CFRMF Neutron Field Flux Spectral Characterization

The Coupled Fast Reactivity Measurements Facility (CFRMF) is a zoned-core critical assembly with a fast-neutron spectrum zone in the center of an enriched ²³⁵ U, water-moderated thermal “driver” zone. The neutron spectrum of the fast zone has a mean energy of ∼0.72 MeV with an energy distribution of interest in fast breeder reactor dosimetry and reaction rate studies. The CFRMF neutron field has become one of the standard spectra for testing fast-neutron reactions in fissile and nonfissile materials in an interlaboratory experimental effort. The CFRMF has found much use as a fast-neutron field for the activation of materials of interest in the fast breeder reactor programs because of its well -studied energy spectrum and spatial gradients, intensity [up to 10¹¹ n/(cm² sec)], the precise control and reproducibility of intensity and experimental amenabilities. The neutron energy spectrum at the core center has been measured by proton-recoil spectrometry between ∼3.0 keV and ∼2.5 MeV and calculated by resonance absorption, transport theory, and Monte Carlo multigroup techniques from 10 MeV down. Neutron flux distributions in the fast zone have been examined by activation and fission rate measurements which show that gradients do not exceed ∼0.3% in the region where the experiments are located.