Monitoring MTC using cold leg thermocouple and background neutron detector noise signals in a VVER-440 reactor during a whole fuel cycle

Monitoring of the Moderator Temperature Coefficient (MTC) was performed from the noise signals of cold leg thermocouples and background neutron detectors in a VVER-440 type reactor during a whole fuel cycle. A modified traditional noise based estimator was applied: the estimator was extended in order to take into account the effects of measurement geometry, coolant velocity and the relatively long time constant of the thermocouples. A systematic evaluation of measurement settings and evaluation parameters was carried out in order to determine optimal parameters. Optimal evaluation parameters were determined by considering the frequency dependence of the estimator, and by minimizing the statistical and systematic errors of the results. It can be concluded that the modified estimator provides adequate results which are close to the MTC given by the core design code calculations. It was found that relatively long FFT window sizes are needed to obtain correct results. The method needs long but industrially acceptable measurements for robust operation.