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Karen Kruska1 Weilin Jiang1 Ram Devanathan1

1, PNNL, Richland, Washington, United States

During service in a nuclear reactor, the chemical composition of nuclear fuel (UO2) gradually changes with the chain of fission reactions. The high temperatures and continuous irradiation facilitate high diffusion rates and defect production. Oxide solid solutions, perovskites, metallic Mo-Tc-Ru-Rh-Pd-particles and fission gas bubbles have been observed after burnup. The characterization of radioactive materials is time-consuming and expensive. Therefore, in this study similar degradation processes were characterized in a doped simulated fuel (CeO2). Pulsed laser deposition was used to deposit 1 µm thick polycrystalline CeO2 films from a CeO2 target at 550°C. The target was also containing 2 wt% Mo, 1.5 wt% Ru, 0.75 wt% Pd, 0.5 wt% Re and 0.25 wt% Rh. Polycrystalline yttrium stabilized zirconia was used as a substrate. These doped CeO2 films were irradiated with I+ ions at 610°C and 710°C at a flux of 1016 and 5×1016 I+/cm2 each. Sample preparation for transmission electron microscopy (TEM) and atom-probe tomography (APT) was carried out with the focused ion beam. TEM Energy-dispersive X-ray Spectroscopy revealed the formation of small (5-10 nm) Pd precipitates. In many cases bubbles were observed associated with these precipitates. APT was used to quantify the composition of the metallic nanoparticles. Preliminary data from He+ implanted material showed Pd precipitation preferentially in the vicinity of grain boundaries. Impurity particles containing SiO2Hx observed in these specimens highlight the disastrous effect small contaminations could have on fuel degradation. The absence of other precipitates suggests that Pd is the most mobile atom among the five dopants under the irradiation conditions and the precipitation is mainly driven by irradiation-enhanced diffusion processes. Similar precipitation processes are expected in UO2 fuels in dependence of dose and temperature.

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