Start-up and safety analyses are important issues in core design development of a Super FR. This section summarizes the motivations and objectives of the current study which are based on the general and specific issues of start-up and safety characteristics of the current design of Super FR.
In general, since a Super FR is operated at supercritical pressure, the start-up system will be different from that of LWRs. Analysis of the star-up system is important because it involves transition of the coolant from subcritical pressure to supercritical pressure. There will be a large change of the coolant properties. There is a concern that the MCST might be high at around critical pressure during pressurization. The start-up system must assure that the MCST is low to keep the fuel rod integrity. Scheme of the pressurization from subcritical to supercritical pressure should be also investigated. Besides that, the start-up system should
provide superheated steam due to similar balance of plant (BOP) to that of fossil fuel power plants which the turbines are started by superheated steam. A start-up system with a recirculation had been proposed in the past study to meet these requirements. However, detailed thermal analysis of the start-up system was not carried yet. In the current study, the start-up system of the two-flow pass Super FR with all upward flow is analyzed in thermal view. The start-up procedures are investigated. Maximum cladding surface temperatures both during pressurization and power-raising are also clarified.
In view of the safety characteristics, since the operating pressure and cooling system are different from that of LWRs, the safety principle of a Super FR is also different. Comparison of the cooling system between LWRs and a Super FR is shown in Fig. 1-15. LWRs have coolant circulation system such as recirculation system of BWR and primary system of PWR.
LWRs have water level since these are operated at subcritical pressure. The fundamental safety requirement of LWR is keeping the coolant inventory by maintaining the water level in RPV of LWRs. A Super FR cooling system has no coolant circulation system and water level does not exist at supercritical pressure. The fundamental safety of a Super FR is keeping coolant flow rate instead of the water level. The safety analysis of a Super FR is important due to these differences.
Fig. 1-15 Cooling system of plant systems (Ishiwatari et al., 2005a)
Compared with thermal reactor (Super LWR), a Super FR has higher power density and no moderator. High power density means less water inventory in the RPV. Coolant flow rate
of a Super FR is lower than that of Super LWR. Thus the safety performance of a Super FR will be also different from that of Super LWR.
The single flow pass core of Super FR has some differences from the two flow pass core of Super FR. Gap and lower plenums of the two-flow pass core as shown in Fig. 1-7 are removed in the single-flow pass core design. It leads to be less water inventory in the single-flow pass core. It might influence the safety performance during abnormality. Besides that, a large change of coolant density in axial direction is introduced in the single-flow pass core of Super FR, leading to a high power peaking at beginning of cycle (BOC). In case of abnormality, it might also influence the safety performance. Large difference of flow distribution between Seed and Blanket assemblies leads to a large orifice coefficient in the blanket assembly. In case of flow rate recovery, high pressure drop may happen in the blanket assembly. MCST of the assembly should be clarified. These difference features lead to different safety characteristics of the single flow pass core from that of the two flow pass core of Super FR. Safety characteristics of the single flow pass core of Super FR are important to analyse.
Passive safety system of a Super FR has not studied until now. Passive safety system of a Super FR needs to be studied to improve the maintenance cost of safety system during operation. Passive safety systems do not need an emergency electric power. The current study proposes a concept of passive system for Super FR. Applicability of the passive system to a Super FR is investigated.
The objectives of the current study are as follow:
1. To perform start-up thermal analysis of the two-pass Super FR with all upward flow:
a. to investigate the startup procedures of the Super FR,
b. to perform the detailed time dependent start-up thermal analysis of the Super FR, c. to design the steam drum used for pressurization during the start-up.
2. To perform safety analysis of the single-flow pass core Super FR.
3. To propose a concept of passive safety system of Super FR 1.5 Thesis Organization
This thesis is divided into 6 chapters. Chapter 1 presents the background, core designs
description of the Super FR, review of the past studies, and the objectives of this study.
Chapter 2 presents the methods and codes that are used for the start-up and safety analyses.
Chapter 3 discusses the time dependent start-up thermal analysis of the two-flow pass Super FR with all upward flow. The start-up system and its procedures are clarified. Thermal analysis during pressurization especially at subcritical pressure which potential of high MCST may happen is discussed. Afterwards the thermal analysis during power-raising at supercritical pressure is presented.
Chapter 4 describes the safety analysis results of the single-flow pass Super FR. Its safety characteristics are clarified. Analysis results of abnormalities like accidents, abnormal transients, LOCA and ATWS are discussed. The analyses of accidents are carried both with and without considering the control system.
Chapter 5 presents the passive system of the Super FR. A Passive safety system is proposed for the Super FR. Applicability of the passive system to the Super FR is discussed.
Safety characteristics of the Super FR are clarified using the passive system.
Chapter 6 summaries the conclusions obtained from this study.