A Numerical Analysis on the Correlation between the Core Exit Temperature Distribution and Hot Leg RTD Temperature

The RCS(Reactor Coolant System) flow rate is one of the important plant operating parameters, which is periodically inspected and should be maintained within a pre-designated range to assure a safe plant operation. The heat balance flow rate measurement method, officially used for OPR1000s, needs the hot leg average temperature as an input. If the measured average hot leg RTD(Resistance Temperature Detector) temperature is higher than the actual average hot leg temperature, then the calculated RCS flow rate will be smaller than the actual RCS flow rate as can be seen in the following equation, and vice versa. If measured RCS flow rate goes smaller, then the operation margin and safety margin will also be reduced. m and = Q / CP(TH - TC) where, m and: RCS mass flow rate, Q: core thermal power, CP: coolant specific heat, TH: measured average hot leg RTD temperature, TC: measured average cold leg RTD temperature. The RCS flow rates measured by heat balance method for OPR1000s in the past have shown a considerable change depending on the operation cycle or even within the same operation cycles. Specifically, in recent years, the measurements for YGN 3 and 4 and UCN 3 and 4 by heat balance method unexpectedly showed very low RCS flow rates. However, the flow rates measured by pump DP(Differential Pressure) method, which well represents the trend of flow rate change, showed that relatively constant RCS flow rates were maintained during the operation cycles. As an example, the measured RCS flow rates for UCN 4 calculated by heat balance and pump DP methods are plotted in Fig.1 from beginning of operation up to recent time. This suggests that there was actually no significant change in RCS flow rate. To investigate the reason for the seemingly changing flow rate measured by heat balance method, various operation parameters, such as core thermal power, hot/cold leg temperatures, core exit temperatures, turbine power, etc., were observed. One thing noticed is that there might be a strong relationship between core exit temperature distribution and the hot leg RTD temperature measurements. This suggests that, even though the average core exit temperature is the same, the different patterns of local core exit temperature distribution may cause the different thermal stratification patterns in the hot leg. In this case, the average of hot leg RTD temperatures may not represent the real or actual hot leg average temperature. One notable observation is that as the core exit temperature at core center region goes higher, then the average temperature of hot leg RTDs goes higher. In recent years, low leakage core loading pattern is prevailing to improve the neutron economy. Then the core power distribution is more concentrated in the core center region, and in turn the temperature of core center region may go even higher. This causes the heat balance method to calculate the smaller RCS flow rate than the actual. In this paper, a CFD analysis for the flow field from core exit to inlet of steam generator was performed to quantify the relationship between core exit temperature distribution and hot leg RTD temperatures