Startup Schemes

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). 

Boiling (and dryout) must be prevented in the water rods at subcritical pressures (in sliding pressure startup scheme). 

Fig. 1.18 Calculation model for sliding pressure startup scheme. (Taken from ref. [43] and used with permission from Atomic Energy Society of Japan)...

The sizes of the components required for the startup schemes are assessed. The sliding pressure startup with a steam separator in a bypass line is the best from the viewpoint of weight of the components. A study of the times needed for the startup schemes remains as future work. There is a limitation on the rate due to thermal stresses on thick-walled components such as the RPV. In BWRs, the temperature rise rate of the RPV wall is limited to below per hour. 

In summary, at the subcritical pressure operation during the pressurization phase, thermal criteria are more limiting due to dryout. The startup scheme prior to line switching is mainly determined by thermal criteria. The thermal-hydraulic stability criterion is satisfied by applying a sufficient orifice pressure drop coefficient. The coupled neutronic and thermal-hydraulic stability is also satisfied, since the power to flow rate ratio is low during this phase. 

In this chapter, the startup and stability of the Super LWR plant are described. Since the plant system of the Super LWR differs from those of LWRs, the startup scheme that is suitable for the Super LWR plant needs to be proposed. The reason why the plant startup and stability are described in the same chapter is that the stability is one of the design limitations for the startup procedure. 

In Sect. 5.2, two startup schemes for the Super LWR plant are proposed referring to those of fossil-fuel fired power plants (FPPs). The required sizes and weights of the equipment for these startup schemes are calculated. 

There are two kinds of startup schemes currently used in FPPs [1]. One is the constant pressure startup scheme, in which the boiler operates at constant supercritical pressure after the coolant is pressurized to this point. The other is the sliding pressure startup scheme, in which the boiler operates with variable pressures and the pressure increases with the generation output. 

Since the Super LWR plant system does not have a superheater, the main steam conditions need to be adjusted during startup and low power operations. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. At subcritical pressure operation in the sliding pressure startup scheme, boiling and dry out in the descending moderator water rods are undesirable and should be prevented because they affect the inlet subcooling. 

There is no difference between the pressurization phases of the constant pressure startup and sliding pressure startup schemes because the pressurization phase appears after the line switching to the once-through mode. It is assumed that the core inlet and outlet temperatures are kept equal to their respective values in the normal operating condition. While the reactor core power is increased, the feed-water flow rate is also increased proportionally. MCSTs are calculated for various core powers from 30 to 100% at intervals of 10%, and the calculated results are shown in Fig. 5.11. It is found that MCST satisfies the criterion of 620°C throughout the power increase phase. 

Based on the thermal considerations above, the general startup curves for the constant pressure startup scheme of the Super LWR are designed as shown in Fig. 5.22 [3]. Those for the sliding pressure startup scheme are designed as shown in Fig. 5.23 [3]. 

The startup curve for the sliding pressure startup scheme that is designed based on only the thermal considerations (Fig. 5.23) is redrawn taking the stability considerations into account as well. The constant pressure startup is not discussed here because the partial power operating conditions in the constant pressure startup are covered by the temperature increase phase and power increase phase of the sliding pressure startup. 

Design and Analysis of Procedures for System Pressurization and Line Switching in Sliding Pressure Startup Scheme... 

In the sliding pressure startup scheme, the system pressure of the Super LWR is assumed to be raised by nuclear heating the same as in BWRs. In the thermal and stability considerations for the sliding pressure startup introduced in Sects. 5.3-5.6,... 

From these two background facts, the purposes of this section are to revise the design of the sliding pressure startup scheme, propose its detailed procedures, and assess their feasibility by a system transient analysis. This section covers just the procedures before the power raising phase because the power raising phase itself... 

In this chapter, the plant startup and stability were introduced. Both constant pressure and sliding pressure startup schemes are designed by referring to FPPs. The constant pressure startup system requires a startup bypass system consisting of a flash tank and pressure-reducing valves. The sliding pressure startup scheme... 

Especially for the sliding pressure startup scheme, system pressurization and line switching from recirculation to once-through mode were investigated in detail. The feasibility of the system and procedures for them were assessed by a system transient analysis. 

As described in Chap. 5, the sliding pressure startup is one of the candidate startup schemes. It is necessary to understand the reactor behavior in case of abnormal transients and accidents during the pressurization phase. To do that, SPRAT-DOWN is extended to the SPRAT-DOWN-SUB which can be applied to the transients and accidents during subcritical pressure operation [12]. 

The CR withdrawal at startup is analyzed. The initial condition is the hot standby where kgff is 1.0, the reactor power is 1.0 x 10 of the rated power, and the main coolant flow rate is 20% of the rated flow. The analysis is made assuming adoption of the constant pressure startup scheme described in Chap. 5. The similar analysis for the sliding pressure startup scheme is described in Sect. 6.7.2. The reactivity worth of the withdrawn CR cluster is 2.8%dk/k. The same withdrawal speed as of PWRs is taken. The CR cluster is withdrawn until the reactor period decreases to the scram setpoint (10 s). The inserted reactivity is 0.39. 

The Super LWR and Super FR may be operated at subcritical pressure during startup if the sliding pressure startup scheme is chosen. Also, the pressure decreases from supercritical to subcritical at depressurization events such as a LOCA. The CHF and the post boiling transition (post-BT) heat transfer are important, especially just below the critical pressure because the CHF, the enthalpy of the CHF conditicMi and the post-BT heat transfer coefficient are all substantially smaller than those at a... 

Chapters 3-5 treat the plant system and behaviors. They include system components and configuration, plant heat balance, the methods of plant control system design, plant dynamics, plant startup schemes, methods of stability analysis, thermal-hydraulic analyses, and coupled neutronic and thermal-hydraulic stability analyses. 

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