Investigations of boron transport in a PWR core with COBAYA3/SUBCHANFLOW inside the NURESIM platform

Multi-scale, multi-physics problems reveal significant challenges while dealing with coupled neutronic/thermal-hydraulic solutions. Current generations of reactor dynamic codes applied to Light Water Reactors (LWRs), and in the context of this paper, pressurized water reactors (PWRs), are based on 3D neutronic nodal methods coupled with single or two phase flow thermal hydraulic system or sub-channel codes. This paper describes the extension of the coupling scheme between the 3D neutron diffusion codes COBAYA3 and DYN3D, and the sub-channel thermal-hydraulic code SUBCHANFLOW for the simulation of boron dilution transients. This includes the validation of the boron transport model of SUBCHANFLOW. The coupling of COBAYA3 and DYN3D with SUBCHANFLOW is performed inside the NURESIM platform making use of the novel automatic mesh superposition for code coupling. Boron transport models are needed for the simulation of boron dilution transients following a SBLOCA (after loop-seal clearing). In this case, the mixing is a key mechanism determining the positive reactivity insertion in the core. The results obtained with the coupled codes COBAYA3/SUBCHANFLOW, DYN3D/SUBCHANFLOW will be presented and discussed for two transient scenarios; a homogeneous and a heterogeneous boron dilution problem for a mini-core and for a PWR core defined in the NURISP boron dilution Benchmark.