ISO 7476:2003

Overview

ISO 7476:2003 specifies a precise gravimetric method for determining the mass fraction of uranium in nuclear-grade uranyl nitrate solutions containing more than 100 g/kg uranium. The method converts uranyl nitrate to uranium oxide (U3O8) by controlled ignition and calculates uranium content using a gravimetric conversion factor that depends on the uranium isotopic composition. Non-volatile impurities must be assessed and corrected to maintain accuracy.

Key Topics

• Principle: conversion of uranyl nitrate to U3O8 by ignition in air (900 °C ± 10 °C; alternative 875 °C ± 25 °C) and gravimetric calculation using an isotope-dependent conversion factor.
• Critical controls:
  • Ignition temperature and time (steady heating at 890–910 °C for 1 h is specified for the 900 °C procedure).
  • Sample mass: typically 3 g to 15 g of solution delivering ~1 g uranium.
  • Careful evaporation and use of ashless filter-paper powder or sulfuric acid to avoid spattering during decomposition.
• Impurities: non-volatile impurities influence results. If total non-volatile impurities exceed 0.1 %, overall uncertainty increases. Impurities are determined by suitable techniques (emission spectroscopy, AAS, mass spectrometry) and corrected using gravimetric conversion factors (Annex A).
• Isotopic composition: the conversion factor for uranium depends on the mean relative atomic mass A(U), determined from atom or mass fractions measured by mass spectrometry.
• Quality metrics:
  • Repeatability coefficient of variation about 0.05 % under optimum conditions.
  • Duplicate analyses should not differ by more than 0.15 %.
  • Regular validation against a certified standard uranyl nitrate solution; bias > 0.1 % indicates adjustment needed.

Applications

• Routine laboratory determination of uranium mass fraction in nuclear fuel processing and quality assurance of uranyl nitrate feedstocks.
• Verification of certified reference solutions and validation of concentration scales for analytical laboratories handling nuclear-grade materials.
• Use where high precision and traceable gravimetric results are required, especially when isotopic composition is known and non-volatile impurity levels are low.

Practical advantages of the method include its direct mass-based measurement, traceability to certified reference materials, and well-defined quality-control checks (standard solution analysis, duplicate testing, and impurity correction).

Related Standards

• ISO 10980:1995 - Validation of the strength of reference solutions used for measuring concentrations (normative reference cited in ISO 7476:2003).

Other relevant lab practices referenced in ISO 7476 include requirements for analytical-grade reagents, calibrated muffle furnaces, analytical balances (sensitivity 0.05 mg), and established procedures for reporting results (including buoyancy corrections, sample identification, and method references).

Keywords: ISO 7476:2003, gravimetric method, uranium, uranyl nitrate, U3O8, non-volatile impurities, isotopic composition, mass spectrometry, quality control.