Pressurized water reactors startup

The primary circuit includes the equipment of the main circulation circuit, pressurization system and also auxiliary systems connected to the reactor. The fi ee volume above the reactor water level is used as steam-gas pressurizer for which gas is supplied to the pressurizer before reactor startup. The primary circuit operates in a non-boiling mode. 

The following example apphcable to pressurized water reactors (PWRs) may further illustrate the approach described. One of the SFs relevant for Levels 1-3 of defence in depth is prevention of unacceptable reactivity transients. This SF can be challenged by insertion of positive reactivity. Several mechanisms lead to such a challenge, including control rod ejection, control rod withdrawal, control rod drop or misalignment, erroneous startup of a circulation loop, release of absorber deposits in the reactor core, incorrect refuelling operations or inadvertent boron dilution. For each of these mechanisms there are a number of provisions to prevent its occurrence. For example, control rod withdrawal can be prevented or its consequences mitigated by ... 

T. Nakatsuka, Y. Oka and S. Koshizuka, Startup Thermal Considerations for Supercritical-Pressure Light Water-Cooled Reactors, Nuclear Technology, Vol. 134(3), 221-230 (2001) T. Nakatsuka, Y. Oka and S. Koshizuka, Start-up of Supersritical-pressure Light Wtater Cooled Reactors, Proc. ICONE-8, Baltimore, MD., April 2-6, 2000, ICONE-8304 (2000) T. T. Yi, Y. Ishiwatari, S. Koshizuka and Y. Oka, Startup Thermal Analysis of a High-Temperature Supercritical-Pressure Light Water Reactor, Journal of Nuclear Science and Technology, Vol. 41(8), 790-801 (2004)... 

T. T. Yi, Startup and Stability of a High-Temperature Suptuciitical-Pressure light Water Reactor, Doctoral thesis, the University of Tokyo (2004)... 

In BWRs, provision should be made to control the water level in the reactor pressure vessel during startup and in operational states. 

Low temperature overpressurization was originally identified as a safety issue in the early 1970 s because of numerous incidents of plants exceeding pressure-temperature limits. The majority of these events occurred while in a water-solid condition, during startup or shutdown operations, and at relatively low reactor vessel temperatures. 

The HPCS system can operate independently of normal auxiliary AC power, plant service air, or the emergency cooling water system. Operation of the system is automatically initiated from independent redundant signals indicating low reactor vessel water level or high pressure in the primary containment. The system also provides for remote-manual startup, operation, and shutdown. A testable check valve in the discharge line prevents backflow from the reactor pressure vessel when the reactor vessel pressure exceeds the HPCS system pressure such as may occur during initial activation of the system. A low flow bypass system is placed into operation until pump head exceeds the nuclear system pressure and permits flow into the reactor vessel. 

Schulenberg and Starflinger (2012) reported about a constant pressure start-up and shut-down system for the three-pass core design of the HPLWR, trying to keep the feed-water temperature constant to minimize thermal stresses of the reactor pressure vessel. This concept also includes a warm-up procedure for the deaerator during startup from cold conditions. A battery of cyclone separators is foreseen outside of the containment to produce some steam from depressurized hot coolant of the reactor. 

T. T. Yi, Y. Ishiwatari, S. Koshizuka and Y. Oka, Startup of a High-Temperature Reactor Cooled and Moderated by Supocritical-Pressure Light Water , Proc. GENES4IANP2003, Kyoto, Japan, September 15-19,2003, Paper 1036 (2003)... 

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... 

During subcritical pressure operation in the sliding pressure startup of the Super LWR, a steam-water separator is required to separate the steam and water such that the water can be recirculated to the reactor inlet by recirculation pumps or by additional heaters, in order to maintain adequate core cooling. The size and weight of the steam-water separator are determined by referring to those of sliding pressure supercritical FFPs. The characteristics of the reference 700 MW supercritical boiler and the properties of its steam-water separators are given in Table 5.3. 

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. 

T. Nakatsuka, Y. Oka and S. Koshizuka, Startup thermal considerations for supercritical-pressure light water cooled reactors, Nuclear Technology, Vol. 134(6), 221-230 (2001)... 

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