[en] A Transit Time Flow Measurement System (TTFM) has been developed to accurately measure PWR primary coolant loop flow rate. This method uses the signature of N-16 activity fluctuations to determine the mean coolant transit time between two gamma-ray detectors spaced along the pipe. Cross correlation is used to measure the transit time. When the distance between detectors, the measured transit time and the piping internal diameter are known, the volumetric flow rate in the hot leg of each coolant loop can be determined. The volumetric flow rate can be converted to mass flow rate if temperature (density) is also measured. The TTFM method requires no penetration into the primary system; it requires no flow calibration; and it introduces no pressure drop. The system can be installed on existing nuclear plants and on plants under construction as it requires no primary system design changes. Plant measurements using the TTFM technique at Prairie Island Unit 2 (NRP) have proven the feasibility of the system and show excellent reproducibility, as well as excellent agreement with the results obtained independently using the leading edge flow meter