Claire Le Gall1 Ernesto Geiger2 Olivier Proux3 4 Mauro Rovezzi3 4 Pier Lorenzo Solari5 Myrtille Hunault5 Vincent Klosek1 Chantal Martial1 Jacques Léchelle1 Fabienne Audubert1 Yves Pontillon1 Jean-Louis Hazemann6 4

1, CEA, St Paul-lez-Durance, , France
2, Royal Military College of Canada, Kingston, Ontario, Canada
3, Centre National de la Recherche Scientifique (CNRS), Grenoble, , France
4, ESRF, Grenoble, , France
5, SOLEIL Synchroton, Gif-sur-Yvette, , France
6, Centre National de la Recherche Scientifique (CNRS), Grenoble, , France

Despite the high degree of safety of nuclear power plants in normal operating conditions, the risk of a Severe Accident (SA) is still present. As recently demonstrated by the Fukushima-Daiichi events, the three containment barriers of the reactor might fail leading to the release of highly radioactive elements in the environment. Among them, Barium (Ba) is of particular interest. This highly reactive Fission Product (FP) can interact with many other elements present in the fuel matrix such as U, O, Mo, Zr, Cs and Sr modifying its volatility [1], [2]. Furthermore, if Ba remains in the melted fuel, it can be responsible of up to 20% of the residual heat generated after the accident, and if released, it can severely damage human health. Hence, to better predict the progress and consequences of a SA, it is essential to develop realistic models for Ba behavior in the fuel in these conditions.
The development of such models requires experimental data on Ba speciation in the fuel and their comparison to thermodynamic predictions. Due to the limitations in terms of experiments and characterization techniques available up to now to study FPs speciation in irradiated nuclear fuels, model materials, referenced as SIMFuel, can be used [3], [4]. These SIMFuels are manufactured from depleted UO2 doped with 11 stable oxides as FP surrogates and give access to powerful characterization techniques such as X-ray Absorption Spectroscopy (XAS) [5] [6], [7].
In this work, SIMFuel samples were annealed in different conditions representative of the early stages of a SA (temperatures up to 1700°C in both reducing and oxidizing atmospheres) since the FP behavior during this phase is crucial regarding the final stage. XAS experiments coupled with SEM-EDX analyses were performed after each annealing tests in order to study the evolution of Ba phases. The results tend to show that Ba remains under a zirconate form in reducing conditions whereas its local environment is modified to a molybdate form in oxidizing conditions above 1000°C.

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