ERVC is widely adopted as a part of in-vessel retention (IVR) in severe accident management strategies. In this paper, two-phase flow instability in natural circulation loops of external reactor vessel cooling (ERVC) system in a large size power PWR (CAP 1700 with a thermal power 5000 MWt) is simulated and evaluated by the RELAP5 code. Under certain conditions, flow instability of ERVC system are obtained. It is a kind of density wave oscillation that occurs in non-equilibrium boiling in the heat section and void flashing in the riser at low equilibrium quality and low system pressure. The calculation results show such oscillation course clearly. And several parameters affecting the flow stability are discussed.

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