Experimental investigation on hydrodynamic phenomena associated with postulated vapor explosion scenarios in the Phenix reactor

摘要

This work proposes an experimental investigation on interface instabilities between accelerated immiscible fluids with very different densities. These phenomena are representative of hydrodynamic phenomena associated with vapor explosion scenarios postulated to occur within narrow rectangular subchannels of experimental assemblies deployed in the Phenix core and suspected to be the cause of the four automatic reactor trips at the end of the nineties. A versatile experimental device has been conceived and operated to investigate the onset and the development of interface instabilities. These are triggered by a pressurized gas pushing on a liquid layer. A pressure probe detects the pressure in the gas phase, whereas a high speed video camera is used to track the interface. High speed video records have revealed corrugations on the interface initiated by Rayleigh-Taylor instabilities, which evolve into a chaotic phase as long as the liquid phase is expelled from the channel. These experiments are intended to face innovative issues in the creation of a two-dimensional instability, the quantitative estimation of the interface area up to later stages of Rayleigh-Taylor instabilities (non-linear phase) and the qualitative analysis of the chaotic, final phase. The test matrix spans different initial overpressures, from 2 to 7 bars, corresponding to a maximum interface acceleration of about 250 m/s~2.