This paper discusses the development of a microprocessor-based instrument to measure boron concentration in the cooling water of a pressurized water reactor. The technique used to develop the boronimeter is neutron transmission entailing the use of a neutron source and a bank of detectors to measure the absorbed neutrons in a sample of borated water. A unique feature of the boronimeter is the inclusion of a servo-operated absorber sleeve which is automatically positioned to compensate for changes in boron concentration. The sleeve is positioned to keep the count rate constant and the position of the sleeve is then used to determine the concentration of the sample. The null operation feature makes the boronimeter particularly adaptable to on-line operations owing to the improved counting statistics. Tests completed on the boronimeter demonstrate its usefulness for accurate, rapid analysis of boron concentration. The system was calibrated over the concentration range 0-2500 ppm boron. At a concentration of 1000 ppm the standard deviation was ± 2% for an analysis time of < 4 minutes. / Master of Science