Main coolant flow

The primary circuit incorporated inside the RPV includes the main coolant flow path, intended for direct heat removal fi om the core and its transfer to the secondary circuit in the two internal once-through steam generators (SG), and the pressurization system intended to create and maintain preset pressure in the primary circuit. The 45 m steam-gas pressurizer is located under the reactor head. Nitrogen is used for initial pressurization. Additional pressure is created by steam generated fi-om the primary coolant. 

Direct the main coolant flow to and from the fuel assembhes, so as to ensure the adequate removal of core heat. 

The safety principle of the Super LWR and Super FR is compared with those of PWRs and BWRs in Table 1.7. The main coolant flow rate and turbine inlet pressure are monitored and used for the emergency signal, instead of the water level ofLWRs. 

The relation between the levels of abnormalities and the safety system actuations are shown in Table 1.8 [54]. A decrease in the coolant supply is detected as low levels of the main coolant flow rate. The reactor scram, the AFS and the ADS/LPCl are actuated sequentially depending on the levels of abnormaUty. The reactor is scrammed at level 1 (90%) and then the AFS is actuated at level 2 (20%). Level 3 (6%) means that the decay heat cannot be removed at supercritical pressure, so the reactor is depressurized. 

Monitored Water level in the Water level in the reactor Main coolant flow rate. 

The covering of all major abnormal transients by these proposed models are confirmed by comparing the results obtained by them with results obtained from detailed fuel rod analyses modeling each abnormal transient event. The following eight abnormal transient events are analyzed for confirmation inadvertent startup of the auxiliary feedwater system (AFS) loss of feedwater heating loss of load without turbine bypass withdrawal of control rods at normal operation main coolant flow control system failure pressure control system failure partial loss of reactor coolant flow and loss of offsite power. 

Loss of feedwater flow is the same as loss of reactor coolant flow for the Super LWR. BWRs have the recirculation system and large coolant inventory in the reactor vessel. PWRs have the secondary system. Therefore, the feedwater of the Super LWR is more important for its safety than that of LWRs. In this chapter, feedwater flow, feedwater system, and feedwater pump of the Super LWR are called main coolant flow, main coolant system, and reactor coolant pump (RCP) , respectively, to distinguish them from those of LWRs. The main coolant flow rate is equal to the core coolant flow rate and the main steam flow rate at the steady-state. 

Main coolant flow rate low (level 1) Reactor power high (120%)... 

Since the function of the AFS is to keep the main coolant flow rate in the event of the unavailability of the RCPs, its actuation signals should be released by detecting an abnormality in the RCPs or a decrease in the main coolant flow rate. Reactor coolant pump trip and main coolant flow rate low are taken as the AFS signals. Loss of offsite power, condensate pump trip, turbine control valves quickly closed, main stop valves closure, and MSIV closure are also taken as the AFS signals because these abnormalities cause a trip of the RCPs. 

Fig. 6.25. The main coolant flow rate decreases linearly to 50% of the rated flow. Flow rate low level 1 is detected and the scram signal is released at 1 s. Although the trip of the RCP itself would release the scram signal, it is conservatively neglected. The cladding temperature increases until 3.6 s due to the decrease in the flow rate and then decreases due to the decrease in the power. The increase in the hottest cladding temperature is 60°C which is the highest among the abnormal transients. It is sensitive to the coast-down time and the scram delay as shown in Table 6.11.

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... 

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. 

Control rod withdrawal at normal operation is analyzed. The reactivity worth of the withdrawn CR cluster is conservatively assumed as 1.3%dk/k. The same withdrawal speed as of PWRs (114 cm/min) is taken. The CR cluster is withdrawn until the reactor power reaches the scram setpoint (120% of rated power). The inserted reactivity is 0.69. The calculation results are shown in Fig. 6.32. The power increasing rate is small due to the reactivity feedbacks from the water density and fuel temperature. The cladding temperature increases by only 10°C because the main coolant flow rate is increased by the control system so as to keep the main steam temperature. If the control system is not considered, the increase in the temperature is about 110°C. The influence of the CR worth is small because the inserted reactivity before the reactor scram is almost the same [5]. 

Only the partial loss of reactor coolant flow is accompanied by a decrease in the main coolant flow rate before initiating the ADS. The ADS is actuated at 5 s by the ATWS signal which is reactor coolant pump trip and reactor power ATWS permissive for 5 s . The calculation results are shown in Fig. 6.48. A decrease in the flow rate leads to an increase in the coolant temperature due to the power and flow mismatch. The cladding temperature increases due to the coolant heat-up and a decrease in the heat transfer coefficient. The net reactivity and the reactor power decrease due to coolant density feedback. The increase in the cladding temperature is about 120°C, which is the highest value of all the ATWS events with the alternative action. 

Abnormality type Typical ATWS event Density coefficient (dk/k/(g/cm )) Loss of flow Loss of offsite power AMCST (°C) Pressurization Loss of turbine load without bypass " Peak pressure (MPa)/peak power (%) Reactivity insertion Uncontrolled CR withdrawal Peak fuel enthalpy (cal/g) Flow increase Main coolant flow control system failure Peak power (%)... 

The analysis results of the partial loss of reactor coolant flow (transient) are shown in Fig. 7.98. The inlet flow rates of the three hot channels similarly decrease with the main coolant flow rate. The highest cladding temperature appears in the upward flow seed channel due to its higher value initially. The AMCST in the upward flow seed channel is about 80°C, which is higher than that in the Super LWR by 20°C. This is because the Super FR has a smaller heat capacity and also because the smaller reactivity feedback from the coolant density makes the decrease in the power slower before initiating the reactor scram. However, there is still a margin of over 60°C to the criterion. 

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