Measurements of neutron energy spectra in the Fast Breeder Blanket Facility

Neutron energy spectra have been measured for various locations in the Fast Breeder Blanket Facility (FBBF) for the energy range from approximately 2 keV to 2 MeV. The FBBF is a subcritical facility designed and constructed at Purdue University for studies on fast-reactor blankets. Hydrogen- and methane-filled cylindrical proportional counters were used for the spectrum measurements, with two-parameter pulse-shape analysis for discirmination against gamma-ray event in the detectors. Corrections were made to the proton-recoil distributions for electric-field, finite-range and carbon-recoil efects, using the PSNS data analysis codes. Neutron energy spectra were derived by numerical differentiation using the PUNDIT code. Experimentally-determined spectra are compared with spectra calcualated by the 2DB multigroup diffusion code and with other experimental data obtaned for the FBBF. The multigroup calculation predicts softer spectra in the blanket than what was found experimentally. In addition, the magnitude of the total flux falls off with blanket radius much more quickly for the calculated spectra than for the experimentally-determined spectra, in agreement with other experimental data for the FBBF. The calculations and measurements agree more closely near the steel support plate than for other blanket locations. It is apparent from the experimental results that the quality of the calculations is not adequate for fast reactor blankets