Thermal-Hydraulic Stability Considerations

4 Design of Startup Curves Based on Thermal Considerations 

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]. 

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Thermal-Hydraulic Stability Considerations 5.4.1 Mechanism of Thermal-Hydraulic Instability... 

Thermal-Hydraulic Stability Considerations Mass conservation ... 

Coupled Neutronic Thermal-Hydraulic Stability Considerations... 

From the thermal considerations in Sect. 5.3.3.2, the core power, feedwater flow rate and feedwater temperature are designed as 20 and 35% of the rated values and 280°C, respectively. The decay ratios of coupled neutronic thermal-hydraulic stability are then calculated with these conditions. The results are shown in Fig. 5.55 [10]. The stability criterion is satisfied with sufficient margin during... 

The power raising phase in the plant startup was designed in consideration of the thermal and thermal-hydraulic stability criteria. Due to the two-pass flow scheme, the power to flow rate ratio became locally large in the downward flow channels during certain power and flow conditions. Thus, the flow rate required to satisfy those criteria was determined by the MCST and the decay ratio of thermal-hydraulic stability in the downward flow channels. 

Considerations of distributions across the flow channel, transverse to the primary flow direction, were first included in basically one-dimensional models by approximating the temperature distribution in the fluid parallel to the flow direction. Recently there is an increasing application of CFD to various single- and two-phase thermal-hydraulic analyses, including NCLs and supercritical fluid states, in nuclear power systems. These approaches also allow for resolution of the thermal stratification in horizontal and vertical sections of the loop as well as resolution of gradients normal to the primary flow direction and the consequent effects on calculated stability. Fully three-dimensional analyses are becoming the norm, but only for simple idealized single-phase cases. 

The plant dynamics code for the analysis of plant control and startup thermal considerations are described in ref. [115]. The subchannel analysis code and the analysis are found in refs. [116, 117]. Thermal-hydraulic and coupled stability calculations at supercritical and at subcritical pressure as well as startup considerations are described in ref. [118]. 

Compared with the mineral-based fluids, these fluids exhibit reduced variation in viscosity as a function of temperature, which removes the requirement for a VI improver to be added. PAO-based hydraulic fluids also demonstrate outstanding reliability under mechanical stress and excellent chemical and thermal stability. The fluid developed was given the NATO designation H-537 and is controlled by the US military specification MIL-PRF-83282. In contrast to the mineral-based H-515 this fluid has a PMC flash point in the region of 215°C which is a considerable increase. 

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