Modeling of droplet diameter changes during reflood and its application to RELAP5

Abstract

Water droplets are generated in the reflood period of a large break loss-of-coolant accident by either shattering of liquid ligament or interfacial shear force. As generated droplets experience vaporization, collision and merging with other droplets, and further breaking up by spacer grid and aerodynamic force, their diameters could vary. Droplet diameter plays an important role in estimating reflood heat and mass transfer, especially in dispersed flow film boiling regime. System thermal-hydraulic analysis code like RELAP5 in which Sauter mean diameter is used as an average diameter in general, however, does not consider mechanistic variation of the diameter. The average diameter is often adjusted as a function of local Weber number in some situations. In this study, mechanism of droplet size variation and how to accordingly determine Sauter mean diameter are proposed assuming lognormal distribution. And it is validated against RBHT data using RELAP5/Mod3.3 patch 5 code. Proposed model was found to quite well predict Sauter mean diameter of droplets and cladding temperatures of RBHT, especially in droplet-prevailing region. And standard deviation of their uncertainty was evaluated as ±0.259 mm. Average difference between measured and calculated cladding temperatures along the core of ten RBHTs was reduced from 42.1 K to 21.7 K with less scatter by 31.7 K.