Pressure Evolution of a Bituminized Washing Water Concentrate During Leaching
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Pressure Evolution of a Bituminized Washing Water Concentrate During Leaching Alexander H. Waellisch Section Dismantling and Waste Management, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland ABSTRACT In this investigation the swelling pressure of confined samples of a real waste form, consisting of bitumen containing washing water concentrate, was measured. The theoretically reachable swelling pressure was estimated as high as ~ 273 bar (27 MPa). A first simple experimental set-up was a sample of bituminized radwaste embedded in a cylinder of sintered bronze with a piezo-resistive pressure transducer. This was put into a leaching medium. A maximum pressure of 2 bar (0.2 MPa) was measured. The leaching rates of these confined samples were similar to those of unconfined ones. The second set-up was a sample of bituminized radwaste embedded in a cylinder of stainless steel, also equipped with a pressure transducer. The open bottom of the cylinder was closed with a disc of porous cement with water/cement ratio of 1.3. In two similar experiments with this set-up the swelling pressure reached in one case a value of 9 bar = 0.9 MPa and in the second improved one about 15 bar = 1.5 MPa. INTRODUCTION In Switzerland it is required that liquid radwaste be solidified prior to deposition into an interim storage or a final repository. The solidification of washing water concentrate with bitumen is a common procedure. Bitumen matrices with washing water concentrate or other salt containing waste usually show a strong swelling tendency during leaching in deionised water if they are not hindered sterically [1]. In this case the measurable swelling pressure would be very low, because the body has the possibility to expand. So the question arose, what would be the pressure if expansion of the sample of bitumen with radwaste is prevented. Hence the aim of this investigation was to measure the swelling pressure of samples of a real bituminized waste form during static leach tests. For this purpose two experimental set-ups were built. THEORETICAL CONSIDERATIONS If a bituminized waste form is leached in deionized water and swelling is inhibited, a theoretical maximal reachable osmotic pressure can be calculated. For this rough estimation a few assumptions have to be done. One assumption is the bitumen should behave as a perfect semi-permeable membrane. Since the washing water concentrate consists predominantly of sodium phosphate (Na3PO4), it is assumed that all the waste is sodium phosphate. The last important assumption is that the bituminized waste is behaving like an aqueous solution of the sodium phosphate.
For a concentration of 38 wt.-% sodium phosphate in the bitumen and a temperature of 22°C a possible osmotic pressure of 273 bar was calculated with the following equation: POsm = Z · [A] · R · T
(1)
whereas Z = number of particles, which occur if one formula unit of salt is dissolved in water [A] = concentration of the salt A in the solution R = universal gas constant, 83,144 mbar · l / mol · K or 8,3144 J / mol · K T =
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