241AmBe SEALED NEUTRON SOURCE ASSESSMENT STUDIES FOR THE FISSILE MASS FLOW MONITOR

In this paper the assessment studies performed on the fissile mass flow monitor (FMFM) system for possible use of 241AmBe sealed neutron sources is presented. The FMFM uses 252Cf neutron sources for fission activation of a UF6 gas stream for 235U fissile mass flow rate measurements. Present FMFM 252Cf sources are replaced about every two years due to their relatively short half-life (~2.65 years). This is a costly process not only for the source fabrication but also because of the associated costs of transportation, installation, and storage. In addition, the sources are calibrated with the previously installed sources to ensure proper and seamless performance. The long half-life (~433 years) of 241AmBe would provide an almost constant level of neutron flux from the source over the lifetime of the equipment (~20 years). The FMFM is calibrated once after the source installation, practically eliminating any interference on facility operation for the lifetime of the equipment. The standard size U.S. 241AmBe source (3 Ci in a 0.75-inch-diam, 2-inch-long double-sealed stainless steel capsule) fits into the present FMFM moderator source housing. This source provides ~6.6 ?? 106 neutron/s, which is equivalent to ~3 micrograms of present 252Cf source neutron output, with an average neutron energy of ~4 MeV, a factor of 2 higher than 252Cf neutrons. Experiments to characterize 241AmBe sources were performed on the FMFM Test Facility, using a 3He-neutron counter and a fission chamber. Thermal neutron production, which is one of the key FMFM performance parameters, and dose rate were evaluated. These experiments are designed to provide data to assess the possible use of 241AmBe for the FMFM. The assessment caveats are that the FMFM performance needs to be maintained and the facility dose rate requirement must be met (i.e., maximum of 0.3 mrem/h at 1 m from the equipment) without significantly altering the FMFM design. The experimental results on the performance comparison obtained from the 252Cf and 241AmBe sources as well as the overall assessments are presented.