  In this paper, we present a formal Fourier analysis (FA) of Picard iteration
for the coupled neutronics/thermal hydraulics (N/TH) problem and derive
theoretical predictions for the spectral radius of Picard iteration for such
coupled calculations as a function of the temperature difference between the
fuel and coolant, temperature coefficients of cross sections (i.e., Doppler
feedback), scattering ratio, and core height. An optimal underrelaxation factor
is also derived based on the Fourier analysis.
