The safety criteria of high flux research reactor with downward parallel channels: onset of flow instability and critical heat flux

This paper focuses on safety criteria for high flux reactors with parallel downflow channels: critical heat flux (CHF) and onset of flow instability (OFI). Sudo93 CHF correlations, a set of correlations widely used in high flux research reactors, are used to predict CHF. Flow excursion is especially important for high flux reactors as it can lead to premature CHF. Therefore, OFI prediction is a must for high flux reactors. In this paper, the database of Sudo93 CHF correlations is studied and analyzed. It’s found that the data in the CHF database have the possibility of being affected by flow excursion. In addition, Sudo93 CHF correlations are used to evaluate the collected OFI experimental data. It is shown that Sudo93 CHF correlations can predict downflow OFI under certain conditions. The reason is given by comparing Sudo93 CHF correlations with OFI prediction correlations. The –33% uncertainty of Sudo93 CHF correlations can also be accounted for from the perspective of OFI. Furthermore, this paper analyzes the deviations between the predicted and measured values in Sudo CHF database. It is confirmed that OFI influences the lower limit of Sudo93 CHF correlations. Eventually, this paper proposes the improved downflow Sudo93 CHF correlations by introducing the flow instability factor, which improves the economic benefits while ensuring safety. When the improved Sudo93 CHF correlations are used for high flux reactors, additional OFI prediction is unnecessary. This further reduces unnecessary safety margin. Therefore, this study has great significance for the design and safety analysis of high flux research reactors.