Calculation and analysis of fast neutron fluence of pressure vessel in AP1000 reactor

The structural integrity of the reactor pressure vessel plays an important role in the safe operation of AP1000 nuclear power plant. Radiation damage of fast neutrons is the main factor leading to pressure vessel material deterioration. Therefore, it is necessary to accurately calculate the fast neutron fluence in the pressure vessel. Discrete ordinates method radiation shielding analysis code cosSN is used to calculate fast neutron fluence in the pressure vessel and the calculation result is compare to measurement of actual AP1000 power plant. Bondarenko method is adopted to calculate multi-group cross section in resonance energy region, and Bell-Hansen-Sandmeier(BHS) transport correct method is used to deal with anisotropic scattering effect. To improve computational efficiency, cosSN adopts the diffuse comprehensive acceleration and the multi-grid technique to accelerate source iteration, and adopts parallel computing to solve the storage limitations in large-scale calculations. For fast neutron fluence calculation, the calculation uncertainty comes from input parameters and approximation of theoretical model. The results indicate that the calculated specific activity is conform to the actual measured value of the power plant. The calculation uncertainty is +/- 13.28%, which meets the requirements of China domestic energy industry standards NB/T 20576-2019 and NRC RG 1.190.