Spectroscopic, pressure and temperature measurements of the reactant mixing process in sodium-water reaction

•The runaway of sodium reaction with liquid water is a very fast process triggered within 1 ms.•The key to understanding this runaway is the reactant mixing process, which may occur either in liquid or gas-phase.•The presence of sodium vapor has been identified thanks to optical measurements.•The sodium temperature at runaway onset is about 240 °C.