Reactor primary coolant flow rate determination from observation of the ratio of gamma to neutron radiation levels near the primary line.

Abstract

Routine operations and health physics measurements at a research reactor commonly include monitoring of gamma and neutron radiation levels in the coolant pump room area. Gamma fields are heavily weighted by emissions from 7.1 sec. 16N and neutron levels are commonly attributed to 4.1 sec 17N decay. For a fixed flow rate measurements of either gamma or neutron levels made at a particular point downstream from the core are indicative of a particular power level. The work described here demonstrates the use of the ratio of gamma to neutron measurements as a quantitative indicator of primary coolant flow rate. The quantities of 16N and 17N present at a particular location in the primary coolant line are dependent on in core coolant irradiation time and the time required for coolant to move from the core to the monitoring point. Since 16N and 17N have appreciably different half lives variations in coolant flow rate will affect the quantities of each species present at a particular location such that the ratio of the gamma (16N) to neutron (17N) measurement also changes. The ratio is constant, independent of power level for a fixed flow rate, but varies smoothly with increasing or decreasing flow rate over a range of interest; it can thus serve as a secondary measurement of the important operating parameter, primary coolant flow rate.