Analysis of operating characteristics of IPWR under natural circulation

Many integrated pressurized water reactor (IPWR) designs using natural circulation operation mainly to enhance their inherent safety. The operating characteristics of primary coolant are completely different without mechanical pumps. The designs and safety analysis of forced circulation reactors are widely researched, but the natural circulation characteristics of IPWR have not been well studied by literatures. The present work discussed the thermal-hydraulic characteristics of IPWR under natural circulation conditions by using the best estimate codes RELAP5. And the effect of system parameters on natural circulation characteristics of IPWR is also studied. The results show that, the primary coolant average temperature and steam pressure are two key parameters that affect the natural circulation stable operating load. The set value of primary coolant average temperature effects the core outlet temperature and the steam temperature, but the primary coolant flow is basically the same under different primary coolant average temperature but same load conditions. The smaller steam pressure is more conducive to produce superheated steam, but there is risk of two phase flow instability in OTSG secondary side. The rapid load change process under natural circulation indicating that the reactor has a good load tracking characteristics under natural circulation, but the rapid change of primary coolant temperature will cause oscillations in system parameters.