The aim of these experiments is to investigate the potential reduction of U(VI) carbonate and hydroxide complexes by aqueous Fe(II). This reduction phenomenon could be important under the disposal conditions of spent fuel. If groundwater enters the copper/iron canister, alpha radiolysis of the water may locally induce oxidizing conditions on the surface of UO2 fuel, leading to the dissolution of UO2 as more soluble U(VI) species. A potential reducing agent in the intruding water is Fe(II)(aq) from anaerobic corrosion of the copper/iron canister. The reduction of U(VI) to U(IV) would substantially decrease the solubility of U as well as co-precipitate other actinides and radionuclides. The interaction experiments were conducted in 0.01 M NaCl and 0.002 M NaHCO3 solutions using an initial uranium concentration of either 8.4 • 10-8 or 4.2 • 10-7 mol/L with an initial Fe(II) concentration of 1.8 • 10-6 in the NaCl solutions and 1.3 • 10-6 mol/L in the NaHCO3 solutions. Only after an equilibration period for U(VI) complexation was Fe(II) added to the solutions. The reaction times varied from 1 week to 5 months. For extra protection against O2, even inside a glove-box (N2 atmosphere), the plastic reaction vessels were closed in metallic containers. The concentrations of U, FeTOT and Fe(II) were analysed as a function of time for unfiltered, micro- and ultrafiltered samples. In addition, the precipitate on the ultrafilters was analysed with ESEM-EDS. The evolution of pH and Eh values was measured. The oxidation state of U in solution was preliminarily analysed for chosen periods. The results of the tests in 0.01 M NaCl showed an initial rapid decrease in U concentration after the addition of Fe(II) to the solution. The U found on test vessel walls at the end of the reaction periods, as well as the ESEM-EDS analyses of the filtered precipitates from the test solutions, showed that precipitation of U had occurred. The oxidation state analyses showed the presence of U(IV) in the solutions after 3 and 6 weeks’ reaction time. These results suggest that the reduction of U(VI) by Fe(II) occurred in NaCl solution. The observed change in U concentration in 0.002 M NaHCO3 solution was small and very slow. Uranium(IV) was also found in solution in this test, showing that some reduction occurred in spite of the stabilising effect of carbonates on U(VI) solutions.
|Place of Publication||Olkiluoto, Eurajoki|
|Number of pages||35|
|Publication status||Published - 2011|
|MoE publication type||D4 Published development or research report or study|
|Series||Posiva Working Report|
- anoxic conditions
- spent fuel