Constant Pressure Startup System of the Super LWR

The constant pressure startup is proposed with reference to that of FPPs. Nuclear heating starts at supercritical pressure, and the pressure is kept constant during load change. Because the reactor operates at a constant supercritical pressure, the coolant in the fuel channels is single phase and steam-water separation is not necessary. The constant pressure startup system for the Super LWR is shown in Fig. 5.3 [2]. It is required to establish a sufficient flow rate to prevent the 

The constant pressure startup system can be classified into five phases, which are briefly described below. 

The core outlet coolant enthalpy must be greater than that of the saturated steam from the flash tank to prevent the main steam temperature from decreasing through the line switching. The relation between the required core outlet temperature and the flash tank pressure is shown in Fig. 5.4 [3]. If the flash tank pressure is taken to be 6.9 MPa (the same as that of supercritical FPPs), the coolant temperature at the core outlet must be greater than 420°C. This core outlet temperature is readily achievable in the present design (1,000 MWe class) of the Super LWR. 

The flash tank and pressure reducing valves are necessary initially during constant pressure startup of the Super LWR. The flash tank is designed such that the moisture content in the steam at turbine inlet is less than 0.1%. The dimensions of the flash tank required for startup are determined by using the correlations for the droplet entrainment and carryover from a boiling pool by a streaming gas, developed by Kataoka and Ishii [4]. 

In this region, the entrainment is independent of height and gas velocity. It consists of aU the droplets entrained at the pool surface and is given by 

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Startup schemes of the Super LWR are considered by referring to those of supercritical FPPs [43—45]. The constant pressure startup systems of the Super LWR and a supercritical FPP are shown in Fig. 1.16 [41]. The register tube and flash tank are installed on the bypass line. The supercritical steam is depressurized at the register tube and used for heating up the turbine during the startup (Table 1.6). 

Fig. 1.16 Constant pressure startup systems of the Super LWR and supercritical FPP. (a) Super LWR (b) Supercritical FPP...

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