Tightly coupled multi-physics simulations for Pebble bed reactors

We have developed a simulation tool for Pebble-bed reactor (PBR), a type of Very High-Temperature gas-cooled Reactor (VHTR). The simulation tool, PRONGHORN, takes advantages of the Multi-physics Object-Oriented Simulation library, and is capable of solving multidimensional thermal-fluid and neutronics problems implicitly in parallel. Expensive Jacobian matrix formation is alleviated by the Jacobian-free Newton-Krylov method, and physics-based preconditioning is applied to improve the convergence. Validity of the thermal-fluid solver has been tested and the comparison has revealed a limitation of the current flow model (Darcy's flow) near boundaries, while the neutronics solver agrees well with other codes on the PBMR400 benchmark problem. (authors)