The Spectral Distribution of Neutrons and Neutron Reaction Cross Sections in an Unreflected Uranium-235 Critical Assembly and the Fission Neutron Spectrum

Publisher Website
Google Scholar
Abstract
Previously reported inconsistencies between activation detector and n, p scattering/time-of-flight (TOF) measurements of the thermal-neutron-induced 235U fission spectrum prompted a comparison of such measurements in the core center and on the surface of a bare 235U assembly, referred to as Godiva. For the present study, TOF measurements and multiple foil measurements of the core and surface spectra of the APFA-III-Godiva are compared. Comparison of the integral fluxes above specified energies for the two methods shows agreement to within ∼5% at core center. Results obtained at ½ in. from the core surface, however, show disagreement between the multiple foil and TOF (and previously reported photo-plate data) which is similar to that found for measurements in the fissionspectrum. Results for the fission spectrum are re-analyzed using the same evaluated energy-dependent cross sections as used for the Godiva study but with a larger number of foil reactions than previously available. A Monte Carlo error analysis code is used for the assignment of errors for the activation results for the Godiva and fission spectrum studies. It is concluded that if the activation measurements of the Godiva and fission spectrum remain firm, and significant changes in current evaluated reaction cross sections are not effected, then increases up to as much as 10 to 15% in the mean energy of the Godiva leakage and fissionspectrum must be considered.