Suitable high temperature PCMs for thermal energy storage are characterized and proposed as a promising candidate for nuclear reactor safety applications in the containment structure in the event of LOCA accidents. PCM with phase transition temperature in the range 70 °C to 90 °C and latent heat of around 200 kJ/kg or higher can be utilized to reduce temperature and pressure in the reactor containment during accidents. Reviewing the available data for high-temperature PCMs indicated significant deficiencies in published data and further investigations revealed that a material misrepresentation has occurred in some cases. Octadecanoic acid was identified as the most promising PCM for this application and the temperature-dependent thermo-physical properties and phase change behavior of octadecanoic acid were studied in detail. The results closed the gaps in the existing experimental database, studied the feasibility of utilizing several PCM candidates, measured the currently unknown temperature-dependent properties for optimum PCM, measured the specific heat capacity in the overlap and phase transition regions, and provided a valuable database for future design concepts, experiments and predictive simulation and modelling models.