Remote sensing of Pu in uranyl nitrate crystals using reflectance spectroscopy and chemometrics

Remote quantification of Pu(vi) (0-5 mol%) co-crystallized with U in uranyl nitrate hexahydrate (UNH) crystals was achieved in a glove box using reflectance spectroscopy coupled with chemometric modeling. Reflectance spectra were also acquired for Pu(iv) and Np(vi) (0-5 mol%) crystallized with UNH; revealing spectral features consistent with their solution-phase analogs. Principal component analysis revealed Pu(iv/vi) and Np(vi) concentrations as the primary source of variation in the data, informing the development of a supervised partial least squares regression model for Pu(vi). The resulting calibration demonstrated robust performance, with replicate root mean square errors near 10% and quantifiable limits near 0.2 mol% Pu(vi) relative to U. The Pu(vi) remained stable in the crystalline UNH matrix for at least one week with minimal reduction to Pu(iv). Notably, Pu(vi) and Np(vi) incorporation in UNH quenched U(vi) fluorescence while Pu(iv) did not. This study presents a noninvasive, spectroscopic approach for solid-state Pu quantification, with direct implications for material accountability and nuclear nonproliferation monitoring.