Effect of the steam generator on temperature fluctuations of the primary circuit coolant in VVER-440 reactors

•Transfer properties of the steam generator were calculated for temperature fluctuations.•Perturbations broaden due to coolant mixing in the steam generator.•Maximum of their distribution is located at slower transit time than the average.•Magnitude of the fluctuations drastically decreases above 1 Hz.

Reactor noise diagnostics is based on statistical investigation of the fluctuations of various reactor parameters in steady state of the reactor. In PWRs small reactivity fluctuations are partly induced by temperature fluctuations of the coolant passing through the reactor core. The main source of these temperature fluctuations traveling with the coolant is the reactor itself. Perturbations arisen in the core and circulating in the primary loop return into the reactor after being attenuated in the steam generators. This kind of feedback depends on circulation period, phase and time history of the perturbations. Time history of a perturbation is mainly determined by the transfer properties of the steam generator. Measured circulation period of the perturbations may differ significantly from the result of the calculation based on the total mass and mass flow rate of the coolant. According to our investigation this difference can mainly be attributed to the structure of the steam generator.