1 Recent Progress in the Development of Lead Slowing-Down Spectroscopy for Direct Measurement of Pu in Spent Fuel L. Eric Smith*, Sonya Bowyer, Mark Shaver, Derek Haas, Mary Bliss Pacific Northwest National Laboratory, Richland, WA 99352 Avigdor Gavron, Los Alamos National Laboratory, Los Alamos, NM 87545 Yaron Danon, Rensselaer Polytechnic Institute, Troy, NY 12180 George Imel, Idaho State University, Pocatello, ID 83204 Denis Beller, University of Nevada, Las Vegas, Las Vegas, NV 89054 * corresponding author: tel: (1) 509-376-1599 , email:eric.smith@pnl.gov ABSTRACT Accurate and direct measurement of Pu in spent nuclear fuel remains a key challenge for safeguarding nuclear fuel cycles. Lead slowing-down spectroscopy (LSDS) is an active nondestructive assay method that has the potential to provide independent, direct measurement of Pu and U isotopic mass with an uncertainty considerably lower than the approximately 10% typical of today’s confirmatory assay methods. Studies of LSDS viability reported previously were based on simulated assays of pressurized water reactor assemblies, and results indicated that LSDS could provide direct assay of Pu-239 and U-235 (and possibly Pu-240 and Pu-241) with uncertainties less than a few percent. Realizing this potential and improving upon it will require the development of new time-spectra analysis methods that can overcome the nonlinear effects of neutron self- shielding, development of high-efficiency threshold neutron sensors for collecting the fission- neutron assay signal, and the use of interrogating neutron sources capable of short pulses and high intensity. This paper describes recent work aimed at addressing these LSDS technical challenges, including MCNP modeling of pressurized water reactor assemblies, advancements in time-spectra analysis methods used to calculate isotopic mass, development of threshold neutron sensors tailored for LSDS, and benchmarking measurements to be performed at LSDS facilities in the United States. INTRODUCTION At present, total Pu mass in spent fuel assemblies is inferred from a combination of passive measurements of easily measured isotopes (e.g. Cs-137 and Cm-244) and computational predictions of isotopic inventories using burnup codes. International Atomic Energy Agency (IAEA) experience shows that the Pu uncertainty for these confirmatory methods is approximately 10% on total Pu concentration, when compared to destructive analysis results [Peter]. In a single assembly, this translates to as much as 0.5 kg of Pu. In high-volume storage or reprocessing facilities, this could equate to ―unaccounted Pu mass‖ of more than 1000 kg per year. A nondestructive assay technology is needed that can provide timely, independent (i.e. no requirement for operator-declared information such as initial U-235 enrichment or burnup), and direct measurement of Pu mass, and do so with uncertainties lower than those achieved with today’s indirect, confirmatory methods. Lead slowing-down spectroscopy (LSDS) is an active interrogation technique that has been used for several decades in cross-section measurements, but has seen relatively limited application to the nondestructive assay of nuclear materials. Prior work in the application of LSDS to spent fuel demonstrated its potential for direct measurement of fissile isotopes such as U-235, Pu-239 and Pu-