Void-drift mechanism and application to the subchannel analysis of boiling water nuclear reactors

With the help of the recent results on the interfacial turbulence in two-flows, the mechanism of the void drift is exposed. The mechanism is based on the following explanations: 1) The net turbulent mixing in the vapor phase is due to the conventional turbulent mixing only. 2) The net turbulent mixing in the liquid phase is due to the conventional turbulent mixing and to the interfacial turbulence contribution which lead to the void drift phenomenon, i.e., to a void coring and to an accumulation of the liquid phase in the corner subchannels. It is shown that this void drift is in fact the direct consequence of the interfacial turbulence driven secondary flows. Thereafter, a mechanistic model of the void drift was derived, it is mainly based on the introduction of a length scale characteristic of the turbulence diffusion near the interface (in the liquid phase) and not on the conventional turbulent mixing length theory of Prandtl. Moreover, and conversely to the previous correlations, this mechanistic model takes also into account the existing non-proportionality between the void fraction difference (αi-αj)FD and the mass flux difference (Gi-Gj)FD in the fully developed flow conditions in the adjacent subchannels i, j. Systematic tests using THERMIT2 code exhibits a good agreement with the experimental data of GE 3 x 3 rod bundle. (author)