Development of an efficient tightly coupled method for multiphysics reactor transient analysis

Picard Iteration is a widely used coupling method for multiphysics simulations. This method allows one to directly leverage existing and well-developed single-physics programs without re-writing large portions of the codes. In Picard Iteration, single-physics codes just iteratively pass solutions to each other as inputs until each code has reached a converged solution. However, multiphysics computation linked by Picard Iteration is susceptible to over-solving, which can make the overall computation much less efficient. Over-solving means that each single-physics code provides an accurate solution in each Picard Iteration, which is not necessary in practice. Solving the single-physics codes in an inexact manner, i.e. with relaxed termination criteria, can help avoid this problem. This work develops a modified Picard Iteration coupling method with adaptive, inexact termination criteria for the underlying single-physics codes. Also, nested within the inexact Picard Iteration, inexact Newton methods were applied in the single-physics codes. The effect on the overall computation efficiency due to the inexact (relaxed) termination criteria at both levels is investigated by applying them to solve reactor transient problems. A reactor dynamics problem with temperature feedback in one-dimensional slab geometry is used to scope the behavior of nested inexact solvers. Then these methods are applied to a larger two-dimensional Boiling Water Reactor (BWR) problem. Computational time savings reach 55% for the two-dimensional problem. Additionally, applying an inexact termination criterion (inexact Newton method) to each single-physics code results in a further time savings of up to 18%.