Two-phase critical flows through some breaks of pressurized pipe systems are calculated by an improved critical flow model. These phenomena have many difficulties in predicting the two-phase critical flow rates at the break points mainly due to the inaccuracies of the critical flow model. Further, the case of critical flow through the small bottom break of horizontal pipe, the accuracy of a quality prediction model is also important as well as the critical flow model itself. In this study, an improved critical flow model is introduced which is based on a new sound speed criterion derived from the hyperbolic two-fluid model for non-equilibrium flow and this model is implemented to a system analysis code. Following to the vertically upward flows with quality variation and Marviken tests as benchmark problems, the small bottom break of a pressurized horizontal pipe is calculated and discussed in some details. The assessment results on the critical flow test through a small bottom break in a horizontal pipe show the importance of the accuracy of the critical flow model as well as that of the quality prediction model to reduce the relative errors.

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