Development and verification of PWR-core nuclear design code system NECP-Bamboo: Part III: Bamboo-Transient

To analyze the rapid change of Pressurized Water Reactor (PWR) core states in the time scale of seconds, a transient analysis code named Bamboo-Transient has been developed to provide neutronics and thermal-hydraulics coupled simulation with distinguished prompt and delayed neutrons. These transient processes can be initialized by control rod movements, core inlet coolant temperature perturbation and/or inlet coolant flow perturbation. Different from the existing codes, it adopts the Predictor-Corrector Improved Quasi-Static Method (PC-IQS) to ensure the computational efficiency, and employs the heterogeneous Variational Nodal Method as its three-dimensional neutron diffusion solver. Three advantages can be provided by HetVNM due to the elimination of the transverse integration approximation. Firstly, control rod cusping effect can be eliminated by directly treating heterogeneous cross sections and discontinuity factors. Secondly, physical adjoint neutron flux can also be obtained by constructing and solving the multi-group adjoint equations with discontinuous factors. Thirdly, the detailed nodal precursor density distribution can be directly provided by the detailed nodal fluxes from VNM to ensure the kinetics calculation accuracy. In this paper, these methods and the Bamboo-Transient code were verified by the NEACRP rod ejection accident and uncontrolled control rod withdraw accident in BEAVRS reactor core.