An experimental study on debris bed formation behavior at bottom-heated boiling condition

Highlights: • Typical flow regimes are reproduced under boiling despite local differences. • Similar effect of particle properties under boiling is confirmed. • Boiling can affect regime boundary and characteristics due to vapor bubbles generated. • Non-monotonous effect of water depth under boiling environment is newly recognized. • Validity of using gas-injection to simulate boiling is justified. - Abstract: Motivated to further understand the effect of sodium boiling on the debris bed formation behavior that might be encountered during a core disruptive accident of sodium-cooled fast reactors, in this work a series of new experiments has been performed under the bottom-heated boiling condition. It is found that the four kinds of flow regimes (namely the particle-suspension regime, the pool-convection dominant regime, the transitional regime and the particle-inertia dominant regime), as observed from previous non-bubbling experiments, can be generally reproduced under current boiling condition, despite the existence of some local differences. Although a similar influence of particle properties (size, density and shape) on the regime transition is observable, the regime boundary is confirmed to be changed due to the steam bubbles generated from boiling. Even for the experimental cases without regime transition, the generated bubbling is verified to have an evident impact on the bed characteristic quantities during the debris bed formation process. As for the experimental parameter of water depth, different from previous non-boiling experiments, in this work for a given heating power, a non-monotonous effect is found to exist, due to the much diminished bubbling rate at rather higher water depths. To be comparable with previous experiments using gas-injection, based on energy conservation a quantity of effective bubbling rate is suggested. By using this quantity, the validity of gas-injection to simulate sodium boiling is justified, thereby providing us enhanced confidence for future studies over a much larger range of gas velocities.