Verification of Multiphysics Software: Space and Time Convergence Studies for Nonlinearly Coupled Applications

High-fidelity modeling of nuclear reactors requires the solution of nonlinear coupled multi-physics stiff problems with widely varying time and length scales that need to be resolved correctly. A numerical method that converges the implicit nonlinear terms to a small tolerance is often referred to as nonlinearly consistent (or tightly coupled). This nonlinear consistency is still lacking in the vast majority of coupling techniques employed today. We describe a tightly coupled multiphysics framework that tackles this issue and present code verification and convergence analyses in space and time for several models of nonlinear coupled physics.