Abstract:Based on the design and test data of a 1 000 MW nuclear power plant,the fuel assembly is divided into several control volumes along the axis,and the critical heat flux(CHF) characteristic is calculated by MATLAB method.The calculation results show that under the normal operating condition,from the first control volume at lower part of the fuel rod to the upper volume,the coolant temperature increases slowly firstly and then increases rapidly,and finally grows slowly,and the temperature of coolant at core outlet is the maximum.The temperature changes at fuel rod surface are all corresponding to the nuclear power;moreover,the heat flux is larger in the middle area of the fuel rod with larger nuclear power.The departure from nucleate boiling ratio(DNBR) of upper-middle volume under each nuclear power condition is the minimum value,and the DNBR value shows the trend of large at both ends but small in the middle.Meanwhile,under the maximum nuclear power condition,the minimum value of DNBR is close to 1.3,which means the CHF is close to the actual heat flux,and the probability of occurrence for CHF increases,so overpower operation should be avoided during reactor operating.