Numerical Simulation of a Highly Compressed Saturated Water Flashing Flow

Abstract

Transient fluid velocity and pressure fields in a pressurized water reactor (PWR) steam generator (SG) secondary side during the blowdown period of a feedwater line break (FWLB) accident were numerically simulated employing the saturated liquid flashing model. This model is based on the assumption that compressed water in the SG is saturated at the beginning and decompresses into the two-phase region where saturated vapor forms, creating a mixture of steam bubbles in liquid by bulk boiling. The numerical calculations were performed for two cases where the outflow boundary condition is different from each other; one is specified as the direct blowdown discharge to atmospheric pressure and the other is specified as the blowdown discharge to an extended calculation domain with atmospheric pressure on its boundary. To effectively simulate the saturated water flashing from the SG following the FWLB accident, the physical SG model was simplified as a vertical once-through SG to which a feedwater pipe is attached. However, the physical geometry of the analysis model was modeled as realistically as possible in terms of the SG tube bundle height, the SG inner diameter and porosity, the inner diameter and length of broken feedwater pipe part, etc. It was considered that the SG shell-side and the attached feedwater pipe were initially filled with high pressure saturated water. The pressure in the steam space was 7.5 MPa. For the calculation of the two-phase flow during high pressure saturated water flashing from the SG through the broken feedwater pipe, the inhomogeneous two-fluid model was used. The present simulation results were discussed through a comparison with the predictions using a simple non-flashing model neglecting the effects of phase change. Based on the comparative discussions, the applicability of each of the non-flashing liquid discharge and saturated liquid flashing discharge models to the confirmatory safety evaluations of new SG designs was examined.