First determination of dissolution rates of oriented UO 2 single crystals
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.41
First determination of dissolution rates of oriented UO2 single crystals S. BERTOLOTTO1,2, S. SZENKNECT2, S. LALLEMAN1, R. PODOR2, L. CLAPAREDE2, A. MAGNALDO1, P. RAISON3, A. MESBAH², B. ARAB-CHAPELET1 and N. DACHEUX2 1
CEA, DEN, DMRC, Univ Montpellier, Marcoule, France ICSM, CEA, CNRS, ENSCM, Univ Montpellier, Site de Marcoule, BP 17171, 30207 Bagnols-Sur-Cèze cedex, France 3 JRC Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany 2
Abstract: Millimetre UO2 single crystals were cut and oriented at JRC Karlsruhe. The orientation of each face of the parallelepiped single crystals was determined with Laue diffraction and the corresponding surface area by geometric measurements. Then, the (111), (100), (110) faces of each single crystal were polished to optical grade and characterized by XRD in order to confirm the surface orientation. The dissolution of the three single crystals was achieved in nitric acid media under dynamic conditions, at room temperature. Two dissolution regimes were observed for all samples. The normalized dissolution rate measured in the first step was not influenced by the crystallographic orientation of the faces. However, during the second step, (110) oriented faces were found to dissolve 4 times faster than the (100) faces. One explanation could involve the atomic composition of each oriented surface in the fluorite-type structure.
INTRODUCTION After its stay for several years in a nuclear reactor, French spent nuclear fuel (SNF) is reprocessed in order to recover uranium and plutonium. The head-end step of the process is the dissolution of the SNF in concentrated and hot nitric acid. SNF is heterogeneous in terms of microstructure, elementary composition and distribution, which could significantly
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influence the dissolution rate. Currently, the dissolution mechanism is described using a simplified solid/liquid interface and a succession of two steps. The first one was defined by Hermann et al1., according to equation (1): 8
2
3
3
UO2 + HNO3 → UO2(NO3)2 +
4
NO + H2O 3
NO(g) + HNO3(aq) → NO2(g) + HNO2(aq)
(1) (2)
2
While the second one can be described as : UO2 + 2 HNO2 + 2 H+ → UO22+ + 2 NO + 2 H2O
(3)
The first step is associated to a slow rate of reaction but leads to the formation of strongly oxidising species like dissolved NOx gases or nitrous acid. According to reaction (3), the latter is involved in the second step of the mechanism that corresponds to the fast oxidative dissolution of UO23. Usually, the multiparametric study of the kinetics of dissolution of UO2 is performed using polycrystalline materials. The heterogeneous attack of the solid/solution interface was observed during the dissolution of UO 2 polycrystalline samples4-6. Microstructural featur
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