Local nuclear reactor power measurements using gamma-ray fluctuations

Local measurements of the inherent fluctuations in the gamma-ray density at specific locations in the core of the Pennsylvania State Triga Reactor have been conducted using two externally located detectors. Local power was determined for positions up to the center of the core from a distance of approximately 200 cm by measuring the zero-time cross correlation function in small intervals. Voids introduced by the removal of single fuel elements were readily located and mapped. Single collimator dc scans along diagonal rows in the core were successful in resolving the individual rows of fuel in the core. A comparison between the neutron distribution predicted by two dimensional multi-group diffusion theory and a single collimator measurement showed very good agreement in their respective shapes. Cospectral densities were obtained for the two cases where the collimators focused on and off a volume of fuel in the core. The frequency response data indicated, when interpreted in terms of an analytical model, that the observed output fluctuations were caused by temperature and coolant flow input fluctuations.