| In the design of the next generation of nuclear reactors and in the safety assessment of the currently operating nuclear power plants, it is necessary to evaluate the possibility of experiencing a degraded-core accident and to identify the key strategies to follow in order to mitigate the possible consequences. The ex-vessel cooling by bottom water injection strategy presumes the failure of the reactor pressure vessel.This project is focused on determining the influence of water mass flow rate, system pressure and co-injected gas flow rate on the volumetric heat removal rate, flow stability and molten pool void fraction of the system. An apparatus has been designed, which uses molten lead at a temperature of 500°C as a corium simulant. A wide array of experiments are being performed focused on co-injection of 1--6 g/s of water and up to 30 slpm of Argon at system pressures of up to 5 bar, typical of that expected in accident scenarios. Important parameters for the heat transfer characteristics are measured and flow instability is investigated.The experiments indicate that the heat removal rate from the melt increases with increasing water injection rate. The void fraction increases with increasing water mass flow rate as expected. An increase from low to medium pressure (0--2.5 bar) decreases the void fraction, while there is a modest decrease due to a pressure increase from medium to high pressures (2.5--5 bar). Increasing the water flow rate results in a shift towards an unstable flow regime. An increase in system pressure suppresses the observed flow instabilities. The presence of non-condensable gas also stabilizes the system. Beyond some limit, the heat removal rate appears to be independent of the water injection rate. These findings suggest that there exist an optimum water mass flow rate for melt quenching between 0.3 and 1.0 kg/m2-s under the current composition, geometric scale, and anticipated accident condition pressures. Suggestions on how to implement these findings into current ex-vessel cooling by bottom injection concepts are discussed. |
