Measurement of average circulation period of temperature fluctuations in the coolant of VVER-440 reactors' primary circuit

Enhancing reactor noise diagnostics investigations requires modeling the circulation in the primary circuit of the reactor. An important part of the modeling is the investigation of feedbacks and the determination of transfer properties involving the estimation of the circulation period of the coolant. A brief estimation can be obtained from the total mass of the coolant and the mass flow rate in the primary circuit, but approximate knowledge about the volume of stagnant parts of the coolant and some other factors affecting and biasing the final result imply uncertainties. In VVER-440 reactors noise diagnostics provides a measurement option by calculating the correlation between temperature noise signals. It was found that only the cross correlation between average cold and hot leg temperature noise signals provides useful results mainly because of the mixing of the coolant coming from different loops and because of the dampening effect of the steam generator. The circulation period of perturbations traveling with the coolant calculated from correlations was found to be at least 15% smaller than the brief estimation from the total mass and mass flow rate of the coolant.