Fig. 5-4 x-ray diffraction pattern of molten UCl3 (1,150 K) Octahedral coordination (UCl6)3-, which in the melt is about 0.6 nm in size (determined by FSDP (First Sharp Diffraction Peak) analysis).

Fig. 5-5 Images of larger scale systems The size of the basic cell to be used in the simulation depends on the length of the structural order. In the case of a linkaged structure via a Cl- ion as shown in the figure, the size of the dimer is 1.2 nm. Large systems, of more than 10,000 ions, should be used to obtain precise structural information.

Fig. 5-6 Partial correlation functions gU-U(r) of molten UCl3 The curve shows the radial distribution density of U as a function of distance from U. A value of gU-U(r)=1 means no structural order between U and U. Cut-off distance for each curve corresponds to the size in Fig. 5-5. In a small system (1,000-ion system), a significant oscillation remains at a cut-off distance of 1.58 nm.

A molten salt is a high temperature liquid. In the liquid state, the atomic arrangement has a disordered distribution. There is, however, a short-ranged structural order in many liquids. Many physical properties of the liquid are affected by the structural order. We have studied the correlation between molten structure and physical properties to contribute to new technologies, for example, a nuclear fuel cycle based on pyrochemical reprocessing. In molten uranium trichlorides (UCl3), (1) The U3+ ion is surrounded by six chloride ions to form an octahedron (UCl6)3-. (2) This melt has a longer structural order than simple liquids such as water. In trichloride melts, a Cl- ion must belong to two neighboring octahedra simultaneously, because of a lack of Cl- ions due to the chemical stoichiometry. Such a network structure may be assigned to a medium-range structural order in molten UCl3 (Fig. 5-4). The structure of molten salts has been widely studied by computer simulation. The number of ions used in many simulations is around 1,000, corresponding to 3 nm of basic cell as shown in Fig. 5-5. This is insufficient to treat networking of a ~ 0.6 nm octahedron. We used a parallel computational technique in the simulation using larger systems of more than 10,000 ions. The medium-ranged structural order can then be precisely evaluated by the simulation (Fig. 5-6). We plan to obtain more precise structural information experimentally, and to improve the accuracy of the model in the computer simulation. Our goal is to clarify physical property of molten salts systems containing actinides.

Reference Y. Okamoto et al., Structure and Dynamic Properties of Molten Lanthanum Tribromide, Z. Nat. Forsch., A:Phys. Sci., 54a (2), 91 (1999).

Select a topic in left column