A strong influence of minor lead additives on the liquid sodium microstructure is revealed in the molecular dynamics (MD) simulation of the Na0.98Pb0.02 alloy. The obtained results can be explained by the existence of lead-sodium clusters in liquid sodium built up by ionic bonds, Na+ , due to essential distinction of the alloy components in the electronegativity. On this reason, MD simulation of the Na0.98Pb0.02 alloy is carried out within the framework of a three-component bipolar model, Na + Na+ + , with Na Na+ recharging the nearest-neighbor particles of solvent in every 3 ps (an optimal period) during the numerical run. 1. Introduction A nonideal solution of lead in liquid sodium, as an intermediate element in the alkaline-metal group, takes place in the whole composition range of Na-Pb alloy, but the strongest changes of thermodynamic properties are observed in the neighborhood of 20 and 50% at. of lead concentration [1–3]. It is connected with the presence of two kinds of clusters in the alloy: (Na4Pb)n and (NaPb)m, having an ionic bond, Na+–Pb？, in diluted solutions and the covalent one, Na÷Pb, for the other compositions. They have explained this by the existence of Zintl’s cluster, (Na+)4(Pb4)4？ [3–7]. The anionic tetrahedron, (Pb4)4？, is considered as a united particle with sodium cations, Na+, on the four faces of this tetrahedron. However such anion is not in a steady state in liquid sodium [6, 7]. Therefore the application of Zintl’s cluster model [5] for understanding properties of the Na-Pb alloy is not enough. A new MD study as well as the experimental one of its microstructure and atomic dynamics by the method of neutron scattering is necessary. The knowledge about the Na-Pb alloy properties can be useful to develop a concept for improving the composition of the sodium coolant in particular by reducing its chemical activity in the environment and an automatic shut-down of sodium fires when sodium is used as a coolant of a fast nuclear reactor [8]. 2. The Cluster Model of Na-Pb Alloy It is known [9, 10] that the mixture of sodium and lead relates to strongly interacting systems with covalent bonds that are illustrated by a long homologous series of compounds, NanPbm, where n and m are integers. They influence the structural and thermodynamic properties of the binary alloy [1–7, 11, 12]. This effect can be displayed in analyzing the structural factors, SNN(q), SNC(q), and SCC(q), measured and calculated on various models [2, 5, 7]. It is also characterized by a sharp change of the alloy entropy [1] or an increase of the alloy electric
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