Decrease in Feedwater Temperature

The feedwater temperature decreases stepwise from 280 to 270°C. The results are shown in Figs. 4.33 and 4.34. At the beginning, the volume flow rate at the reactor vessel inlet decreases because the density of feedwater increases. It temporarily decreases the flow rate at the fuel channel inlet, and hence the main steam temperature increases and the power decreases. This behavior is one of the characteristics of the Super LWR with the once-through coolant cycle which differs 

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Decrease in Feedwater Temperature, Increase in Feedwater Flow, Increase in Steam Flow, and Inadvertent Opening of a Steam Generator Relief or Safety Valve... 

Feedwater system malfunctions causing a reduction in feedwater temperature were not modelled as described above rather, the transient was analysed by calculating conditions at the feedwater pump inlet following the removal of a low-pressure feedwater heater train from service. The feedwater conditions were then used to recalculate a heat balance through the high-pressure heaters. This heat balance gives the new feedwater conditions at the steam generator inlet. The decrease in feedwater temperature transient so calculated was less severe than (and therefore... 

A decrease in feedwater temperature at the steam generator inlet to 300 K. 

Feedwater system malfunctions that result in a decrease in feedwater temperature. 

Fig. 4.33 Response to stepwise decrease in feedwater temperature with control system (1)...

Fig. 7.80 Responses to 10°C decrease in feedwater temperature setpoint with four control systems. (Taken frcnn [31] and used with permission from Atomic Energy Society of Japan)...

The feedwater temperature can be gradually decreased by removing the HP feedwater heaters [6]. When the feedwater temperature is 280 "C, the BOP requires four HP and four LP low pressure feedwater heaters. When it is 210°C, the cycle has two HP and four LP feedwater heaters. There are only four LP feedwater heaters in the case of ISO C. 

In supercritical FPPs, high thermal efficiency is favorable because the fuel cost occupies a large fraction in the total power cost. Fuel cost fraction, however, is a relatively small part of the NPP cost. Lowering the feedwater temperature reduces the number of feedwater heaters. This decreases the size of the BOP. The decrease in the coolant flow rate per electric output will be more effective in decreasing the capital cost than increasing the thermal efficiency in the Super LWR. The flow rate of the Super LWR with the outlet temperature around 500°C decreases by 31% when the feedwater temperature is 210 0 and by 35% when it is 150°C, compared with that of the ABWR [7]. The size of the turbines and the capacities of the pumps will decrease with the feedwater flow rate. 

A positive reactivity of 0.1 is inserted stepwise as a reactivity perturbation. The feedwater flow rate and the turbine control valve opening are kept constant. The results are shown in Figs. 4.9 and 4.10. The power quickly increases to 111% of the initial value. It is consistent with the analytical solution of prompt jump. Then, the power decreases due to reactivity feedbacks from Doppler and coolant density. The main steam temperature changes by following the power. The main steam pressure and the core pressure increase due to increases in the temperature and hence the volume flow rate of the main steam. The fuel channel inlet flow rate changes with the core pressure due to the relation between the feedwater flow rate and the core pressure shown in Fig. 4.4. The plant almost reaches a new steady state in 40 s. 

The power setpoint decreases stepwise from 100 to 90%. The results are shown in Figs. 4.29 and 4.30. The control rods are inserted so as to decrease the power. The power reaches the new setpoint without oscillation. The main steam temperature decreases with the power. The feedwater flow rate is gradually decreased to 90% of the initial value so as to keep the main steam temperature 500°C. The main steam pressure is kept crmstant by the turbine control valves. The pressure loss in the main steam lines decreases because of the decrease in the main steam flow rate. As a result, the core pressure decreases by about 0.1 MPa. After 80 s, the plant is settled at a new steady state. The variation of the main steam temperature is around yC. 

The power to flow rate ratio must be low enough to keep MCST below the criterion and to prevent boiling in the water rods. The maximum allowable powers are calculated with various feedwater flow rates and various feedwater temperatures. Figs. 5.12 and 5.13 [3] show the results at two pressures of 10 and 20 MPa, respectively. The maximum allowable power increases with increasing flow rate, or decreasing feedwater temperature. Figure 5.14 [3] shows the maximum allowable powers as a function of the pressure. The maximum allowable powers for various flow rates during pressurization from 8 to 25 MPa with constant feedwater temperature of 280°C are shown in Fig. 5.15. 

Unlike FPPs, the Super LWR has no superheater. Thus, the main steam conditions during startup of the Super LWR need to be adjusted so that they are suitable for steam turbines. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. Herein, it is assumed that 5% of the rated power is necessary for turbine startup. The minimum required powers are calculated with various feedwater flow rates and feedwater temperatures. The calculated results at 10 MPa are shown in Fig. 5.16 and those at 20 MPa are shown in Fig. 5.17. It is found that the minimum required power decreases with decreasing flow rate or increasing the feedwater temperature. Figure 5.18 [3] shows the minimum required powers as function of the pressure. 

