ISO 24459:2021

Overview

ISO 24459:2021 specifies a quantitative method to determine uranium content in solutions and solids from the nuclear fuel cycle using L‑absorption edge spectrometry (X‑ray transmission at the uranium L‑edge, E ≈ 17.17 keV). The standard covers measurement principle, instrument and cell requirements, spectrum acquisition and processing, calibration, quality control and uncertainty evaluation for both process control and high‑accuracy (safeguards) applications.

Key topics and technical requirements

• Scope and sample types: Applicable to nitric acid and TBP‑diluent (e.g., TBP‑kerosene) solutions and to solid materials that can be fully dissolved in nitric acid.
• Concentration ranges:
  • Process control: 10 g/L to 300 g/L U.
  • High‑accuracy (safeguards): 100 g/L to 220 g/L U (nitric acid solutions).
• Interference limits for high accuracy: neptunium, plutonium and neutron poisons (e.g., gadolinium, erbium) should be <1% of uranium content to avoid significant interferences.
• Measurement principle: A collimated X‑ray beam passes through a well‑defined path length; the sharp transmission jump at the uranium L‑edge is used to derive concentration via calibration and spectrum‑fitting algorithms.
• Instrument and cell:
  • X‑ray tube voltage > 25 kV, stable current (adjustable, typically <100 µA).
  • Detector: high‑resolution solid‑state (example: silicon drift detector) with energy resolution better than 135 eV (Fe 5.9 keV).
  • Sample cells: examples include disposable 4 mm PC cell for process control and 2 mm path flow‑through cell (≈125 µL) for high accuracy.
• Data processing: reference spectrum acquisition (matrix matching), background subtraction, optional smoothing, transmission calculation (sample/reference), fitting across defined regions of interest and corrections for isotopic composition and temperature when required.
• Counting statistics: For high‑accuracy measurements a minimum total number of counts (example given: 2,000,000 counts in the fitting window) is required to meet statistical uncertainty targets.
• Calibration and QA: Gravimetric preparation of reference solutions, use of certified reference materials where possible, replicate measurements, and formal uncertainty evaluation.

Applications

• On‑line and laboratory process control of uranium-bearing streams in reprocessing and fuel fabrication facilities.
• Safeguards and accountability measurements requiring high accuracy and traceability.
• Quality control of solid uranium materials after dissolution.
• Rapid, non‑destructive (liquid) screening and routine monitoring where X‑ray L‑edge sensitivity is suitable.

Who should use this standard

• Analytical laboratories in nuclear fuel cycle facilities
• Safeguards and regulatory bodies
• Nuclear materials accounting and quality assurance teams
• Instrument vendors and service providers specifying L‑edge spectrometers for uranium analysis

Related standards

• Documents produced by ISO/TC 85 (Nuclear energy, nuclear technologies and radiological protection) and guidance on certified reference materials and uncertainty evaluation are complementary to ISO 24459:2021. Users should also follow applicable national radiological protection and transport regulations when implementing the method.

Keywords: ISO 24459:2021, uranium determination, L‑absorption edge spectrometry, nuclear fuel cycle, nitric acid solutions, TBP‑diluent, safeguards, X‑ray spectrometer, uranium concentration, calibration, quality control.