Monte Carlo modeling and application study of Hybrid L-Edge/L-XRF densitometry

Abstract: Precise measurement of uranium (U) and plutonium (Pu) concentrations is critical in spent nuclear fuel reprocessing for nuclear material accounting and process control. The hybrid L-edge/L-XRF densitometry (HLED), which combines the L-edge densitometry technique with the X-ray fluorescence (XRF) technique, can be applied to measure the concentration of U and Pu in both pure and mixed forms. In this study, a Geant4-based simulation model of the HLED system was established to optimize critical parameters including X-ray tube voltage, primary filter configuration, XRF detection angle and collimator design. The simulation results reveal that the optimal parameters for the X-ray tube are an X-ray source voltage of 30 kV and a 0.15 mm Ni filter, with an XRF detection angle of 45°. An HLED platform was also developed based on the simulation model for practical validation. The working curve was calibrated using standard uranium samples spanning a concentration range of 14 g/L to 162 g/L, yielding a coefficient of determination (R2) of 0.999. Measurement validation showed relative errors of less than 2% and measurement precision of less than 0.5%. To address the low measurement precision for both low-concentration samples that produce weak L-edge absorption, and high-concentration samples that exhibit strong L-edge absorption, an optimal path length and a current correction model of X-ray tube were presented, respectively. Furthermore, a uranium solution of 1.807 g/L and a thorium (Th) solution of 200 g/L was prepared to verify the precision performance. The experimental results indicate that the measurement precision of the 1.807 g/L uranium solution can be improved from 2.356% to 1.588% by increasing the path length. An optimal path length of 11 mm for samples with concentration below 10 g/L was obtained by simulation experiment. Moreover, the precision of the 200 g/L thorium solution was increased by 35%, with relative standard deviation decreasing from 2.35% to 1.588%.