Determination of the fuel heat transfer dynamics via CFD for the purpose of noise analysis

In this paper, a 3-dimensional CFD model of a PWR fuel pin surrounded by its cooling water was developed using the Fluent code. The full heterogeneity of the system was accounted for both radially and axially, and the temperature-dependence of the thermal properties of the different constituents was explicitly taken into account. The turbulent character of the flow was modeled via a k-e model. Thereafter, sinusoidal fluctuations of the power density around its mean value were induced for oscillation frequencies ranging from 0.01 Hz to 10 Hz. The transfer function between the fluctuations of the volume-averaged fuel temperature and the fluctuations of the volume-averaged power density, and the transfer function between the fluctuations of the volume-averaged moderator temperature and the fluctuations of the volume-averaged fuel temperature were then numerically determined. While the former can be approximated by a first-order system, the estimation of the analytical expression of the latter is rather complicated, due to the spatial dependence of the temperature and velocity fields in the coolant. This paper nevertheless gives numerical estimates of these transfer functions that can be used in any axially-homogenized lumped thermal-hydraulic model.