Feedwater controller

Feedwater controller failures decreasing flow or 30wer LoSvH of FW l.OE-l 10 X... 

This is briefly described in Section 23.1.11. Its principle is based on a special feedwater control system, which... 

Main feedwater control valves fully closed 4,695... 

Moderate Screen tube Overheating when instrument lines exposed to cold weather froze, making feedwater controls and dmm level indicator inoperative or unreliable. 

The boiler is provided with a three-element feedwater control system, which was described in detail in Chapter 2, Section 2.2, in connection with Figure 2.1. The boiler is also provided with a variable-speed feedwater pump station, having the controls described in Chapter 2, Section 2.17.2 (see Figures 2.123 and 2.124). 

Analysis of limit cycling on a boiler feedwater control system. Boiler Dynamics and Control in Nuclear Power Stations 3, Proceedings of the third international conference held in Harrogate,... 

Excessive feedwater flow may result from improper operation of the feedwater controlling device. The fast controlled power reduction and FPR systems protect the reactor against overfilling of the coolant circuit, acting on the corresponding set points for water levels in the drum separators. 

LEADIR-PS 200 has a graceful and safe response to all anticipated transients. For example, an overcooling event (as could be caused by loss of feedwater control or spurious opening of steam relief valves in combination with control system failure) causes the core inlet temperature (normally 350°C) to fall as the freezing point of 327°C is approached the coolant viscosity increases, coolant flow decreases, and in the absence of any control system action, the negative temperature coefficients of the fuel and moderator reduce reactor power. Heat removal is maintained by natural convection. 

The Feedwater Control System (FWCS) controls the flow of feedwater into the RPV to maintain the water level in the vessel within predetermined limits during all plant operating modes. 

Main feedwater flow during plant startup is only delivered to the economizer inlet feedwater nozzles of the steam generators at temperatures at or above 200°F, which minimizes the probability of condensation-induced water hammer in the economizer sections of the generators. Below a predetermined power level, main feedwater is delivered to the downcomer feedwater inlet nozzles. Changeover to the economizer nozzles at this power level is effected by the Main Feedwater Control System. Emergency feedwater is always delivered to the downcomer inlet nozzles. (See CESSAR-DC, Sections 10.4.7.2.ID and 7.7.1.1.4.)... 

The reactor feedwater control system automatically controls the flow of reactor feedwater into the reactor vessel to maintain the water in the vessel within predetermined levels during all modes of plant operation. The control system utilizes signals from reactor vessel water level, steam flow, and feedwater flow. 

The reactor feedwater control system provides the signal for the reduction of reactor water recirculation flow to accommodate reduced feedwater flow caused by failure of a single feedwater pump. 

The MBRU-12 has maintained a conservative approach, providing for the shuffling of fuel under a closed guard vessel cover, which could help achieve early market availability. The 4S reactor, however, incorporates a small-diameter core of high neutron leakage rate with moving reflector control of bum-up reactivity loss, as a way to assure negative sodium void worth under all conditions. The reflector in the 4S is located outside the core and the power control is executed via the feedwater control from the steam-water power circuit. Some further related R D is required on these features (ANNEX XIV). 

A mismatch between reactivity added by the reflectors and the reactivity lost via fuel bum-up is adjusted by the feedwater control of the water/steam system. Therefore, the reactivity control is unnecessary at a reactor side and this is an important factor to simplify the reactor operation. 

Since the digital technology is considerably different than analog technology, the criteria appropriate for the safety review of digital computer based system are different. Such systems are being considered for use in the reactor protection, ECCS actuation, feedwater control systems, etc. 

BWRs use reactor level instrumentation to perform a number of functions including control functions, such as feedwater control, and protective functions, such as automatic scram and autostart of emergency core cooling system. 

Feedwater system malfunctions causing an increase in feedwater flow (two cases were modelled the accidental opening of one feedwater control valve with the reactor just critical at zero load conditions and the accidental opening of one feedwater control valve with the reactor in automatic control at full power). This fault models the failure of one protection division as the limiting single failure. This is fault 4.2.2 in the fault schedule. 

An isolation valve is installed downstream of each feedwater control valve, whose main purposes are to stop hot pressurised feed water leaking into the containbment in the event of a main feed line break inside the containment, and to isolate the steam generators in the event of a steam generator tube mpture closure of the isolation valve uses compressed nitrogen as its energy source. 

