Application of Ultra-High Temperature XAFS Measurement System for Actinide Materials

In order to clarify accident progression scenarios in the Fukushima NPP accident, it is essential to evaluate the behavior of fuel heated to high temperatures, and information on fuel behavior in the "ultra-high temperature region (around melting point)" is required. However, there is little information on the structural and electronic properties of actinides in this high temperature region. In this study, we developed a new measurement cell that enables high-temperature heating and constructed an in-situ observation system using synchrotron radiation XAFS and XRD to obtain information on the structure and electronic state of materials around the melting point. The validity of machine learning was also verified by comparison with the structural and electronic state information predicted by theoretical calculations using NN potentials. In this study, we mainly report the results obtained by synchrotron radiation analysis of yttrium-stabilized zirconia around the melting point as first step. The measurements were performed at the JAEA beamline BL22XU at SPring-8. The incident light intensity was measured with an ion chamber placed just in front of the sample, a CCD detector and imaging unit for X-ray diffraction, and a silicon drift detector for XAFS, each of which was placed horizontally displaced from the optical axis downstream or upstream of the sample, taking into account the position of the window in the measurement chamber. The heating system was a pulsed current heating furnace using W as the electrode, which is capable of performing structural analysis experiments up to ~3500K. In the radial structure function by XAFS, the distance of the first peak originating from Zr-O tends to shorten as the temperature increases, especially around 2000 K. This indicates a phase transition of zirconia, corresponding to the phase transition of zirconia from Tetragonal to Cubic. On the other hand, the second peak originating from Zr-Zr shows a trend of increasing distance as a whole, and the peak became so small that its order is almost unobservable near the melting temperature. The energy of the absorption edge also showed a tendency to shift to the lower energy side with the structural change due to the temperature increase.