Fissile Mass Flow Monitor Source-Strength Calibration Using the ORNL Neutron Detector System

Abstract

This paper describes the neutron detector system (NDS) developed to measure the neutron source strength of the 252Cf neutron sources used in the fissile mass flow monitor (FMFM). The FMFM measures the 235U fissile mass flow in the UF6 gas streams and uses 252Cf neutron sources for the fission activation of the UF6 gas. Four FMFMs are operational in three Russian facilities for the U.S. Department of Energy's Highly Enriched Uranium Transparency Program. The 252Cf sources are replaced about every 2 years due to their short half-life (~2.65 years). During FMFM source replacement, the new 252Cf sources are calibrated and verified with the previously installed sources (i.e., a relative source mass check) to ensure proper and seamless FMFM performance. The NDS consists of a neutron detector (a commercially available high-efficiency 3He proportional counter) and the electronics, which are commercial nuclear instrument modules. A measurement time of 10 s is sufficient to take the data; the NDS yields about les 0.5% error from the average count taken up to 100 s. Measurement repeatability is good (< 1%), and it is not sensitive (< 0.5%) to the orientation of the source plug inside the NDS polyethylene source plug holder. The NDS is then calibrated for the measured source mass strength, and it is established that the sources can be measured with an overall uncertainty of 1.4% for a typical FMFM source mass of 3 mug. A detailed description of the NDS, its performance characteristics, and results of measurements performed on the latest FMFM sources are presented.