Determination of thermochemical properties of the molten PuCl3-NaCl eutectic mixture by high-temperature drop calorimetry

Despite more than 70 years of research on the thermodynamic properties of molten salts, there are still limited experimental data towards understanding the phase stability relations of molten mixtures containing PuCl₃. While recent thermodynamic measurements of PuCl₃-NaCl yielded some data, there have been significant differences in the reported values of heat capacity of PuCl₃-NaCl in the liquid, or molten state. In this work, we conducted transpose temperature drop calorimetry of the PuCl₃-NaCl eutectic using a commercial Tian-Calvet twin microcalorimeter and the Ni encapsulation technique developed by us previously. The transpose temperature drop enthalpy (ΔH_ttd) was measured to be 155.31 ± 8.03 kJ∙mol⁻¹ at a temperature of 975.77 ± 0.08 K. To verify this value, a critical assessment of the literature was performed to determine that the enthalpy increment, ΔH_T-726, of molten PuCl₃-NaCl eutectic is 101.5 ± 8.1 kJ∙mol⁻¹, which agrees well with the heat capacity (C_p) derived by Karlsson et al. [26] using differential scanning calorimetry. The original C_p equation of Karlsson et al. was then extended with confidence to 993 K (C_p = 101.9 ± 2.1 J∙mol⁻¹∙K⁻¹). In addition, the excess heat capacity ($C_p^{\mathrm{ex}}$) of molten PuCl₃-NaCl eutectic was determined to be 4.2 ± 2.1 J∙mol⁻¹∙K⁻¹, which was then used to determine the enthalpy of mixing (ΔH_mix) to be –5.3 kJ∙mol⁻¹. These results provide the basis for modeling the thermodynamic stability of molten PuCl₃-bearing chlorides for nuclear energy and other applications.