Assessment of the relevance of Coffinite formation within the near-field environment of spent nuclear fuel geological di
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Assessment of the relevance of Coffinite formation within the near-field environment of spent nuclear fuel geological disposals 1
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Robit-Pointeau V.1, Poinssot C. , Vitorge P. , Grambow B. , Cui D. , Spahiu K. and Catalette 5
H. , 1
CEA Nuclear Energy Division, Department of Physics and Chemistry, DPC/SECR, F-91191 Gif Sur Yvette cedex, France. 2
SUBATECH, Ecole des Mines de Nantes, 41 rue Alfred Kastler, BP20722, F-44307 Nantes Cedex, France. 3
Studsvik Nuclear AB, E-61182, Nyköping, Sweden. 4
SKB, P.O.Box 5864 102040, Stockholm, Sweden. 5
EDF, R&D division, Les Renardières, Moret sur Loing, France. ABSTRACT Experiments were performed in anoxic gloves box in an attempt to synthesise Coffinite both in representative near-field conditions, and in conditions which were expected to favour its precipitation according to thermodynamic calculations. The experimental results did not confirm the predictions. However, a new mineral was observed instead of Coffinite. In addition, accurate characterization of various natural samples demonstrate the permanent presence of U(VI) within Coffinite contradictory to its theoretical composition. Our observations raise the question on the validity and applicability of available –actually estimated- thermodynamic data of Coffinite. Based on kinetic hindrance of Coffinite formation, coffinitization of spent nuclear fuel in geological disposal is not anticipated to be a dominant short term process. INTRODUCTION Most of the reference sites for geological disposal of nuclear waste are characterised by strongly reducing conditions (Eh < -150 mV/ENH) in which uranium is mainly at the +4 oxidation state. Spent nuclear fuel alteration in these conditions may proceed by local oxidising conditions at the fuel / water interface under the influence of alpha irradiation. However, due to the strong redox buffer capacity of the near-field materials (especially the canister, the corrosion products and hydrogen generation), most of the near-field environment will remain reducing. In these conditions, uranium may reprecipitate in U(IV) secondary phases. Due to the relative high concentration of silica in such system, Coffinite USiO4·n(H2O) may be a relevant phase to consider as it has been suggested from the natural observations of the natural reactors (Oklo) and uranium ores (Cigar Lake for example). The aim of this work was to determine the relative stabilities of the UO2 and USiO4·n(H2O) solids phases. For this, we wanted to study the aqueous precipitation and leaching of Coffinite. We also simulated the near-field repository conditions to observe the nature of the newly secondary phases formed.
EXPERIMENTAL Attempts of synthesizing Coffinite Quite few authors published protocols for synthesizing Coffinite, but to our knowledge only one paper from Fuchs showed the XRD pattern of the solid phase produced by this its preparation [1]. We have repeated its preparation procedure: a 1 mmol equimolar mixture of UCl4 and Na2SiO3 was buffered by NaHCO3 in an anoxic glove box, where a 0.5 M
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