Boiloff pattern transition during core uncovery in a small break loca

Through the core uncovery experiments, it has been known that the low power and high power core boiloff patterns are observed in the high pressure core uncovery following a small-break loss-of-coolant accident. A criterion for the prediction of annular to intermittent flow transition in vertical flow is developed and applied to the classification of low power boiloff and high power boiloff patterns. The instability of the interface wave on the liquid film is considered as the real physical mechanism for the flow pattern transition and the instability is explained by the concept of the hyperbolicity breaking in the characteristic equation. The applicability of the developed cliterion to the rod-bundle geometry is demonstrated using the flow pattern transition data taken by Bergles et al. and Venkateswararao. Finally, it is shown that the developed criterion well predicts the boundary between low power boiloff and high power boiloff through the comparisons of the predicted annular to intermittent flow transition conditions with the boiloff experiment data taken by Anklam and Osakabe.