A Multi-Spectra Neutron Irradiation Technique for the Nondestructive Assay of Fissionable Materials

A technique has been developed for the nondestructive assay of fissionable materials which makes use of the variations with energy in the fission cross section for a given isotope in order to produce a signature characteristic of that isotope. The technique consists of irradiating the sample with neutrons from an accelerator and measuring the delayed-neutron response from the induced fission reactions in the sample. Different neutron irradiation energies were obtained by surrounding a 14-MeV neutron source with various moderating assemblies. Assay results obtained using this delayed-neutron technique include the following: (a) uranium samples with enrichments ranging from ∼3 to 98%; (b) plutonium-uranium oxides; (c) ²³³u-²³²Th salt mixtures; (d) “spent” reactor fuel elements; and (e) scrap containers ranging in size from small vials to one-gallon cans containing plutonium or uranium scrap. In addition, measurements have been made on the influence of various nonfissionable matrix materials on the assay results.