Nondestructive Co-assay Of U And Pu Fissile Traces By Combined Delayed Neutron And Delayed Gamma-ray Fission Product Neutron Activation Analysis

The Delayed Neutron Delayed Gamma (DNDG) technique provides a new analytical capability to the International Atomic Energy Agency (IAEA) for detecting undeclared nuclear activities. IAEA’s Long Term R&D (LTRD) plan has a stated high urgency need to develop elemental and isotopic signatures of nuclear fuel cycle activities and processes. The United States National Nuclear Security Administration (NNSA) is sponsoring research aimed at expanding the capabilities of rapid nondestructive safeguards measurements by using the combination of delayed neutron and delayed gamma analysis techniques at the Oak Ridge National Laboratory’s (ORNL) High Flux Isotope Reactor (HFIR). The pneumatic transfer facility at the HFIR Neutron Activation Analysis (NAA) laboratory is used to conduct rapid irradiations and subsequent measurements of samples containing fissile material. The NAA facility at HFIR has supported the IAEA’s Network of Analytical Laboratories (NWAL) for nearly a decade by providing mass and enrichment characterization on pre-inspection check (PIC) samples collected by the IAEA inspectors in the field. It is recognized that the distribution profile of heavy fission products (atomic mass 125 to 145) remains fairly invariant for the fissile isotopes such as 235U and 239Pu while the distribution of light fission products (atomic mass 85-105) varies from one isotope to another. By empirically calibrating the ratio of the net full energy peaks as a function of known concentration of the binary mixture, one can determine the relative fraction of fissile isotopes in an unknown sample. Combining this gamma-ray method with Delayed Neutron Activation Analysis (DNAA) provides quantification of trace fissile mass. The DN/DG method was successfully tested using fissile masses approaching the masses of pre-inspection check swipe samples. In addition, signal processing chain of the DN counter at the HFIR NAA laboratory has been upgraded, lowering the uncertainty in the 235U equivalent mass by one-third. Quantification of 238U in PIC samples using gamma spectrometry provides another vital piece of safeguards information because it allows verification of the enrichment of the uranium sample. A Compton Suppression System was configured to improve the sensitivity of 238U quantification. The results of these multi-faceted approaches to improve safeguards are discussed and results are presented.