Assessment of TRAC-PF1 Condensation Heat Transfer Model for Analysis of ECC Water Injection Transients

Abstract

Direct contact condensation of vapor occurs after injection of emergency core cooling (ECC) water during a loss-of-coolant accident (LOCA) in a light water reactor (LWR). Despite that vapor condensation may have large influences on system thermal-hydraulic responses, it is difficult to predict the condensation rate accurately during ECC water injection. This difficulty sometimes results in considerable discrepancy between predicted and measured system overall transients. For instance, analyses of PWR reflood tests with the TRAC-PF1/MOD1 code predicted non-physical large flow oscillations because the code largely overpredicted the condensation rate. In an attempt to resolve this shortcoming of TRAC, a sensitivity study was carried out on the condensation heat transfer Stanton number, that is assumed to be a constant in TRAC, by analyzing the broken-loop hot leg responses to ECC injection in a combined injection test conducted at the Cylindrical Core Test Facility (CCTF). Based on this sensitivity study, it is recommended to use a Stanton number of 0.0035, which is about six times smaller than the value used in the current version of TRAC, to predict LWR system responses during an ECC water injection transient.