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New experiments on air‐water countercurrent flow limitations in a vertical pipe are presented. The experiments were carried out to investigate the effect of test procedure on the occurrence of the two countercurrent flow limitations; namely the onset of flooding and complete carry‐up limits. The results revealed the existence of significant hysteresis effects at both limits particularly for high liquid feed rate. For low liquid feed rate, hysteresis effects were only observed at the complete carry‐up limit. Some associated visual observations are reported and the results were empirically correlated and compared with recent data obtained by others.

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The paper describes briefly a best estimate plus uncertainty analysis (BE+UA) methodology and presents its prototyping application to the power pulse phase of a limiting large Loss-of-Coolant Accident (LOCA) for a CANDU 6 reactor fuelled with CANFLEX® fuel. The methodology is consistent with and builds on world practice [1], [2]. The analysis is divided into two phases to focus on the dominant parameters for each phase and to allow for the consideration of all identified highly ranked parameters in the statistical analysis and response surface fits for margin parameters. The objective of this analysis is to quantify improvements in predicted safety margins under best estimate conditions.

This paper deals with the use of a two‐fluid model for predicting the thermal‐hydraulic characteristics of the rewetting and refilling of hot horizontal tubes. The two‐fluid model equations were simplified and solved using an explicit finite difference scheme. A set of constitutive equations is used for closure. A new film boiling model and a model for evaluating the rate of latent to total heat in film boiling were incorporated in the two‐fluid model. A newly proposed rewetting criterion, based on vapour film collapse, was used to mark the end of the film boiling regime. The model is capable of predicting the rewetting and refilling process in horizontal tubes under conditions of stratified or inverted annular refilling flow. The model predictions were found to be in good agreement with the experimental results.