ISO 18256-1:2019

Overview

ISO 18256-1:2019 - Nuclear fuel technology: Dissolution of plutonium dioxide powders specifies a laboratory method to dissolve PuO2 powder samples to produce aliquots suitable for subsequent elemental concentration and isotopic composition analysis. The standard covers sample handling, choice of apparatus and reagents, dissolution procedures (including heating and reflux), solution adjustment and basic quality control, with explicit safety considerations for handling plutonium and corrosive reagents.

Key topics and technical requirements

• Scope: Dissolution of plutonium oxide (PuO2) powders for analytical measurements (elemental and isotopic).
• Sample mass guidance: Typical sample masses are chosen to minimize radiological risk and waste; commonly used ranges noted in the standard include a few hundred mg up to several grams (examples: ~0.1 g to 2 g commonly appropriate; powders up to ~5 g in some procedures).
• Reagents and mixtures:
  • Nitric acid (about 8–14 mol/L) combined with a catalytic fluoride source: hydrofluoric acid (~0.05–0.1 mol/L) or ammonium hydrogen bifluoride (~2 g/L).
  • Preparations for mixed nitric–HF and nitric–NH4HF2 solutions are described.
• Apparatus: Analytical balance (≤0.001 g), dissolution vessels (glass, HDPE, PTFE, Nalgene), PTFE-lined caps, reflux caps/watch glasses, shielded hotplate with temperature control, PTFE-coated tweezers.
• Procedure highlights:
  • Weigh sample in pre-weighed vessel, add appropriate volume of mixed acid (guideline: minimum ~5 mL ±1 mL per 1 g solid), heat (typical setpoint 90 °C ±5 °C) or apply reflux to accelerate dissolution.
  • Special handling to avoid loss or contamination (minimize powder exposure, work in dry inert atmosphere).
  • Avoid adding concentrated HF to plutonium nitrate solutions (can precipitate insoluble PuF).
• Analytical considerations:
  • Gravimetric dissolution methods are recommended for highest assay accuracy; volumetric methods acceptable for less critical assays.
  • Solution adjustment/dilution should use nitric acid (not water) to avoid Pu colloid formation.
• Refractory (high-fired) PuO2: Alternative techniques such as ceric nitrate in nitric acid or sodium carbonate fusion are indicated for very refractory materials.
• Safety: Plutonium must be handled in gloveboxes or hot cells; HF and concentrated nitric acid require strict safety controls.

Applications and users

• Laboratories performing nuclear fuel analysis, safeguards and safeguards verification, isotopic and elemental plutonium assays, and radiochemistry.
• Use cases include preparing samples for K‑edge densitometry, potentiometric titration, spectrometric assays, isotopic abundance measurements, and impurity determination.
• Regulatory bodies, quality assurance teams, and nuclear facility analytical services will use this standard to ensure reproducible, safe dissolution and reliable analytical results.

Related standards

• ISO 18256 is a multi‑part series - consult the ISO catalogue for other parts of the ISO 18256 series and for complementary ISO standards on laboratory water (ISO 3696) and radiological safety practices.