Nd- and Ce-doped ceramic-glass composites: chemical durability under aqueous conditions and Surface alteration in a mois
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Nd- and Ce-doped ceramic-glass composites: chemical durability under aqueous conditions and Surface alteration in a moist clay medium at 90°C P.J. Mcglinn1, T. Advocat2, E. Loi1, G. Leturcq1 and J.P. Mestre2 1 ANSTO, New Illawarra Road, Menai, NSW, Australia 2234 2 CEA/SCD - Marcoule BP 171 30207 Bagnols/Cèze ABSTRACT Zirconolite-glass and sphene -glass specimens, doped with REE as simulants for trivalent actinides, were leached under two conditions. The first was a solubility test using a powdered sample in deionized water at an SA/V ratio of 200 cm-1, to examine the long-term leaching behavior of the composite materials. The other test was carried out in the presence of moist clay, to assess the degree of surface alteration of the composites in the presence of potential geological repository materials. Both tests were carried out at 90°C. The specimens leached in clay showed signs of preferential attack on the glassy matrix along zirconolite and sphene grain boundaries. EDS results showed no gross changes in composition of the constituent phases as a result of leaching. For the solubility tests, steady state conditions of elemental release were attained within 7 days of leaching, suggesting development of a surface passivation layer hindering movement of reactive species between the surface of the material and the leachant. Calcium, Si and Al releases were similar between composites. Titanium and Ce releases were also similar between composites, and were two orders of magnitude lower than those for Ca, Si and Al. The actinide simulants partitioned into the glass phase and into the crystalline component of the composite materials in approximately similar proportions. Although the surrogates were contained in the less durable glassy phase at these levels, this was not reflected in the release of Ce, for example, which was similar to that for a single-phase zirconolite. INTRODUCTION Glass-ceramics are important candidate materials for nuclear waste immobilization. The potential advantages of these materials are their excellent durability and relative ease of fabrication. They represent a compromise between the superior chemical durability of crystalline materials and the processing flexibility of glass. Durability testing has been carried out on a zirconolite-glass and a sphene-glass composite. Zirconolite and sphene are titanate minerals which are potential host phases for actinides and certain fission products. Zirconolite is particularly durable and is the principal host phase in synroc nuclear waste forms [1], whilst sphene is used in Canadian glass-ceramics.
EXPERIMENTAL METHODS Fabrication of materials The glass-ceramics were fabricated by melting oxide mixtures in a platinum crucible at 1450°C for one to two hours, pouring the material into an alumina crucible for heat treatment at 1200°C for 12 hours (devitrification of the parent glass), then furnace cooling to ambient temperature. This thermal sequence was considered to be optimal for obtaining the desired crystalline phase on cooling of the melt [2]. Fou
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