ISO 12795:2004

Overview

ISO 12795:2004 - "Nuclear fuel technology - Uranium dioxide powder and pellets - Determination of uranium and oxygen/uranium ratio by gravimetric method with impurity correction" specifies a gravimetric laboratory method to determine the uranium mass fraction (U) and the oxygen-to-uranium atomic ratio (O/U) in hyperstoichiometric uranium dioxide (UO2+X) powders and pellets. The method is intended for nuclear-grade UO2 and includes provisions for impurity correction when non-volatile impurities exceed specified limits.

Key topics and requirements

• Principle: Oxidise a weighed UO2+X sample by controlled heating in air to produce stoichiometric U3O8 (under specified conditions), then weigh to calculate U mass fraction and O/U atomic ratio. Verification with a precise U determination or a reference material is required for method validation.
• Sample and apparatus:
  • Typical sample masses: ≥ 8 g for powders, ≥ 5 g for pellets.
  • Muffle furnaces (up to 550 °C and up to 950 °C), platinum crucibles, desiccator, analytical balance (capable of 0.1–0.5 mg resolution).
• Thermal procedure:
  • Pellet pre‑ignition: ~500 °C ±50 °C for ≥ 2 h (powders omit).
  • Final ignition: ~900 °C ±50 °C for 3 h, repeat until constant mass.
  • Cooling constraints specified (e.g., crucible cooling times).
• Impurity correction:
  • Apply correction if total non‑volatile impurities (expressed as oxides) exceed 1 500 µg/g of sample.
  • Impurities below this level affect O/U by < 0.0005 and may be neglected.
  • Non‑volatile impurities must be measured by an appropriate analytical technique and converted to oxide form using gravimetric conversion factors provided (table of factors for Ag, Al, Fe, Si, rare earths, etc.).
• Calculations:
  • Includes stoichiometric factor (F) accounting for uranium isotopic composition, and a correction (C) for non‑analysed impurities.
• Precision and accuracy:
  • U mass fraction: for powders (≤4 000 µg/g impurities) and pellets (≤300 µg/g), precision ±0.02 % and accuracy ±0.03 % (2σ) for U.
  • O/U ratio: powders (2.06–2.15) precision ±0.005 and accuracy ±0.008 (2σ); pellets with ≤300 µg/g impurities precision ±0.0026 and accuracy ±0.003 (2σ).

Applications and users

• Quality control and acceptance testing of nuclear fuel (UO2) powders and pellets.
• Fuel fabrication and materials laboratories for batch certification and production control.
• Regulatory and inspection bodies requiring standardized U and O/U determinations.
• Research & development groups validating material stoichiometry, oxidation behavior, and enrichment-dependent conversions.

Related standards

• ISO 9989 - Determination of uranium in UO2 by titrimetric methods (referenced for verification).
• ISO 3696 - Water for analytical laboratory use (reagents).
• Use of certified reference materials (e.g., CBNM EC standard No. 110) is recommended for method verification.

Keywords: ISO 12795:2004, uranium dioxide, UO2+X, O/U ratio, uranium mass fraction, gravimetric method, impurity correction, nuclear fuel quality.