Phase transitions of sodium peroxide investigated by DSC

To simulate the sodium fire phenomenon that is expected to occur in an accident of sodium-cooled fast reactor, it is necessary to obtain the thermodynamic parameters that describe the structural phase transition and melting of sodium peroxide (Na₂O₂). Nevertheless, the thermodynamic database and existing literature provide only limited information on this topic. The objective of this study was to ascertain the thermodynamic parameters associated with the phase transitions of Na₂O₂ through the use of differential scanning calorimetry (DSC). Due to the high chemical reactivity of Na₂O₂ at elevated temperatures, particular precautions were necessary for the DSC measurement, including the selection of an appropriate crucible material, the preparation of a custom-made crucible with a specialized geometrical configuration, and the meticulous calibration of the recorded temperature and enthalpy change. Furthermore, all experimental procedures for the DSC measurement were required to be conducted under controlled atmospheric conditions of inert gas. Despite the aforementioned difficulties, we were able to successfully determine the transition temperatures and enthalpy changes associated with the structural phase transition and melting of Na₂O₂ using DSC. The reliability of these thermodynamic parameters was validated by comparing them with previously reported values.