The JRC was among the pioneers in the field of development, production and certification of Large Sized Dried (LSD) spikes. LSD are certified reference materials (CRMs) used for measuring the U and Pu isotope amount content in mainly spent nuclear fuel solutions by isotope dilution thermal ionization mass spectrometry (ID-TIMS). LSD play a very important role in safeguarding nuclear reprocessing plants.
Embedding matrices to extend the shelf life of reference materials
A recent achievement in the further development of certified reference materials was the successful implementation of an exploratory research project for extending the shelf‑life of LSD spikes (see more on Nuclear Reference Materials). This transdisciplinary research effort enabled to interact beyond the nuclear sciences with the fields of polymer physics, chemistry and material science focusing on sustainability.
Shelf-life plays a crucial role in the safe transport, usability and storage of any actinide material. Therefore, a stabilizing matrix is applied that embeds the actinides of dried spike CRMs and fixes them at the bottom of the vial. This matrix must guarantee sufficient shelf-life, while dissolving easily in nitric acid solution and not interfering during actinide separation techniques or ID-TIMS measurements. It is not straightforward to find suitable matrices that meet all these requirements at the same time.
Embedding matrices with excellent resistance to radiolysis, oxidation and hydrolysis are likely to offer limited usability of the spike CRM.
Cellulose acetate butyrate (CAB) with dioctyl phthalate (DOP)
Since 2002, a solvent cast polymer embedding film of cellulose-acetate butyrate (CAB) has been used as embedding matrix for the IRMM-1027 LSD spikes having the following characteristics:
• Good adherence to the glass.
• Easy to prepare and dispense.
• Readily dissolves in hot HNO3.
• Drawback: Tendency towards crack formation and flaking after a few years.
The addition of 20-30% Dioctyl phthalate (DOP) and triethyl citrate (TEC) plasticizer were tested on U/Pu samples for their mechanical resilience and ageing mechanisms with the result that DOP outperformed TEC in terms of elasticity and toughness. DOP boosts the initial mechanical properties of the cellulose based embedding films and reduces the internal stress at the polymer-embedded material interface:
• No tendency towards crack formation and flaking after a few years
Carboxymethyl cellulose (CMC)
Carboxymethyl cellulose (CMC) is used as a viscosity modifier or thickener, and to stabilize emulsions in various products, both food and non-food, including ice cream, cakes, tooth paste, fabric, detergents and in advanced battery applications.
Embedding IRMM-1027 LSD spikes with CMC foam shows:
• Good adherence to the glass
• Feasible to prepare and dispense
• Readily dissolves in hot HNO3
• No tendency towards flaking
CAB reconditioning
Flaked spike CRMs treated with H3PO4 can be transformed into a homogeneous amorphous material and used for measurements
• Reconstructs homogeneity and integrity.
• Easy to prepare.
• Dissolves quickly in hot HNO3.
• Stability > 6 months.
Achievement and future objectives
- A shelf-life of at least 5 years can be achieved for dried actinide CMRs if CAB/DOP or CMC is used as embedding matrix.
- The validity of the certificate can be further extended via post certification monitoring, using isotope dilution thermal ionization mass spectrometry.
- Flaking dried actinide CRMs can be reconditioned with phosphoric acid.
- Recently, a European patent application was filed on an entire portfolio of organic embedding matrices that will further optimise the characteristics of LSD spikes and other actinide CRMs to meet contemporary demands of policy partners and stakeholders.