Year
1997
Abstract
The fissile mass-flow monitoring system functions to measure the flow rate and uranium-235 content in liquid or gas streams. The system is non-intrusive in that it does not penetrate the process piping. Fissile mass flow of the fluid is monitored by using a moderated fission neutron source to periodically introduce a “burst” of thermal neutrons into the fluid stream to induce fission. Delayed gamma emissions from the resulting fission fragments are detected by high-efficiency scintillators downstream of the neutron source. The fluid flow rate is measured from the time between initiation of the thermal neutron burst and detection of the fission product gamma emissions, and the U-235 content is inferred from the intensity of the gamma burst observed by the detectors. Design of the fissile mass flow monitor requires satisfaction of several competing constraints. Efficient operation of the monitor requires that source-induced fission rate and detection efficiency be maximized while the source-induced background rate is simultaneously minimized. Near optimal nuclear design of the system was achieved employing numerous Monte Carlo calculations and measurements. This paper primarily addresses calculational aspects of the physics design for the system applied to UF6 gas.