Abnormal Transient and Accident Analyses at Subcritical Pressure

5 Abnormal Transient and Accident Analyses at Subcritical Pressure 

The core designed by Kamei et al. [31 ] is analyzed here. During the pressurization from 8 MPa to 22 MPa, the feedwater temperature is raised from 150°C to 280°C linearly with the pressure. The reactor power and the feedwater flow rate are kept at 20% and 40% of the rated value, respectively. These conditions are slightly different from those described in Chap. 5 due to the difference in the core characteristics. 

The pressure control system and the power control system designed in Chap. 4 are used. However, the main steam temperature control system cannot be used because the core outlet temperature is the saturation temperature. Therefore, a feedwater controller for subcritical pressure operating conditions is needed. Dining subcritical pressure operation, the feedwater flow rate is regulated in order to keep the water level in the steam water separator, instead of regulating the main steam temperature. A combined proportional and derivative controller (PD controller) is found to be suitable for that purpose [12]. 

The transients and accidents shown in Table 6.5, excluding LOCA events, are analyzed. As representative cases, the initial conditions at 8.3 MPa, 15 MPa, and 21 MPa are selected. The same criteria as described in Sect. 6.5 are applied. The MCST during the pressurization phase is lower than that of the normal operating condition by 350°C at 8.3 MPa, 310°C at 15 MPa and 30 C at 21 MPa. These differences of the MCST are added to the allowable increases in the cladding 

The calculation results of the total loss of reactor coolant flow at 15 MPa are shown in Fig. 6.56 as an example of the loss of flow events. Due to the coast-down and hence the decrease in the core coolant flow rate, departure-from-nucleate-boiling (DNB) occurs at 3 s and the cladding temperature quickly increases. However, the peak value is much lower than the criterion. Although the water source effect of the water rods is small at subcritical pressure, the core coolant flow can be kept by natural circulation in the recirculation loop before the start of the AFSs. 

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