Research on transient fixed-source term for three-dimensional space-time neutron kinetics simulation of hexagonal nodes based on conformal-mapping technique

Given the importance of core transient simulation for both accident analysis and innovative techniques in operating commercial pressurized water reactors (PWRs), a method for calculating three-dimensional (3D) space–time neutron-kinetics for hexagonal nodes has been proposed in this study. After time discretization, the transient fixed-source problem (TFSP) was solved using a conformal-mapping technique. To estimate the distribution of the conformally-mapped transient fixed-source term, the Standard Quadratic Fitting (SQF) and Mapping-enhanced Quadratic Fitting (MQF) methods were respectively implemented. Specifically, SQF fits the entire term using a quadratic function, whereas MQF enhances this approach by multiplying the quadratic function with a mapping scale function.Leveraging these methods, a transient solver for hexagonal nodes has been developed in our home-made core-analysis code SPARK, which has been evaluated against the IAEA-Hex kinetics benchmark, AER-DYN-001 benchmark and a dynamic rod-worth measurement (DRWM) process for a VVER-1000 unit. In IAEA-Hex kinetics benchmark, the result by SPARK with MQF shows good agreement with the reference solution, while SQF predictions exhibit a trend of deviation from the reference solution. In AER-DYN-001 benchmark, the transient power level predicted by SPARK with both SQF and MQF shows good agreement with results obtained from other computer codes. In the DRWM process, the transient responses calculated by both SQF and MQF agree well with each other and with the measured results as well. Therefore, the proposed transient method can accurately simulate the transient process for hexagonal nodes. In conditions involving significant core variations, MQF yields more accurate results. By contrast, under normal operating conditions, the results from both SQF and MQF are nearly identical.