A Multilevel Quasi-Static Kinetics Method for Pin-Resolved Transport Transient Reactor Analysis

Three-dimensional (3D), full-core transport modeling with pin-resolved detail for reactor dynamic simulation is important for some multiphysics reactor applications. However, it can be computationally intensive due to the difficulty in maintaining accuracy while minimizing the number of time steps. An innovative Predictor-Corrector Quasi-static Method (PCQM) is introduced that is based on a Transient MultiLevel (TML) methodology. Two levels of couplings are used between 3D-transport/3D-CMFD (coarse-mesh finite difference) and 3D-CMFD/EPKE (exact point-kinetics equation). In each level, the original flux equation is solved in the coarse predictor step and then is factorized as an amplitude and a shape function in the corrector step, where the predicted solution is adjusted using multiple fine steps. In the first-level 3D-transport/3D-CMFD coupling, the angular and subpin flux shape functions in the Boltzmann transport equation are assumed to vary slowly over time, and the CMFD cellwise amplitude function is solved using multiple steps by the 3D-CMFD transient equation. In the second level, the CMFD scalar flux calculated in the last step is further corrected by a whole-core-wise amplitude function generated by the EPKE solver. The utilization of hierarchical multilevel neutronics transient solvers achieves the goal to balance the numerical accuracy and computational efficiency. In addition, a new iteration scheme with pin-resolve thermal-hydraulic feedback and theoretical proof for the accuracy of PCQM are also presented. Finally, a stripe assembly case adopted from the SPERT (Special Power Excursion Reactor Test) transient tests is used to demonstrate the accuracy and efficiency of the TML method.