Using Nuclear Resonance Fluorescence for Nondestructive Isotopic Analysis

Using Nuclear Resonance Fluorescence for Nondestructive Isotopic Analysis Bernhard A. Ludewigt 1) , Vladimir V. Mozin 2,3) , Andrew R. Haefner 1,2) , and Brian J. Quiter 1,2) Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA Los Alamos National Laboratory, NM 87544, USA Abstract Nuclear resonance fluorescence (NRF) has been studied as one of the nondestructive analysis (NDA) techniques currently being investigated by a multi-laboratory collaboration for the determination of Pu mass in spent fuel. In NRF measurements specific isotopes are identified by their characteristic lines in recorded gamma spectra. The concentration of an isotope in a material can be determined from measured NRF signal intensities if NRF cross sections and assay geometries are known. The potential of NRF to quantify isotopic content and Pu mass in spent fuel has been studied. The addition of NRF data to MCNPX and an improved treatment of the elastic photon scattering at backward angles has enabled us to more accurately simulate NRF measurements on spent fuel assemblies. Using assembly models from the spent fuel assembly library generated at LANL, NRF measurements are simulated to find the best measurement configurations, and to determine measurement sensitivities and times, and photon source and gamma detector requirements. A first proof-of-principal measurement on a mock-up assembly with a bremsstrahlung photon source demonstrated isotopic sensitivity to approximately 1% limited by counting statistics. Data collection rates are likely a limiting factor of NRF-based measurements of fuel assemblies but new technological advances may lead to drastic improvements. Keywords: Nuclear resonance fluorescence, nondestructive analysis, nuclear safeguards, isotopic assay, MCNPX. I. Introduction Quantifying the isotopic composition of spent fuel and other fuel cycle materials is an important goal in nuclear safeguards. A key challenge is the determination of plutonium mass in spent fuel with nondestructive assay techniques. New techniques are sought to address key safeguard needs including independent verification of plutonium mass, determination of plutonium mass for input accountability of a reprocessing facility, and the detection of possible shipper/receiver differences. Nuclear resonance fluorescence (NRF) has been studied as one of the nondestructive analysis (NDA) techniques currently being investigated by a multi-laboratory collaboration with support from the Next Generation Safeguard Initiative[1]. NRF is a well-known phenomenon in nuclear physics. In recent years NRF-based methods have been developed for use in cargo screening and nuclear nonproliferation applications[2,3]. NRF is the excitation of nuclei by energetic photons and the subsequent decay of the excited states by emission of gamma rays with characteristic energies. Gamma spectra measured at backwards angles exhibit isotope specific, narrow lines that allow the identification and quantification of isotopic content. Nuclear resonances for 235 U, 238 U, and 239 Pu have been found in the 1.5 MeV to 2.5 MeV range with cross sections that are typically several to tens of eV⋅barn[4,5,6]. Since photons with energies around 2 MeV can penetrate a fuel assembly, NRF potentially enables the non-destructive, direct