The core inlet temperature affects the temperature and velocity distributions in the core. When it is lower, the coolant temperatures and density ratio between the core inlet and outlet become smaller which has a stabilizing effect. On the other hand, it also leads to lower coolant flow velocity and core pressure drop, resulting in a longer time delay which destabilizes the system. The net effect depends on the individual operating conditions. Here, the core power and flow rate are kept constant and the effect of inlet feedwater temperature on stability is investigated. As shown in Fig. 5.42 [11], under the operating conditions of the present parametric study, decreasing the inlet temperature leads to lower oscillation frequency and decay ratio which stabilizes the reactor. 

The net effect on the stabihty depends on the balance between the above two effects. In the present analysis, the neutronic feedback effect is found to be dominant over the hydraulic feedback effect. Here, the core power and flow rate are kept constant and the decay ratios are calculated with various feedwater temperatures. The decay ratio is found to decrease when the core inlet temperature decreases as shown in Fig. 5.61. 

Loss of one stage of the feedwater heating will cause a 35°C drop of the feedwater temperature. In the safety analysis, it is conservatively assumed as 55°C as is done in the safety analysis of ABWRs. The result is shown in Fig. 6.29. At the beginning of the transient, the fuel channel inlet flow rate decreases because the coolant density increases, and hence the volume flow rate decreases upstream from the fuel channel. This is one of the characteristics of the once-through coolant cycle without recirculation. The cladding temperature increases and the reactor power... 

Three units of the AFS are assumed to start. The AFS flow (12% of rated value, 30°C) is added stepwise to the main coolant flow at 0 s. The results are shown in Fig. 6.30. The main coolant flow rate and the fuel channel inlet flow rate increase due to the AFS startup. At the beginning, the fuel channel inlet flow rate is lower than the main coolant flow rate because the feedwater temperature, which is the same as the loss of feedwater heating transient described above, decreases. The... 

A positive reactivity ( 0.1) is inserted stepwise by withdrawing the CRs. The feedwater pump speed and the turbine control valve stroke are kept constant. The results are shown in Fig. 7.68 [31]. The reactor power increases about 10% almost stepwise due to the prompt jump and then gradually decreases due to the reactivity feedbacks from the fuel temperature and coolant density. This behavior implies that the Super FR also has inherent self controllability of the reactor power despite the much smaller density reactivity coefficient compared to that of the Super LWR. The main steam temperature increases, which leads to an increase in the main steam and core pressures because the specific volume of the main steam increases. The increase in the core pressure leads to a decrease in the feedwater and core flow rates, which increases the main steam temperature further. As a result, the maximum increase in the main steam temperature is nearly 40°C while that in the Super LWR is only 9°C (see Fig. 4.10). 

The improved control system with the feedwater controller described as (7.28) is designated Control system (A). The gain A p2a is tuned by analyzing the plant dynamics against the 10% stepwise decrease in the setpoint of the reactor power. The results are shown in Fig. 7.73 [31]. The changes in the power to flow rate ratio and the main steam temperature can be decreased from the case with the reference control system. 0.4 is chosen as Kp2 so that these changes are minimized. 

The results are shown in Fig. 7.78 [31]. In order to increase the main steam temperature, the power to flow rate ratio needs to be increased by decreasing the feedwater flow rate. The main steam temperature does not reach the new setpoint with control system (A) because the second term of (7.28) tries to make the flow rate follow the power. With other control systems, the main steam temperature settles to the new setpoint... 

With Control system (C), the feedwater flow rate decreases further at the beginning and hence the change in the main steam temperature is larger than the reference case. This is because the derivative control term of (7.30) decreases the feedwater flow rate by detecting the initial decrease in the reactor power. 

In this event, the feedwater temperature decreases stepwise by 10°C and does not recover. In the beginning, the main steam temperature increases and the reactor... 

The decrease of steam temperature at the SG outlet of about 20 K, consequence of the pressure decrease, prevents dryout occurrence on the fuel rod clad surface, calculation NI03. In this case, no attempt has been made to optimize the combination between feedwater temperature and SG pressure keeping the goal of maintaining the core in nucleate boiling. 

The cold feedwater causes a temperature drop which decreases the volume of the entrained vapor bubbles. This leads to a decrease of the liquid level of the boiling water, following first-order behavior (curve 1 in Figure 12.4b), that is, -K / Z S + 1). 

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. 

Even in the case that all the steam and feedwater lines are instantaneously isolated, so that the SGU heat transfer capability is quickly zeroed, the secondary side of the SGU reaches thermal equilibrium with the primary side with a pressure increase up to about 115 bar which is below the design pressure of the secondary system. At the same time, heating-up of the primary water occurs with associated decrease of nuclear power caused by the reactivity feedback of the moderator temperature. 

Although Si(OH) is nonvolatile at ordinary temperature and polymerizes quickly when heated, nevertheless at elevated temperature and pressure in-water its solubility is greatly increased and it can exist in equilibrium as the vapor phase in the steam, as shown by Kennedy (25). This is of importance in very high pressure boilers in power plants where deposits build up on turbine blades unless all silica is removed from the feedwater. Brady (26) supposes the volatile species is Si(OH)4 o., (HO)3SiOSi(OH)3. Astrand (27) found that volatility increased with decreasing alkalinity in experiments conducted up to 350°C and 300 atm. This, of course, suggests that Si(OH)4 is more volatile than the silicate ion. Wendlandt and Glemser (28) reviewed evidence from earlier workers and calculated the equilibrium constants involved whence the species in the vapor were related to the density of the water vapor ... 

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