Nuclear reactor primary coolant loop flowmeter with phase shift tracking compensation

An improved nuclear reactor primary coolant loop flowmeter having a first detector positioned substantially adjacent to the coolant loop at a first location, which is responsive to the gamma radiation emanating from the Nitrogen-16 activity within the reactor coolant at the first location to provide a representative electrical output, a first detector channel for conditioning and communicating the first detector's output, a second detector positioned substantially adjacent to the coolant loop at a second location downstream of the first location, which is responsive to the gamma radiation emanating from the Nitrogen-16 activity within the reactor coolant at the second location to provide a representative electrical output, a second detector channel for conditioning and communicating the second detector's output, and means having inputs from the first and second detector channels' outputs for cross-correlating the first and second detectors' outputs and delaying the first output with respect to the second until the cross-correlation function approaches a maximum so as to determine the delay time between corresponding points in the first and second outputs, wherein the improvement comprises means for switching the first detector output to the second detector channel and the second detector output to the first detector channel and means for subtracting the delay times monitored between corresponding points in the first and second outputs by the cross-correlating means with the first and second detectors connected in both switching positions to provide an output proportional to the phase shift error between the outputs of the first and second detector channels.