Complex Structure of Molten NaCl-CrCl_x Salts: Octahedra Network and Intermediate-Range Order

摘要

In the last decade, there has been a sustained resurgence of the Molten-Salt Nuclear Reactor (MSR) concept. As the MSRs operate, chemical compositions and physical properties of salts change because of the creation of fission products, and the effects of corrosion and radiolysis. Therefore, computer simulations of the microscopic structure of multi-component molten salts are necessary to predict changing thermophysical properties. However, first-principles simulations inevitably involve simplifying approximations and therefore require careful experimental validation. Here we present detailed experimental and simulation studies of the structure of molten NaCl-CrCl_3 and NaCl-CrCl_2 near-eutectic mixtures. These compositions have been chosen to elucidate the behavior of Cr ions in the melts since Cr is the most important corrosion product. Although the corrosion will result in relatively small quantities of Cr in the melt, we used much larger Cr concentration to measure the structure around Cr ions with high resolution. The results of X-ray and neutron diffraction with isotopic substitution, X-ray absorption spectroscopy, and ab initio molecular-dynamic simulations are in agreement. In NaCl-CrCl_3 we found CrCl_6~(3+) octahedra and their networks, as well as intermediate-range order, which manifests itself in the prepeak at ～ 1 A~(-1) with non-monotonic temperature behavior.