The Oxidative Dissolution Mechanism of Uranium Dioxide. The Effect of pH and Oxygen Partial Pressure

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7KH2[LGDWLYH'LVVROXWLRQ0HFKDQLVPRI8UDQLXP'LR[LGH7KH(IIHFWRIS+DQG2[\JHQ 3DUWLDO3UHVVXUH  1 1 1 Joan de Pablo , Ignasi Casas , Javier Giménez , Frederic Clarens1, Lara Duro2 and Jordi Bruno2 1 Department of Chemical Engineering, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647 H-4, 08028, Barcelona, Spain. 2 ENVIROS Spain, Avda. Rubí 29-31, 08197 Valldoreix, Spain. $%675$&7 The dissolution of the unirradiated uranium dioxide, UO2, is largely affected by chemical variables like pe, pH and carbonate concentration in solution. In a previous paper [1], we studied the kinetics of the dissolution of UO2 as a function of pH and oxygen partial pressure and the experimental dissolution rates obtained showed a fractional dependence on both parameters. These fractional orders indicate a dissolution occurring via a surface mediated mechanism. In this work we have developed a mechanism accounting for the oxidative dissolution of UO2, which depends on the oxygen concentration in solution and on the pH and a general rate equation has been deduced. This mechanism has been used to explain the results obtained by different authors and to explain and predict the dissolution of the spent nuclear fuel matrix.  ,1752'8&7,21 The near-field performance assessment of the spent nuclear fuel disposal is very dependent on the redox conditions considered on the final repository and on the pH of the deep groundwaters. Under oxidising conditions, UO2 is thermodynamically unstable and an oxidation process on the surface of the solid occurs. Several uranium oxides with O/U stoichiometric ratio between 2 and 3 can form and furthermore the precipitation of secondary uranium phases can also take place under certain conditions. In this context, the de-convolution of the overall process is complicated, the effect of parameters such as pH and oxidants concentration is still controversial, and the dissolution-oxidation mechanism is not completely understood [2]. The objective of this work is to define a general mechanism for the oxidative dissolution of the uranium dioxide depending on the oxygen concentration in solution and on the pH of the contacting water, which can explain the experimental results obtained before and to use this general mechanism to explain and predict the dissolution of the spent nuclear fuel matrix under oxidising conditions. '(6&5,37,212)7+(02'(/ The proposed oxidative dissolution mechanism can be illustrated by the following processes taking place in the system: 6WHS2[LGDWLRQRIWKHVXUIDFHRIWKHVROLG    k1 >UO2 + ½ O2 ' >UO3 k-1

(1)

1

6WHS6XUIDFHFRRUGLQDWLRQRI+ IDYRXUHGDWDFLGLFS+  K2 k2 >UO3 + H+ ⇔ >UO3-H+ → UO2(OH)+(aq)

(2)

6WHS6XUIDFHFRRUGLQDWLRQRI+2 IDYRXUHGDWQHXWUDODONDOLQHS+  K3 k3 >UO3 + H2O ⇔ >UO3-H2O → UO2(OH)2 (aq)

(3)

Considering these steps, the expression of the dissolution rate, U, is given by: r=

d[U(VI) aq ] dt

= k 2 ·{> UO 3 − H + } + k 3 ·{UO 3 − H 2 O} =

[ ]

(

[ ])

(4)

= k 2 ·K 2 ·{> UO 3 }· H + + k 3 ·K 3 ·

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