Investigation of Anderson acceleration in neutronics-thermal hydraulics coupled direct whole core calculation

Abstract

The Anderson Acceleration (AA) scheme is applied and investigated in neutronics and thermal-hydraulics (T/H) coupled direct whole core calculation as an effort to improve the convergence of the alternating calculation of the neutronics and T/H fields which are nonlinearly coupled with each other. This is an approach to overcome the limited convergence rate of the conventional Gauss-Seidel type fixed-point iteration scheme which might reveal oscillatory convergence behaviors in local solutions. The scheme is to represent the solution at a fixed-point iteration as a linear combination of the previous solutions. The AA solution scheme is implemented in the direct whole core calculation code nTRACER in which both a simple closed channel T/H module and the pin-level drift-flux based T/H solution module ESCOT are embedded. A series of coupled 3-dimensional problems are solved with increasing complexity starting from a single assembly steady-state problem to a full core depletion problem via checker-board problems. The convergence behavior is examined in terms of the true error reduction by comparing the intermediate fission source distribution with the fully converged reference solution obtained applying a very tight convergence criterion. It turns out that the number of neutronics-T/H iterations is reduced considerably because the local oscillatory behaviors can be smoothed and convergence is reached earlier so that the computing time of the coupled calculations is reduced by about 25% retaining the solution accuracy. (C) 2020 Elsevier Ltd. All rights reserved.