Effect of Secondary Phases Formation in the Leaching of UO₂ Under Simulated Radiolytic Products

Abstract

The leaching of the spent fuel matrix (UO2) is function of the radiolytic products formation. The effect of each radioiytic product on the leaching process is not totally understood. In the literature, the influence of H2O2 on the dissolution process is described from the qualitative point of view, and most of the studies were performed for pH values from 8 to 12. In this paper we report on the effect of the H2O2 in the leaching process of UO2 by dissolution experiments at various H2O2 concentrations. Also, it was tested the influence of S/V ratio (surface area exposed to the leaching media) on the UO2 leaching and secondary phases formation. It was identified the formation of secondary phases on the UO2 surface. Solid phases characterization was carried out by x-ray Photoelectron Spectrometry (XPS), x-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. By XPS studies the secondary phase formed corresponded to a U(VI) phase. By XRD analyses the solid was identified as studtite, UO4 - 4H2O, (card n0 16–206, [I]). A comparison of the U(VI) phases formed in spent fuel and UO, leaching experiments in various media has been carried out.

Oxidation of UO2(s) in aqueous solution

“…There are several other reports of the formation of (meta)studtite upon H 2 O 2 oxidation of UO 2 [8,[13][14][15][16][17][18] and of the formation of schoepite by O 2 oxidation [6,19]. The mechanical weakness of the studtite layer has also been reported [11,16].…”

“…Several studies report the formation of a partially oxidized surface film when investigating the dissolution of UO 2 under O 2 oxidizing conditions at alkaline pH (7)(8)(9)(10)(11) (with no or little carbonate) [4][5][6][7][8][9]. XPS results presented by Torrero et al [4] suggested a stoichiometry of UO 2.25 , whereas other authors [5][6][7] Arocas et al [8] and de Pablo et al [9] studied the formation of a surface film during oxidation by H 2 O 2 and found stoichiometries of UO 2.33 and UO 2.36 , respectively, at alkaline pH. For oxidation to less than UO 2.33 the fluorite structure is maintained and the oxidation occurs by the uptake of O 2-at interstitial sites, whereas oxidation beyond UO 2.33 destroys the fluorite structure and leads to major structural changes, increased dissolution and formation of secondary precipitates.…”

Addition versus radiolytic production effects of hydrogen peroxide on aqueous corrosion of UO2

“…Nevertheless, few works [14][15][16][17][18][19][20][21][22] on the H 2 O 2 effects on UO 2 corrosion have been carried out. The present work aims to compare the effects of H 2 O 2 addition and radiolytic production on the evolution of UO 2 /H 2 O interfaces.…”

The electrochemical reduction of hydrogen peroxide on uranium dioxide under intermediate pH to acidic conditions

“…The influence of H 2 O 2 on fuel corrosion has been extensively studied [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and much of the early literature has been reviewed [5,24]. The cathodic reduction of H 2 O 2 on UO 2 surfaces was found to proceed via a coupled chemical-electrochemical process in which H 2 O 2 chemically creates U IV -U V donor-acceptor sites on the UO 2 surface…”

“…Consequently, under natural corrosion conditions, H 2 O 2 could not only oxidatively dissolve the UO 2 , but also catalyze its own decomposition, and claims that decomposition occurs have been advanced [14][15][16][17]. However, under neutral to alkaline conditions, when the solubility of U VI (as UO 2 2+ ) is low [25,26], corrosion is blocked at sufficiently high H 2 O 2 concentrations by the formation of insulating U VI surface deposits [5,10,20].…”