Incore neutronic fluctuation measurements in a large graphite moderated power reactor

Neutronic fluctuation measurements were made in various locations in the N Reactor core. Although the reactor is very quiet, some interesting features were observed: 1) autospectra that increase in amplitude at low frequencies and exhibit a relatively broad resonant peak at about 0.3 Hz, 2) coherence between all measured locations that shows a relatively broad peak with a value of about 0.8 at 0.3 Hz, 3) zero phase shifts between detectors placed at a common position along the flow steam, 4) linear phase shifts at low frequencies for detectors placed in a upstream/downstream configuration whether or not the detectors are in the same radial location and regardless of the radial locations of the detectors, and 5) a space and time dependent resonant feature at about 1.4 Hz which has strong coherence between selected detectors. Computer calculations were made to explain some of these observations with only limited success. The results achieved up to this time are discussed. (author)