ISO 18256-2:2019

Overview

ISO 18256-2:2019 specifies a validated procedure for the dissolution of MOX pellets and powders (mixed oxide fuels containing plutonium dioxide) to produce aliquots suitable for subsequent elemental concentration and isotopic composition analysis. The standard addresses sample size, reagent selection, apparatus, dissolution conditions and safety measures needed in plutonium-bearing sample preparation for nuclear fuel technology and radiochemical laboratories.

Key topics and technical requirements

• Scope and purpose: Dissolution methods tailored to MOX pellets or powders to generate representative solutions for Pu/U assay and isotopic measurements.
• Sample mass: Typical sample masses recommended are 0.1 g to 10 g depending on radiological hazards and analysis needs.
• Reagents: Use analytical-grade reagents. Recommended dissolving media include mixed nitric–hydrofluoric acid (≈8–14 mol/L HNO3 with HF ≈0.05–0.1 mol/L) or nitric–ammonium bifluoride (≈14.4 mol/L HNO3 with ~2 g/L NH4HF2 producing ~0.07 mol/L HF).
• Apparatus: Analytical balance (0.001 g), PTFE-lined vials or glass vessels, reflux caps or condensers, shielded hotplates and fume hood. Handling in glove boxes or hot cells is mandatory.
• Dissolution conditions:
  • Powders: heat at 70–110 °C; add acid slowly due to rapid reaction.
  • Pellets: dissolve under reflux/boiling; dissolution may require ≈18 h or more.
• Container material trade-offs:
  • Glass: attacked by fluoride (tare mass changes) but volatile SiF4 formation can reduce final fluoride and limit PuF precipitation.
  • PTFE/HDPE: inert to HF (tare stable) but higher risk of insoluble plutonium fluoride if oxidation state changes.
• Quality control: Gravimetric dissolution recommended for highest assay accuracy; volumetric methods acceptable for less critical assays. Mass-loss checks (e.g., <1% loss criterion) and formulae to estimate heavy-metal concentration and acidity are provided for solution adjustment.
• Safety: Strong emphasis on radiological control and chemical hazards-HF and concentrated HNO3 are highly corrosive; specific precautions and local safety procedures required.

Applications and users

ISO 18256-2:2019 is intended for:

• Nuclear fuel manufacturing and quality-control laboratories (MOX fabrication facilities)
• Radiochemistry and safeguards analysts performing Pu/U assay and isotopic analysis
• Regulatory and inspection bodies assessing sample-preparation best practice
• Research labs working on plutonium-containing materials and analytical method development

Practical uses include preparing dissolution aliquots for spectrophotometry, potentiometric titration, K-edge densitometry, mass spectrometry and impurity/trace-element determination.

Related standards

• ISO 18256 series - other parts of the series cover dissolution of different plutonium dioxide–containing materials (a full list of parts is available on the ISO website).
• Refer to local and international radiation-safety and chemical-hazard guidance for handling plutonium, HF and concentrated acids.

Keywords: ISO 18256-2:2019, MOX pellets, MOX powders, plutonium dioxide, dissolution, nuclear fuel, sample preparation, isotopic analysis, plutonium assay, nitric–hydrofluoric acid, ammonium bifluoride.