Pulsed Neutron Activation for Determination of Water Flow in Pipes

Doctoral thesis, 2005

In a Pulsed Neutron Activation (PNA) flowmeter neutron induced activity is used to measure water flow in pipes. The water in the pipe is bombarded with neutron pulses which introduce activity into the pipe. The activity is then transported and mixed with the flow. Gamma radiation emitted from the activity is measured with a detector downstream from the activations point. The average velocity of the water is calculated using the time-resolved detector signal. The ultimate purpose is to develop a portable and non-intrusive flowmeter that can be used as a calibration tool. The objectives of this work were to investigate some parameters that affect the accuracy of the measurement. Both experimental and numerical investigations have been performed. Developments were made in the transport and mixing of the activity. A Computational Fluid Dynamics (CFD) code was used for the first time in a model of the PNA flowmeter. With the CFD code it was examined how the activity distribution changes with time and its average velocity at different times after the neutron pulse was also calculated. The results showed that there is a substantial discrepancy between the average velocity of the water and the average velocity of the activity. One factor that influences this discrepancy is how the activity is distributed in the pipe directly after the neutron pulse. It is desirable that the initial activity distribution is homogeneous and in this work it was investigated if the homogeneity could be improved. A neutron collimator and an increased distance between the neutron source and the pipe wall were shown to improve the activity distribution in the pipe. Background radiation is a problem in many nuclear applications and the PNA flowmeter is no exception. The background in a PNA measurement changes both the shape and position of the peak and if not treated correctly it will decrease the accuracy of the flowmeter. A method for identifying the origin of the background as well as a method for subtracting it was developed in this work.