Improvements to the prediction of boiling transition during boiling water reactor transients

The ability to accurately predict the time required to reach a boiling transition in postulated BWR (Boiling Water Reactor) loss of coolant accidents is an important step in analyzing the system response to these transients. As part of the effort to develop an advanced BWR system analysis computer code capable of simulating these postulated transients, a study has been undertaken to improve the boiling transition calculations associated with LOCA-type (Loss of Coolant Accidents) phenomena. In particular, the transition from nucleate to film boiling under high steam quality conditions and the film to nucleate boiling which is a precessor to fuel rod rewet were examined. Based on comparison with BWR simulation experiments, the results of our study lead to two important conclusions. First, inclusion of the effect of nonuniform heat flux in the prediction of boiling transition is important in BWR transient calculations. Second, use of the homogeneous nucleation temperature as the minimum temperature at which film boiling at fuel rod surfaces may occur may be inadequate in many BWR applications.