The oxidative dissolution mechanism of uranium dioxide. I. The effect of temperature in hydrogen carbonate medium

The oxidative dissolution of uranium (IV) dioxide has been experimentally i nvestigated as a function of hydrogen carbonate concentration at 4 differen t temperatures (10, 25, 45, and 60 degrees C) by using a continuous thin-la yer flow-through reactor. The experimental results have been interpreted as evidence for a bicarbonate-promoted oxidative dissolution mechanism which can be differentiated in to 3 steps: 1) initial oxidation of I:he uranium d ioxide solid surface; 2) binding of HCO3- at the U(VI) sites of the oxidize d layer; and 3) detachment of the U(VI)-carbonato surface complex. From this mechanism a general rate equation has been derived: r= k(1)k(2){>UO2}(tot)[O-2][HCO3-] divided by k(-1) + k(2)[HCO3-] + k(1)[O- 2] Which allows to rationalization of some of the discrepancies found in the l iterature concerning the dependence of the dissolution rate of uranium diox ide on the hydrogen carbonate concentration. The application of this bicarb onate-promoted oxidative dissolution mechanism allows us to predict unirrad iated UO2 and spent nuclear fuel dissolution rates which are in satisfactor y agreement with rates determined experimentally. Some differences observed in spent fuel dissolution rates have been attributed to radiolysis effects . Copyright (C) 1999 Elsevier Science Ltd.