Study on NC in primary loop and reverse flow in SG during SBO combining with SBLOCA

Marine nuclear power plants can rely on Natural Circulation (NC) to remove the heat generated by the reactor under normal operation or accident condition, which is the key to keeping the reactor safe. Based on the steady-state operation of single-phase NC and Forced Circulation (FC), the characteristics analysis of NC in primary loop and the reverse flow in the Steam Generator (SG) are analyzed employed by RELAP5/MOD3.3 code under the conditions of the Station Blackout (SBO) accident overlapped with 5-mm Small Break Loss of Coolant Accident (SBLOCA) in the cold leg happened and the Passive Residual Heat Removal System (PRHRS) of the secondary side operated. The results show that, with the development of the accidents, the loops A and B of the SG would go through the flow modes of the single-phase NC, the two-phase NC, and the reflux condensation. The occurrence of the three flow modes and their durations are dramatically different in the two loops for the influence of the pressurizer. The residual heat removal efficiency during the single- or two-phase NC is greater than that during the reflux condensation. The mass flow rate of the primary loop increases modestly, as the single-phase NC converting to the two-phase NC. During the NCs, the mass flow rate of the obverse flow in the SG is greater than that of the reverse flow, however, during the reflux condensation, the two mass flow rate are expected to be very close and the dynamic flow instabilities may appear in the primary loop.