Year
2015
Abstract
Hybrid X-ray fluorescence and K-edge densitometry (HKED) is used at nuclear fuel reprocessing facilities to determine the concentrations of uranium and plutonium in the input accountability vessel. The uranium concentration is determined by K-edge densitometry, that is, from the step-difference in transmission on either side of the K-absorption edge measured using a continuous X-ray spectrum on a vial of the solution defining a well-known geometry. The relative plutonium-to-uranium concentration is obtained from the relative strength of the X-ray-induced K-shell X-ray fluorescence production measured, at the same time, at a backward angle in energy-dispersive mode. As commonly applied at current commercial reprocessing facilities, the plutonium-to-uranium ratio is about 1%, and the uranium concentration is typically in the range 100 to 300 g/L. Under these conditions, optimized HKED instruments routinely provide highly accurate results (with uncertainties of about 0.2% on uranium and about 0.7% on plutonium) more rapidly (combined assay time of about 1 h) and more economically than chemical methods, and without the creation of secondary radioactive waste. The HKED technique was conceived and pioneered at KfK Karlsruhe by Herbert Ottmar and his co-workers [1]. It has proved to be an extremely successful blend of an almost absolute measurement technique (KED) and a robust relative method based on X-ray Fluorescence (XRF). However, the evolving nuclear fuel cycle and changing international safeguards objectives have necessitated enhancements to the HKED analysis to accommodate more complex actinide mixtures and the reporting of additional actinides such as Np, Am and Cm. These new process streams introduce additional measurement challenges. For example, solutions with U:Pu ratios on the order of 1:1 result in interferences are not accommodated by current hybrid analysis techniques. To address these shortcomings, we have developed an analysis method based on fitting extended energy regions of both the K-edge transmission and XRF spectra that mitigates the interferences, provides measurement results for each of the five actinides of primary interest (U, Np, Pu, Am, and Cm) and also offers improved measurement precision relative to current analysis techniques. The new method offers additional advantages such as the potential elimination of the need for extensive experimental calibration of the K-edge transmission measurement and the reduction of required sample volumes. This paper discusses the new analysis approach and presents preliminary performance results.