A novel experimental observation of the gravity-driven flashing of metastable water in a heated pool

Abstract

The gravity-driven flashing of superheated water, a non-equilibrium phase change phenomenon usually associated with geysers, has not been studied yet in the specific configuration of a pool heated from below. However, this vaporization mode is tightly linked with some of the safety issues of the Spent Fuel Pool (SFP) loss-of-cooling accident. In order to fill this gap, an experiment, presented in this article, was designed and provided first results that highlighted some relevant mechanisms behind this vaporization mode. By heating up 40 L of demineralized water at a constant reduced pressure of 25mbar with a heating power of 1000W, it was possible to reproduce the expected phenomenology of a SFP loss-of-cooling accident at the laboratory scale. Starting the test with an initially large supersaturation in dissolved gases within the liquid, the latter, once superheated, vaporized onto the air nuclei that resulted from the liquid degassing. When the dissolved gases content dropped below a threshold value, the nucleation vanished. The liquid continued to heat-up for a while until an energetic equilibrium was reached between the heat supply and the heat removal induced by the free surface evaporation. This equilibrium substantially limited the superheats reached within the liquid.