  In this paper, we consider the coupled N/TH problem, in which the termination
criterion for the neutronics iteration adopts an adaptive tolerance with
respect to the fuel temperature residual at each Picard iteration. We refer to
this coupling scheme as the inexact Picard iteration method. Fourier analysis
is performed to investigate how the convergence behavior of Picard iteration is
influenced by the inexact neutronics solution. It is found that if the
convergence of the inner neutronics iteration is slow, Picard coupling may
become unstable unless a tighter tolerance is used for the neutronics
iteration. Nevertheless, our analysis indicates that a certain amount of
over-solving is necessary for maintaining the stability of Picard iteration if
the iterative solution of the subproblem is not fast enough. However, this
issue has not been addressed in the previous studies.
