Secondary phase formation on UO2 in phosphate media

Abstract

The dissolution of UO 2 (a chemical analog of uraninite and of spent nuclear fuel) has been studied by using a flow-through reactor. The UO 2 dissolution rates at total PO 4 3 - concentrations of 10 −4 , 10 −5 , and 10 −6 mol dm −3 have been determined to be: 1.3 × 10 −10 mol m −2 s −1 , 6.7 × 10 −11 mol m −2 s −1 , and 2.0 × 10 −11 mol m −2 s −1 , respectively. The dissolution rates determined are found to be higher than the ones determined for similar carbonate concentrations. Moreover, the surface of the UO 2 has been studied in static tests by means of the scanning force microscopy technique (SFM) in order to follow the formation of any secondary solid phase on its surface. The formation of chernikovite (H 2 (UO 2 ) 2 (PO 4 ) 2 · 8H 2 O) has been observed at a 10 −4 M total PO 4 3 - concentration, while no uranyl-phosphate secondary phases have been found at lower PO 4 3 - concentrations. In experiments performed in the presence of both carbonate and phosphate, no precipitation of secondary phases has been observed. It is postulated that this is due to the formation of the highly stable uranyl-carbonate complexes in solution.