Liquid Pb-Bi eutectic alloy: Study of short-range order

Liquid Pb-Bi eutectic (LBE) alloy has been selected as a coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). This new reactor type might be one of the possible solutions for the nuclear waste problem because it is able to transmute high level radioactive waste and long lived actinides. The ADS technology however requires special operating conditions. The materials need to resist temperatures ranging between 200-550°C under a high energy neutron flux and in contact with the LBE. This condition increases liquid metal corrosion and embrittlement, in fact the limitation of ADS life is due to the relatively low corrosion resistance of structural materials in the LBE environment. The compatibility of structural materials with liquid LBE at high temperature is one of the key issues for the commercialization of such fast reactors. Possible structural changes of the liquid may affect the interaction of LBE with structural materials. Therefore, in previous works LBE was investigated by internal friction (IF) and dynamic modulus measurements far above the eutectic temperature (125 °C) by using a new method developed by us. After melting the alloy exhibits a steady modulus decrease up to 350 °C, here a remarkable drop is observed covering a temperature range of about 170 °C. Finally, above 520°C, modulus continues to decrease with slope very close to the initial one. In correspondence of the modulus drop two IF maxima were detected: The first centered at -350 °C, the second at -460 °C. Anomalies of the liquid metal have been evidenced by other investigators as well. It is believed that the structure of the alloys is heterogeneous after melting, with residual minor crystals still existing and, when the critical temperature is reached, the residual conglomerates are broken and the uniform structure appears. To better understand the nature of LBE structural evolution vs. temperature High Temperature X-Ray Diffraction (HT-XRD) measurements have been carried out up to 720 °C; from diffraction patterns the radial distribution function (RDF) has been calculated (some G(r) curves are reported in Fig. 1). RDF provides information about possible changes of liquid metal. The average distance rt between the 1st nearest neighbour atoms is of particular relevance: The position (Fig. 2) and shape of 1st RDF peak progressively change as temperature increases with strict correspondence with the dynamic modulus drop previously observed by us. The trend of the ratio r2 / r1 vs. temperature (Fig. 3) showed that just after melting r2 /r1 is ~ 1.41, increases till to reach the value of -1.61 at 720 °C. The result indicates that the short-order of liquid LBE gradually changes from a cuboctahedral configuration (the ratio is ) just after melting to an icosahedral one (the ratio corresponds to the golden ratio φ = 1.618 ) at 720 °C. The two structures of the liquid are shown in Fig. 4. To describe structural re-arrangement of atoms in the liquid from cuboctahedral to icosehedral configuration a model has been realized (Fig. 5). The fitting of the first RDF peak by pair functions Pij(r) permitted to identify and determine the single contributions at increasing temperature. From melting up to 350 °C, the mixed Pb-Bi pairs are not present. They appear at 350 °C and their contribution to RDF 1st peak becomes more important as temperature increases. In fact, the increasing number of Pb-Bi pairs corresponds to a more homogeneous elemental distribution in the alloy.