Determination of Reactor Kinetic Parameters by Photon Observation

Fission rate fluctuations at low power in a reactor with a large fission-product inventory have been observed in the pool-type Ford Nuclear Reactor. A gaseous Cerenkov detector was used to sense the high-energy, prompt-fission gamma rays in the presence of a fission-product gamma field of 10⁵ to 10⁶ R/h. The ratio β/l is determined from the cross power spectral density of the fluctuations in the signals from two of these detectors. Both this spectrum and the power spectral density of the output of a single detector show a large low-frequency component. This is attributed to moderator temperature fluctuations present when the fission-product decay heat is removed by natural circulation of the coolant. The temperature fluctuations as measured with a short-time-constant thermocouple are shown to be correlated to those in the fission rate. The detector is described, as well as a basis for calculating its performance and efficiency. A theory of the gamma noise experiment that reveals the effects of the detector on the measured spectrum is presented.