Analysis of direct contact condensation of flowing steam onto injected water with multifluid model of two-phase flow

When subcooled water in accumulator tanks is injected during a loss-of-coolant accident of a pressurized water reactor, violent condensation takes place in cold legs because of direct contact of steam with water. A flow model based on the multifluid model of the two-phase flow has been developed to formulate the condensation and mixing processes in the injection region. The measured liquid film temperature and pressure profiles in the injection region have been explained quantitatively with the model which includes (a) drop generation arising from atomization of injected water, (b) condensation of steam on the drops, (c) flow contraction resulting from the formation of the dead water region, and (d) deposition of the drops. The calculated results show that the condensation rate depends mostly on the drop diameter generated near the water injection nozzle and the maximum drop mass fraction in the injection region. The present model can predict the effect of inlet thermal-hydraulic conditions on the condensation rate qualitatively, and then it has been confirmed that higher condensation rate with initial steam mass velocity is caused by the formation of finer drops and the higher drop mass fraction in the injection region. (author)