORDI BRUNO*, I. CASAS*, E. CERA*, R.C. EWING**, R.J. FINCH**, AND L.O. WERME*** * Intera (Spain), Parc Tecnolkgic del Vall6s E-08290 Cerdanyola, Spain "**Dept. of Earth and Planetary Sciences, University of New Mexico, USA ***SKB, Box 5864, Stockholm, Sweden. ABSTRACT The long term behaviour of spent nuclear fuel is discussed in the light of recent thermodynamic and kinetic data on mineralogical analogues related to the key phases in the oxidative alteration of uraninite. The implications for the safety assessment of a repository of the established oxidative alteration sequence of the spent fuel matrix are illustrated with Pagoda calculations. The application to the kinetic and thermodynamic data to source term calculations indicates that the appearance and duration of the U(VI) oxyhydroxide transient is critical for the stability of the fuel matrix. INTRODUCTION The performance of a spent nuclear fuel repository has to be assessed for periods up to hundreds of thousands, or even millions of years. Spent nuclear fuel constitutes the innermost barrier of the direct disposal concept. A critical aspect in the evaluation of the safety of such a concept is the effect of potential oxidative alteration of the fuel matrix. Generally, the kinetic and thermodynamic behaviour of the fuel matrix is determined by spent nuclear fuel dissolution experimental programs, which combine carefully obtained microstructural data of used fuel together with a variety of leaching tests. These experimental programs cover reaction time periods up to 20 years, while the conceptual models, tentatively derived from these observations are used to explain the post-closure behaviour (0-1000 years after disposal). After this period the spent fuel waste matrix will become, slowly but surely, less similar to present day spent fuel and more similar to uranium ore. This sequence of events will determine the behaviour of the waste matrix in the time range of
103_106
y. This is
after the expected containment period of the metallic canister. Consequently, there is a need to bridge these two different time-scales and to develop conceptual and numerical models that may be integrated into the performance assessment of spent nuclear fuel. In this context, appropriate natural analogues can be used to provide insight into the critical pathways for the long-term oxidative alteration of uraninite (an analogue of the UO2 spent nuclear fuel matrix). The purpose of this work is to present a thermodynamic and kinetic model that integrates recent experimental data concerning the oxidative alteration of natural uraninites. The implications for the performance assessment of a repository are illustrated by using Pagoda [1] calculations. THE OXIDATIVE ALTERATION PATHWAY OF UO, URANINITE AND SPENT FUEL. The oxidative dissolution of UO2 has been extensively studied in different aqueous media. Electrochemical measurements have
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