Particulate Reference Materials for Nuclear Safeguards—from Hydrothermal Synthesis to Shelf-Life Study

Particle analyses are a key tool for safeguards verification by the International Atomic Energy Agency (IAEA). They mainly consists in measuring the isotopic composition of individual sub-micrometric particles of nuclear materials. Indeed, despite confinement precautions, nuclear processes may release nanometer to micrometer-sized particles inside the facility. Detection and measurement of the isotopic composition of these particles provide precious information on the activities implemented in the facility, for example by assessing the enrichment rate of a fuel through the ²³⁵U/²³⁸U ratio, or the date of the last chemical purification thanks to the ²³⁰Th/²³⁴U radio-chronometer. For this purpose, analytical methods based on mass spectrometry techniques, such as Thermo-Ionization Mass Spectrometry (TIMS) and Large Geometry Secondary Ions Mass Spectrometry (LG-SIMS) have been developed and are nowadays applied by a few specialized laboratories in support of the IAEA’s nuclear safeguards program. However, because of the low number and extremely small size of the particles of interest, such measurements are always an analytical challenge. Hence, laboratories need reference materials representative of the samples analyzed, i.e. actinide oxide particles with well-known sizes, densities and isotopic compositions, for optimization and qualification of analytical methods and instruments.<br/>Therefore, the synthesis of actinide oxide particles is a field of growing interest. Several production methods based on physical processes were developed, but frequently required specific equipment and cannot be easily implemented in standard nuclear chemistry labs. In this frame, we developed since several years original wet chemistry routes, which aim at precipitating directly morphology-controlled actinide oxides from mixtures of solutions. Such methods are mostly based on the hydrothermal decomposition of An(IV) carboxylate, followed by the hydrolysis of the cations which leads to the formation of amorphous An(OH)₄ samples, finally aging to AnO₂.nH₂O.<br/>The first part of this presentation will cover the multi-parametric studies undertaken to monitor the morphology and the size of UOx and mixed (U,Th)Ox particles. Particularly, the impact of pH, aspartic acid concentration, or mechanical stirring was evaluated, leading to prepare monodisperse microspheres in the 400 nm – 2.5 µm diameter range. In both cases, particles were fired in controlled atmosphere to monitor the final O/M stoichiometry of the samples, then extensively characterized, especially to point out any chemical heterogeneity, or residual porosity.<br/>In a second part, alteration tests of the produced particles will be discussed. These latter were undertaken in various media, with the aim to store these samples as suspensions before use. Preliminary results revealed that only a negligible weight loss occurred after several months in ethanol, while significant modification of the general morphology was observed in presence of water. In addition, first measurements of the isotopic composition of uranium by LG-SIMS led to comparable results between raw and leached samples, the behavior of the particles under the SIMS beam being comparable to typical field sample particles. This set of results then demonstrates the feasibility of producing reference actinide oxides particles through hydrothermal wet chemistry methods, with a large range of chemical compositions and sizes, thus offering various possible applications in the field of nuclear safeguards.