Long-term behavior of R7T7-type nuclear glass: Current state of knowledge and outlook
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CC5.7.1
Long-term behavior of R7T7-type nuclear glass: Current state of knowledge and outlook S. Gin (1), N. Godon (1), I. Ribet (1), P. Jollivet (1), Y. Minet (1), P. Frugier (1), E. Vernaz (1), J.M. Cavedon (2), B. Bonin (3,*) and R. Do Quang (3) (1) CEA Valrhô, DEN/DTCD, BP 17171, 30207 Bagnols sur Cèze Cedex, France (2) CEA Saclay, DDIN/HAVL, 91191 Gif sur Yvette Cedex, France (3) COGEMA, COGEMA, BP 4, 78141 Vélizy Cedex, France * Now at : CEA SACLAY DEN/DS, 91191 Gif sur Yvette Cedex, France ABSTRACT This article reviews current knowledge of the long-term behavior of R7T7-type glass during the thermal phase and in geological repository conditions (aqueous alteration). In interim storage R7T7 glass can be considered to conserve its integrity over time. In geological repository conditions, the mechanisms of glass alteration by water have been identified and parameter values have been assigned to the reaction kinetics for wide variations of the influential factors (temperature, pH, flow rate, S/V ratio, etc.). CEA has developed an operational model to obtain robust and reasonably conservative predictions of the glass quantities altered after disposal. Examples of applications of the operational model are discussed, future research topics are also proposed to consolidate this approach.
INTRODUCTION France, like most countries concerned by spent fuel reprocessing and HLW vitrification, is planning for reversible or irreversible disposal of glass packages in a deep geological formation. The scenarios now being investigated generally provide several containment barriers (canisters, engineered barrier, host rock) between the glass and the biosphere. The key issues involved in source term predictions are the glass evolution during the thermal phase and the long-term alteration in contact with water. In 2006, the French parliament will review 15 years of research on this topic and legislate on the disposition of this wasteform. This article reviews the current state of knowledge concerning R7T7-type glass, which accounts for most of the high-level waste in France, and discusses the uncertainties and the options available for progressing in this area.
BASIC DATA CONCERNING R7T7 GLASS R7T7 glass has been produced by COGEMA at La Hague since 1989. Over 10 000 packages, each containing 400 kg of glass, have been produced to date. The total activity contained in this type of glass now exceeds 50 × 106 Ci α and 4 × 109 Ci βγ. The glass composition is given by Pacaud [1]. It is important to note that due to the cooling scenario and the absence of annealing, the glass in the canister is fractured.
R7T7 GLASS EVOLUTION DURING THE THERMAL PHASE The main factors likely to modify glass performance are devitrification and self-irradiation. It has been shown for R7T7 glass that even after extremely severe heat treatment the maximum theoretical crystal fraction in the glass would not exceed 4%, a value with no impact on the glass chemical durability [2].
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Glass self-irradiation studies have yielded the following observati
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