Reactor coolant flow abnormality

The abnormal events related to a decrease in core coolant flow rate are the most important for the Super LWR because the core coolant flow rate is the fundamental safety requirement, as described in Sect. 6.2. Since the coolant cycle of the Super LWR is different from those of both PWRs and BWRs, the events need to be carefully selected and classified. 

The loss of all feedwater flow of LWRs is classified as an abnormal transient. In PWRs, it occurs in the secondary system, and there is a large water inventory in the steam generators. BWRs have the recirculation system, and there is a large water inventory in the RPV. Therefore, the total loss of feedwater does not immediately lead to a low-of-fiow in the core. The total loss of reactor coolant flow corresponds to a trip of all the primary coolant pumps of PWRs and a trip of all the recirculation pumps of BWRs. It is classified as an accident. 

The once-through coolant cycle of the Super LWR is schematically illustrated in Fig. 6.9 [1]. The feedwater pump is the same as the RCP. The loss of all feedwater flow and the total loss of reactor coolant flow are the same incidents. Classification of this event depends on the frequency. Also, the guidelines for Japanese LWRs are followed. A simultaneous sudden trip of all ptunps that have been directly maintaining the core coolant flow rate must be classified as a total loss of reactor coolant flow accident. These pumps correspond to the primary pumps of PWRs and the recirculation pumps of BWRs. Since the RCPs of the Super LWR also maintain the core coolant flow rate, a simultaneous sudden trip of the RCPs is classified as the total loss of reactor coolant flow accident, assuming that its frequency will be less than 10 per year by system separation and high reliability. 

Loss of supply of coolant to the deaerator would also cause a trip of the RCPs because the inlet pressure of the RCPs decreases with the water level in the deaerator. This abnormality is represented by the loss of offsite power transient where the motor-driven condensate pumps stop. Since there is a large amount of water in the deaerator, the RCPs are expected not to stop for some period after the trip of the condensate pumps. The capacity of the deaerator has not yet been determined. If it is 140 m, which corresponds with the typical design of 1,000 MWe class FPPs, the water level in the deaerator would decrease by only 7% in 10 s after the trip of the condensate pumps. In the safety analysis, it is conservatively assumed that the trip of the RCPs occurs 10 s after the condensate pump trip [5]. This transient is less severe than a total loss of reactor coolant flow accident because a reactor scram is possible before the trip of the RCPs. In the safety analysis, the reactor scram by the signal of loss of offsite power, condensate pump trip, or turbine control valves quickly closed is credited. 

Decrease in core coolant Partial loss of reactor coolant flow 0 0 0 

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The reactor coolant flow abnormality is important for the Super LWR because maintaining the core coolant flow rate is the fundamental safety requirement It should be noted that there are two types of reactor coolant flow abnormalities with and without reactor scram before events the former are abnormal transient types... 

For the safety analysis, the abnormal transients and accidents are taken from each category as the reactivity abnormality, pressure abnormality, reactor coolant flow abnormality, and inadvertent start or malfunction of core cooling system in Table 6.4 except the CR assembly misalignment and drop transient, and the depressurization of core cooling system transient. They are shown in Table 6.5. 

Abnormal transients Decrease in core coolant flow rate Partial loss of reactor coolant flow Loss of offsite power Abnormality in reactor pressure Loss of turbine load Isolation of main steam line Pressure control system failure Abnormality in reactivity Loss of feedwater heating Inadvertent startup of AFS Reactor coolant flow control system failure Uncontrolled CR withdrawal at normal operation Uncontrolled CR withdrawal at startup Accidents... 

Decrease in core coolant flow rate Total loss of reactor coolant flow Reactor coolant pump seizure Abnormality in reactivity CR ejection at full power CR ejection at hot standby LOCA... 

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. 

The non-linear dynamics of the reactor with two PI controllers that manipulates the outlet stream flow rate and the coolant flow rate are also presented. The more interesting result, from the non-linear d mamic point of view, is the possibility to obtain chaotic behavior without any external periodic forcing. The results for the CSTR show that the non-linearities and the control valve saturation, which manipulates the coolant flow rate, are the cause of this abnormal behavior. By simulation, a homoclinic of Shilnikov t3rpe has been found at the equilibrium point. In this case, chaotic behavior appears at and around the parameter values from which the previously cited orbit is generated. 

The method has been demonstrated on a continuous stirred tank reactor (CSTR) simulation to identify an abnormal inlet concentration disturbance [340]. The jacketed CSTR, in which an exothermic reaction takes place, is under level and temperature control. An important process variable is the coolant flow rate through the jacket, that is related to the amount of heat produced in the CSTR, and it indirectly characterizes the state of the process. This variable will be monitored in this classification scheme. 

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. 

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. 

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... 

The reactor behavior is similar to that of at the loss of offsite power after the trip of the RCPs at 10 s. The maximum pressure is about 26.8 MPa, the highest among the abnormal transients but is low enough compared to the criterion (28.9 MPa). The hottest cladding temperature increases by about 50°C from the initial value during the flow stagnation caused by the closure of the coolant outlet. 

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