Osmosis: the key process that drives water uptake and swelling of Eurobitum Bituminized Radioactive Waste
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Osmosis: the key process that drives water uptake and swelling of Eurobitum Bituminized Radioactive Waste K. Hendrix1, N. Bleyen1, S. Smets1, W. Verwimp1, X. Sillen2 and E. Valcke1 1 W&D Expert Group, SCK•CEN, Boeretang 200, 2400 Mol, Belgium 2 ONDRAF/NIRAS, Avenue des Arts – Kunstlaan 14, 1210 Brussels, Belgium ABSTRACT In Belgium, the preferred long-term management option for Eurobitum bituminized ILW is its final disposal in a geologically stable clay formation such as the Boom Clay, which is studied as a reference host formation. After disposal, clay pore water will infiltrate the secondary concrete waste containers filled each with ten Eurobitum drums. Eurobitum contains hygroscopic salts, mostly NaNO3 (20-30 wt%) and CaSO4 (4-6 wt%), and thus will take up water and swell. If swelling is hindered, a pressure will be exerted on the concrete container and ultimately on the surrounding Boom Clay, possibly inducing stresses in the clay close to the disposal galleries. To improve our understanding of these processes, water uptake tests are ongoing in which inactive Eurobitum is contacted with 0.1 M KOH (representing young cement water). These tests suggest that the swelling is mainly driven by osmosis. This understanding was validated in the presented research by varying the water activity of the leachant in water uptake tests in both constant stress and constant volume conditions. After a stable swelling rate was reached in contact with 0.1 M KOH, the leachant was switched in the following order: nearly saturated (~7.8 M) NaNO3 – 0.1 M KOH – nearly saturated NaNO3 – 4 M NaNO3 – 0.1 M KOH. The changes in swelling rate and pressure evolution correlated nicely to the changes in water activity. This confirms that osmosis is the key process governing the swelling of Eurobitum. INTRODUCTION Eurobitum is an important Belgian intermediate-level long-lived radioactive waste form produced by Eurochemic/Belgoprocess for the immobilisation of intermediate-level liquid wastes originating from the chemical reprocessing of spent nuclear fuel in the 1960's and 1970's, and from the cleaning of high-level waste storage tanks. Around 13550 drums have been produced, adding up to ~3000 m³ of waste [1]. The bituminized waste (BW) consists of ~60 wt.% hard bitumen Mexphalt R85/40 [2] and ~40 wt.% waste of which ~0.4 wt% are radionuclides, while the main components are NaNO3 (20 – 30 wt.%) and CaSO4 (4 – 6 wt.%) [3]. The preferred long-term management solution pursued by the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) is final underground disposal in a geologically stable clay formation such as the Boom Clay [4]. Geo-mechanical and physico-chemical perturbations of the clay are expected, a result of the large amounts of dehydrated and hygroscopic salts in combination with bitumen acting as a highly efficient semipermeable membrane. Indeed, once the disposal gallery is filled with water (approximately 150 year after its closure [5]), osmosis-induced uptake of water will result in swelling of the BW, w
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