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

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.