Review of the AFD-type CHF mechanistic model and its application to rod bundle

Critical heat flux (CHF) is one of the most important thermal criteria for nuclear reactor design. To obtain a more widely applicable CHF prediction method, many mechanistic models have been proposed based on single channels with a good precision. Since the annular flow may occur in rod bundle in gas-liquid two-phase region for boiling water reactors (BWRs), it is essential to apply the annular film dryout (AFD) mechanistic models to rod bundle geometry. Among many existing AFD mechanistic models that are available to review, two major categories can be distinguished with considerable attention: the three-fluid model and the film analysis model. Many studies show that it is feasible to couple AFD mechanistic model with sub-channel analysis using suitable coupling strategies, but some crucial issues have not been solved. In view of these facts, this paper initially highlights the basis of AFD mechanistic model and its application to rod bundle. Then some factors which have predominant influence on the accuracy of the mechanistic model are reviewed and analyzed, including cross flow, onset of annular flow, inception fraction for droplet entrainment, boiling and spacer grid effect, dryout criteria. Finally, conclusions and prospects in this research field are summarized and proposed.