MTC estimation based on neutron noise and propagating temperature perturbation in VVER-440 reactors

Moderator Temperature Coefficient (MTC) is an important parameter characterizing inherent safety in PWRs. Noise diagnostics provides a theoretically well established method to estimate its value without changing the reactor state with using fluctuation of the temperature and neutron flux measurements in frequency domain. However, several difficulties arise when determining the core average neutron and temperature fluctuation from the real measured signals, due to the specific instrumentation of each reactor type. Coolant temperature fluctuations originate from inhomogeneities traveling with the coolant flow and the treatment of this phenomenon requires an approach using the theory of propagating perturbations. Traditional MTC estimation methods do not consider these effects and they result in substantial under- and overestimations.This paper investigates MTC estimation in VVER-440 type reactors. For the evaluations the traditional H1 estimator was used in a modified form: it was extended with factors taking into account the effects of propagating perturbations through geometry and coolant velocity. The 0.1–1.0 Hz frequency range of the traditional MTC evaluation was narrowed, as well, because of the relatively slow thermocouples and to decrease the effect of feedbacks. With these advancements results were quite close to the calculated values provided by reactor physics.