Development and application of a neutronics/thermal-hydraulics coupling code based on system code and Anderson acceleration

Abstract

In a main steam line break (MSLB) accident, the degree of mixing of coolant with different temperatures in the lower plenum can significantly affect the power distribution of core, which in turn impacts core safety. The point kinetics model used in traditional system codes is unable to simulate spatial variations in power. To investigate this phenomenon in greater detail, it is essential to couple the system code with a 3D neutron dynamics code. In the development of neutronics/thermal-hydraulics coupling codes based on system codes, operator splitting (OS) method and Picard iteration are often used to advance the time-stepping. The operator splitting method has lower accuracy, while the Picard iteration is an implicit coupling method to improve the computational accuracy of neutronic and thermal–hydraulic coupled codes. However, the convergence of Picard iteration is usually poor, the use of relaxation factors is required to ensure numerical stability. The selection of relaxation factors is typically a manual process, and the optimal range varies under different operating conditions. To enhance the convergence and efficiency of alternate calculations between the neutronic field and thermal–hydraulic field, Anderson acceleration was introduced into the coupled transient simulation of the system code NUSOL-SYS. The developed code with Anderson acceleration was used in transient numerical simulations, and the results demonstrated a significant reduction in the number of coupling iterations. For time steps involving rapid transient changes, the iteration count was reduced to approximately 76.5% of the original Picard iteration, while also eliminating the need for manual selection of relaxation factors. Further validation was performed using the NEACRP 3-D LWR core transient benchmark, where the computed results were in close agreement with reference solutions. Subsequently, the coupled code was applied to simulate the MSLB accident in a pressurized water reactor (PWR). The impact of coolant mixing in the lower plenum on transient parameters was investigated. The results revealed that increased coolant mixing led to lower return to power and reduced local power peaks.