There are two start-up feedwater lines, with the same disposition of valves as the two main feedwater lines. Each of these lines supplies its own steam generator through an injection nozzle at the same elevation as the main feedwater nozzle but rotated circumferentially away from the main feedwater nozzle. During start up, feed is supplied through the start-up feed water control valves until the capacity limit of the start-up pumps is qjproached, at which point control is automatically transferred from the start-up feed water control valves to the main feedwater control valves, and the start-up feed isolation valves are then closed. 

The capability to accept a turbine trip from full-power operation without reactor trip. This capability is provided with the normally available systems (sueh as steam dump and feedwater control). 

The pre-boiler cycle consisting of feedwater heaters, feed pumps and feed lines are liable to corrode by the condensate return. The main contributors to corrosion are carbon dioxide and oxygen. Corrosion must, therefore, be minimized. This is done by feedwater control, which reduces the ingress of harmful impurities and gases to the boiler water circuit and also to the steam. The best way to prevent corrosion is by controlling the pH, which should be maintained in the range of 8.5-9.2. 

The Event Class 1 limit in Fig. 1 for failures of feedwater control ... 

The same acceptance criteria as for a loss of feedwater control apply. 7.1L4. Relevant event combinations... 

A load change will appear in the steam system as a change in throttle valve position and, therefore, a change in steam flow and pressure. The feedwater controllers at the inlet to the steam generators will sense these changes and operate to maintain steam pressure constant. The steam flow could also provide an anticipatory signal to the primary and intermediate system pumps to change their speed to suit the load. 

In the other types of abnormalities, the event classification follows those of LWRs because the components such as the valves and the control rod drives are expected to be similar to those of PWRs or BWRs. In the category of the reactivity abnormality, the incidents related to the control rods are taken from those of PWRs. The loss of feedwater heating is taken like BWRs. Most of the incidents of the pressure abnormality are taken from BWRs because the Super LWR also adopts the direct steam cycle. The reactor depressurization is taken from PWRs. The abnormalities categorized into the inadvertent start or malfunction of core cooling system are taken from those of PWRs or BWRs. The inadvertent startup of AFS of the Super LWR corresponds to the inadvertent startup of ECCS of PWRs. The core coolant flow control system failure is the same as the feedwater control system failure for the Super LWR while the two incidents are different in BWRs due to the recirculation system. All the accidents categorized into the loss... 

This is a typical flow increasing transient. The demand of the main coolant flow rate is assumed to rise stepwise up to 138% of the rated flow as is assumed in the feedwater control system failure of Japanese ABWRs. Since increase in the core coolant flow rate is mild in ABWRs due to the large recirculation flow, the feed-water flow rate is assumed to increase stepwise. This assumption is too conservative for the Super LWR. The main coolant flow rate is gradually increased by the control system in the safety analysis. The calculation results are shown in Fig. 6.31. The reactor power increases with the flow rate due to water density feedback. A scram signal is released when the reactor power reaches 120% of the rated power. The maximum power is 124% while the criterion is 182%. The increase in the pressure is small. The sensitivity analysis is summarized in Table 6.15. 

The pressure control system and the power control system designed in Chap. 4 are used. However, the main steam temperature control system cannot be used because the core outlet temperature is the saturation temperature. Therefore, a feedwater controller for subcritical pressure operating conditions is needed. Dining subcritical pressure operation, the feedwater flow rate is regulated in order to keep the water level in the steam water separator, instead of regulating the main steam temperature. A combined proportional and derivative controller (PD controller) is found to be suitable for that purpose [12]. 

Super FR has almost the same enthalpy rise as that in the Super LWR but it has a smaller heat capacity due to no water rods being used. The change in the main steam temperature against various perturbations and operations is expected to be larger in the Super FR. In this section, several concepts for improving the feedwater controller are introduced in order to suppress the fluctuation of main steam temperature of the Super FR against perturbations. The reference Super FR designed in Sect. 7.8.1 is treated here as an example. 

The gain AT in the pressure control system, described as (4.9) and (4.10), is tuned as 0.396 so that the overshoot is minimized against the 1 % stepwise increase in the pressure setpoint. The main steam temperature is controlled by regulating the feedwater flow rate. The equation for the feedwater controller is written again because it is the basis for the improvement in the next section. 

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