Transport of Actinides Through a Bentonite Backfill
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TRANSPORT OF ACTINIDES THROUGH A BENTONITE BACKFILL B. TORSTENFELT, H. KIPATSI, K. ANDERSSON, B. ALLARD AND U. OLOFSSON Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 G6teborg, Sweden. INTRODUCTION Compacted bentonite has been proposed as a suitable backfill material in the Swedish concept for underground storage of high-level waste1 . The backfill barrier will serve both as a physical barrier, preventing convective water flow and allowing radionuclide penetration only by diffusion, and as a chemical barrier capable of chemically interacting with the radionuclides. Previously the diffusion of fission products (Sr, Tc, and Cs) in compacted bentonite has been investigated within the present program2 . This paper presents measurements of the diffusion and sorption of actinides in bentonite and the technique used for these studies. EXPERIMENTAL Radionuclides and clay-water systems The studied systems are given in Table I. The short-lived nuclides 234Th and 233Pa were recovered from 238U and 237Np, respectively. In all experiments Wyoming Bentonite MX-80 (a Na-bentonite) was used 3 The aqueous phase was a synthetic groundwater4 with a total carbonate concentration of 123 mg/l, total salt concentration of 306 mg/l and pH around 8.2, preequilibrated with bentonite. In one of the U-experiments the total carbonate concentration was increased from 123 to 600 mg/l and in another one 10 mg/l of a humic acid5 was added. Some U-experiments were also performed using a clay containing 1% Fe3 (PO4 )2 (s)or 0.5% Fe(s), which were thoroughly mixed with the clay. Diffusion measurements The clay was compacted to a density of (1.9-2)xi0 3 kg/m 3 , which is representative of the backfill barrier proposed in the Swedish concept for 3 final storage of radioactive waste . The compactation was performed with a KBr-pellet press used for IR-spectrometry. To avoid concentration gradients in the compacted clay, due to incomplete equilibrium between the clay and the aqueous phase, the clay was preequilibrated with water and thereafter dried in an oven at 105 C before the pressing.
660 TABLE I Nuclides and clay-water systems Radionuclide 85
Sr 134Cs 234 Th 233 Pa 2 33 U 2 33 U 23 3 U 233U 233U 237Np 2 39 Pu 24 1 Am
Solid phase
Aqueous phase
Bentonitea) Bentonite Bentonite Bentonite Bentonite Bentonite Bentonite Bentonite + 1% Fe3 (P04 )2 Bentonite + 0.5% Fe
G.W.b) G.W. G.W. G.W. G.W. G.W. with 600 mg/l HCO 3 G.W. with 10 mg/l humic acid G.W. G.W.
Bentonite Bentonite Bentonite
G.W. G.W. G.W.
a)Wyoming Bentonite MX-80 3 b)Synthetic groundwater4
A diffusion cell for studies of radionuclide diffusion in compacted clay has been constructed (See the description in ref 2). The diffusion cylinder containing the bentonite, pressed as dry clay (water content 10%), was submerged in preequilibrated water for about 2 weeks for homogenization of the clay sample. Half of the required clay volume was pressed into an empty diffusion cylinder. A thin clay plate containing the radionuclide was placed on top of the cylinder, a
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