Synthesis and characterization of a U/Pu mixed-actinide microparticulate reference material

Abstract

International nuclear safeguards require microparticle reference materials (RMs) with complex uranium (U) and plutonium (Pu) compositions. RMs are needed for quality control within the international nuclear safeguard community governed by the International Atomic Energy Agency, and as qualification specimens for new microanalytical methods. Prior to this effort, no combined U/Pu microparticle specimens existed with sufficient characterization and corresponding qualified analytical measurands for U and Pu isotopic and elemental composition to meet this need. Savannah River National Laboratory (SRNL) produced homogeneous mixed U/Pu, monodisperse, micrometer-scale particles (denoted RM UPu-100A) using the SRNL THermally Evaporated Spray for Engineered Uniform particulateS (THESEUS) particle production platform. The targeted attributes for RM UPu-100A were a monodisperse, spherical particle size distribution centered at 1 μm diameter, n(²³⁸U)/n(²³⁹Pu) = 100; n(²⁴⁰Pu)/n(²³⁹Pu) = 0.30, and n(²³⁵U)/n(²³⁸U) = 0.05. After identifying, purifying, and mixing three uranium and two plutonium endmembers, monodisperse U/Pu microparticulate of the desired elemental and isotopic distributions was produced and electrostatically collected onto silicon planchets. Characterization of the produced microparticulate at SRNL and Los Alamos National Laboratory (LANL) used large geometry secondary ion mass spectrometry (LG-SIMS), multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), scanning electron microscopy (SEM), and alpha spectroscopy. Analysis results demonstrate that a compositionally homogeneous particle population was achieved. Key characteristics of RM UPu-100A include an approximate particle diameter of 1.08 μm and n(²³⁸U)/n(²³⁹Pu) = 103.3 ± 2.8; n(²⁴⁰Pu)/n(²³⁹Pu) = 0.2838 ± 0.0032; and n(²³⁵U)/n(²³⁸U) = 0.05221 ± 0.00024. A SIMS relative sensitivity factor, RSF_Pu:U, was calculated from n(²³⁸U)/n(²³⁹Pu); RSF_Pu:U = 2.282 ± 0.064, and n(²³⁵U + ²³⁸U)/n(²³⁹Pu + ²⁴⁰Pu); RSF_Pu:U = 2.242 ± 0.063.