Experimental study on flow regimes in debris bed formation behavior with mixed-size particles

Abstract During a core disruptive accident of sodium-cooled fast reactors, the fragmented debris released from the reactor core will start to sediment and form debris beds within the lower plenum region of the reactor vessel. To ascertain the characteristics of flow regimes during this debris bed formation process, in our earlier publications based on a great number of experiments conducted using mono-sized spherical and non-spherical solid particles, four typical flow regimes (i.e. the particle-suspension regime, the pool-convection dominant regime, the transitional regime and the particle-inertia dominant regime) were identified. In addition, a flow-regime model, including the base model for predictions of spherical particles and the extension scheme for non-spherical particles, has been successfully developed. In this study, motivated to further understand this behavior at a condition of more realistic particle mixture, a new series of simulated experiments using mixed-size particles is conducted under the gravity-driven release from a nozzle into a static two-dimensional water pool. Through detailed analyses, it is found that the base model, by employing the area mean diameter, can predict the overall regime transition under the condition of mixed-size particles reasonably well. Even if a quantity of effective sphericity is introduced, the previous extension scheme for non-spherical particles lacks the capability to provide satisfactory predictions. To improve the prediction using the form of extension scheme, a new correction scheme based on the volume-mean diameter and the degree of convergence in particle-size distribution is suggested and validated. The conducted analyses in this study also indicate that due to the difference in component inertia, the potential separation and stratification of solid particles are likely to exist during an actual debris bed sedimentation process in a core disruptive accident.