Overview
EN ISO 24459:2023 (ISO 24459:2021) specifies a standardized laboratory method for the determination of uranium content in solutions from the nuclear fuel cycle using L-absorption edge spectrometry. The method applies to nitric acid and TBP‑diluent (for example TBP‑kerosene) solutions that are free of suspension, and it has defined concentration ranges for both routine process control and high‑accuracy (safeguards) measurements.
Key topics and requirements
- Analytical principle: measurement of the sharp transmission jump at the uranium L‑edge (E ≈ 17.17 keV) with a highly collimated X‑ray beam and a solid‑state detector; uranium concentration is derived from the size of the L‑edge jump using calibration and spectrum‑processing algorithms.
- Applicable concentration ranges:
- Process control: 10 g/L to 300 g/L uranium (solutions free of suspension).
- High accuracy / safeguards: 100 g/L to 220 g/L uranium in nitric acid solutions (solutions free of suspension).
- Matrix and impurity limits for high accuracy: neptunium and plutonium < 1% of uranium, and neutron poisons (gadolinium, erbium) < 1% to avoid significant interferences.
- Instrumentation & components: X‑ray tube (max voltage > 25 kV), detector, multichannel analyser (MCA), temperature sensor, collimation and shielding, and sample cells (glass, polycarbonate, or quartz depending on chemical resistance).
- Calibration & quality control: use uranium reference solutions, preferably SI‑traceable certified reference materials with standard uncertainty ≤ 0.06% for concentration; record reference, calibration and sample spectra at the same HV cut‑off and count rate.
- Counting statistics & uncertainty: for high accuracy a minimum total number of counts of 2 000 000 in the fitting window is recommended; the standard details procedures for spectrum evaluation, background subtraction, calculation of transmission and uncertainty evaluation.
- Corrections: isotopic composition and temperature corrections may be required for high accuracy results. Annexes include calculation methods for correction factors and preparation of quality control samples.
Applications and users
- Nuclear fuel cycle laboratories performing process control assays on uranium streams (e.g., reprocessing and conversion facilities).
- Safeguards and accountability laboratories requiring high‑accuracy uranium assays for nuclear material verification.
- Metrology and analytical labs validating or calibrating X‑ray absorption equipment for uranium determination.
- Regulatory bodies and quality assurance groups setting measurement protocols and acceptance criteria.
Related standards
- EN ISO 24459:2023 is the CEN adoption of ISO 24459:2021 (no modification). It sits alongside other ISO/CEN documents on nuclear analytical methodology, nuclear material accountancy and radiological protection developed by ISO/TC 85 and CEN/TC 430.
Keywords: EN ISO 24459:2023, uranium determination, L‑absorption edge spectrometry, nuclear fuel cycle, nitric acid, TBP‑diluent, process control, safeguards, calibration, uncertainty, X‑ray spectrometer.
