Processes Related to the Water Uptake by EUROBITUM Bituminised Radioactive Waste: Theoretical Considerations and First E
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Processes Related to the Water Uptake by EUROBITUM Bituminised Radioactive Waste: Theoretical Considerations and First Experimental Results An Mariën1, Steven Smets1, Xiangling Li2 and Elie Valcke1 1 W&D Expert Group, SCK•CEN, Boeretang 200, 2400 Mol, Belgium 2 EURIDICE EIG, SCK•CEN, Boeretang 200, 2400 Mol, Belgium ABSTRACT According to the present Belgian radioactive waste management program, EUROBITUM bituminised radioactive waste will be disposed of in a geologically stable underground clay formation. The Boom Clay is studied as a potential host formation because of its low diffusion and high retention properties towards radionuclides. The presence of the radioactive waste should not disturb these properties. Due to the presence of hygroscopic salts (25 to 30 weight% NaNO3), EUROBITUM will take up pore water which will result in a swelling and possibly in a very high swelling pressure. First scoping calculations suggest that the swelling pressure exerted to Boom Clay should remain below 7 to 8 MPa to avoid the formation of fractures. If the bitumen in EUROBITUM behaved like a perfect semi-permeable membrane and if no swelling were allowed after the dissolution of NaNO3 into a saturated solution of 10.8 M, osmotic pressures of ~50 MPa could be attained. To better understand the interaction between the swelling EUROBITUM and the host formation, coupled hydro-chemical-mechanical constitutive laws for EUROBITUM have to be developed. To this purpose, water uptake tests under constant volume ('confined') and constant stress ('semi-confined') conditions are being performed. After ~2 years of hydration of small inactive EUROBITUM samples in constant volume conditions, the swelling pressure has raised to ~12 MPa. The volume of samples that can swell against counter pressures of 2.2, 3.3, or 4.4 MPa (constant stress tests) increased with ~5 to 11 volume%, independently of the applied counter pressure. Approximately 10 weight% of the initial NaNO3 content has been leached. INTRODUCTION EUROBITUM is a bituminised radioactive waste form, of which ~3000 m3 has been produced by the EUROCHEMIC/BELGOPROCES reprocessing facility (Mol-Dessel, Belgium) to immobilize precipitation sludges from the reprocessing of spent fuel. This medium-level longlived waste form consists of ~60 weight% hard bitumen Mexphalt R85/401 and ~40 weight% waste, of which the latter is mainly composed of NaNO3 (25 to 30 weight% of EUROBITUM) [2]. According to the present radioactive waste management program of NIRAS/ONDRAF – the Belgian agency for the management of radioactive waste and fissile materials – EUROBITUM will be disposed of in a geologically stable clay formation. The Boom Clay is studied as a potential host formation because of its low diffusion and high retention properties towards radionuclides [3]. 1
The nomenclature for bitumen can be found in [1]
The presence of the radioactive waste in the Boom Clay should not disturb its performance. For that reason the compatibility of EUROBITUM with the Boom Clay formation has to be studied. These c
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