Bituminised Waste Re-treatment: Replacement of the Bitumen Matrix by a Glass Matrix
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Bituminised Waste Re-treatment: Replacement of the Bitumen Matrix by a Glass Matrix Nathalie R.E.N. Impens1, Isabelle M. Tennstedt2, Karel Lemmens1, Ben Gielen1, Jo M.P. Van Laer1, Alfons F. Fonteyne1, Liliane Vos1, Ben Vos1, Ann J.G. Leenaers1, Sven Van den Berghe1, Peter J.P. Van Bree1, Guido Van Oost2 1 SCK•CEN, 200, Boeretang, B-2400 Mol, Belgium 2 Universiteit Gent, Belgium ABSTRACT The vitrification of nuclear waste salts similar to the salts present in Eurobitum produced by the former Eurochemic reprocessing plant, Mol-Dessel, Belgium, is investigated. In order to approach the composition of the high-level waste reference glass SON68, mixtures with different ratios of glass frit to waste salts have been prepared and vitrified. The concentration of the waste salts varied between 17 and 25 % by weight. To compare the short term corrosion behaviour with reference waste glass, leach tests have been performed. During 8 weeks, the glass samples were submitted to leach tests at 363 K using Milli Q water as leachant, at a glass exposed area over leachant volume ratio equal to 100 m-1. During this short period, scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) showed no gel layer formation. This technique did not detect changes in the chemical composition. However, X-ray photoelectron spectroscopy (XPS) showed that the binding energy of the Zr 3d and Si 2p electrons decreased as a function of leaching time, indicating an increasing covalent character of the glass surface. Inductively coupled plasma-mass spectrometry (ICP-MS) of the leachate as a function of time shows that ionic species such as Na and K leach faster than e.g. Si and B. These results together with the XPS data suggest the onset of a gel layer formation. The effect of waste content in the glass on the leaching rate is also studied. The results are compared with similar leaching tests on R7T7 type SON68 glass. Most elements are better retained in the simulated Eurobitum glass than in the SON68 glass. INTRODUCTION According to the present Belgian reference scenario, Eurobitum [1] bituminised radioactive waste must be disposed off in a deep underground repository in a stable geological formation such as Boom Clay. This waste originated mainly from mixtures of nuclear fuel decladding slurries and waste concentrates from the nuclear fuel cycle. As the acceptability of bituminised waste in geological disposal is questioned because of its combustible and organic character, and as a final decision about the disposal of the bituminised waste has not been taken so far, alternative solutions are still conceivable. To support the decision-making we investigate methods to recondition this bituminised waste. For example, a low temperature method to recondition wastes such as Eurobitum has been published [2]. The method is based on the removal of hygroscopic water-soluble salts (mainly NaNO3), and of the majority of the compounds that are the most sensitive towards radiolysis (i.e. saturated hydrocarbons – these are mostly present in the mal
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