Results of neutron noise measurements in a reactor of VVER-440 type

The analysis of the fluctuating (noise) part of signals from neutron detectors placed in the power reactor of VVER-440 type of Nuclear Power Station ''Kozloduj'' in Bulgaria is presented. For in-core measurements eight Self Powered Neutron (SPN) detectors were used with rhodium emitters positioned at half height of the core and for out of-core measurements three ionization current chambers placed 1200 apart. The equipment for registering of signals was set up in the Institute of Nuclear Research using CAMAC blocks and elaboration of signals was done using a small computer of TPA-type (analogous to PDP-8e). The maximal sampling frequency for one detector was equal to 80 c/s and for two detectors simultaneously was two times lower. The analysis of signals consists of statistical (mean value, standard deviation, linear trend, probability distribution) and spectral (power spectral density, phase, coherence) calculations. The results of measurements carried out in December 1977 can be summarized as follows: the spectra from chambers contain significant information in form of single resonances up to 5 c/s and from SPNs up to 15 c/s, the attempt to determine vibrations of the whole core has shown the existence of distinctive direction of core motion, the movement of absorbing rods causes a recognizable change in power spectral density (PSD) of signals from the SPN detectors closest to the rod, this change is limited in the frequency range from 4 to 12 c/s, during one measurement with a SPN detector the appearing of a strong resonance in PSD was observed, the reason of it was not explained but it can be used as an example in preparations of noise diagnostic methods. The conclusions from these measurements may be as follows: the measuring equipment was prepared correctly, it is necessary to increase the number of simultaneously registered signals, the measurements should be carried out systematically during the full fuel cycle, this equipment can be used for noise diagnostics after preparation of suitable programs for which the requirements may be initially prepared. (author)