Microstructural Characterization of U Coprecipitated Phases Formed in Bentonic-Granitic Groundwater and under Anoxic Con
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0985-NN02-08
Microstructural Characterization of U Coprecipitated Phases Formed in Bentonic-Granitic Groundwater and under Anoxic Conditions Javier Quinones1, Eduardo Iglesias1, Jose M. Cobo1, Aurora Martinez Esparza2, and Jose Maria Gomez de Salazar3 1 Energy, CIEMAT, Avda. Complutense 22, Madrid, 28040, Spain 2 Enresa, Emilio Vargas, 7, Madrid, 28043, Spain 3 Materials Science, UCM, Avda. Complutense s/n, Madrid, 28040, Spain ABSTRACT For improving the accuracy of the performance assessment studies related to the spent fuel safety under storage conditions, it is necessary to develop a new matrix alteration model. These models must be based on laboratory experiences and they should be capable of extrapolating storage environmental conditions. The most recent models developed include the oxidation and dissolution process of the spent fuel matrix, but the influence of a possible process of secondary phase formation over the spent fuel surface was not yet taken into account. This is a key process that could produce a reduction of the matrix dissolution rate/radiation shielding behaviour; however, the surface precipitation of the secondary phase could induce a localized corrosion process, in which case the dissolution rate of the spent fuel would be increased. This paper is focussed on microstructural characterization of secondary phases formed in co-precipitation experiments performed under anoxic conditions in granitic-bentonitic simulated groundwater. In order to simulate the influence of the container material, the co-precipitation experiments were performed in the absence and presence of iron powder. The solid phases formed were characterized using the following techniques: XRD; SEM-EDX, and TEM-EDX. The XRD diffraction pattern showed that under anoxic conditions, a mixture of phases were obtained (sodium and potassium uranate and schoepite), whereas uranate phases were detected when only iron was present. The characterization study indicates that the U secondary phase formed (under reducing conditions and in the presence of iron powder) growth from the iron surface. The crystal size of the secondary phase is independent of the presence of iron powder (and it is always less than 3 µm). Furthermore, the microstructural study showed the growing of U phases over iron powder. INTRODUCTION To develop diligent management for radioactive waste implies exhaustive knowledge of the material behaviour versus several environmental aggressions during its manufacture, transport, or storing process. For this reason, in the VI Spanish General Program for radioactive waste, appears the necessity to safeguard this kind of facility; temporary or definitive [1]. This forces the development of working models that would allow us to predict the behaviour of this material in the selected scenario. Nowadays, some models predict the alteration of the spent fuel matrix during its storage [2, 3], but they still do not take into account the effect of the appearance of a U secondary phase over the surface of the pellet. In order to incorporat
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