Sonochemical dissolution of nanoscale ThO2 and partial conversion into a thorium peroxo sulfate

Chronicles of plutonium peroxides: spectroscopic characterization of a new peroxo compound of Pu(IV)

Although hydrogen peroxide (H2O2) has been highly used in nuclear chemistry for more than 75 years, the preparation and literature description of tetravalent actinide peroxides remain surprisingly scarce. A new insight is given in this topic through the synthesis and thorough structural characterization of a new peroxo compound of Pu(IV).

Micrometric drilling of (meta-)studtite square platelets formed by pseudomorphic conversion of UO2 under high-frequency ultrasound

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO2 was investigated in dilute aqueous solutions saturated with Ar/(20 %)O2 (20 °C). Optimized parameter settings enabled the complete conversion of UO2 micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [(UO2(O2)(H2O)2)xH2O] with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H+, in-situ generation of H2O2 and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.

Ultrasonically controlled synthesis of UO2+x colloidal nanoparticles

Actinide colloids and nanoparticles (NPs) currently constitute a topic of strong interest due to their potential role in advanced nuclear energetics and the environmental migration of radioactivity. A better understanding of the physico-chemical properties of nanoscale actinide oxides requires robust synthesis approaches. In this work, UO_2+x NPs were successfully prepared by sonochemistry from U(IV) solutions previously stabilised in a hydrochloric medium (20 kHz, 65 °C, Ar/(10%)CO). Colloidal suspensions were found to be composed of crystalline and spherical NPs showing a UO₂-like structure and measuring 18.0 ± 0.1 nm (SAXS, HR-TEM and PXRD techniques). In comparison with the controlled hydrolysis approach used as a reference, sonochemistry appears to be a simple and original synthesis route providing larger, better defined and more crystalline UO_2+x NPs with a narrower size distribution. These well-defined NPs offer new opportunities for the preparation of reference actinide materials devoted to fundamental, technological and environmental studies.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO₂) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO₂ crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO₂ nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review.

Sonochemistry of actinides: from ions to nanoparticles and beyond

Sonochemistry studies chemical and physical effects in liquids submitted to power ultrasound. These effects arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species. In principle, each cavitation bubble can be considered as a microreactor initiating chemical reactions at mild conditions. In addition, microjets and shock waves accompanied bubble collapse produce fragmentation, dispersion and erosion of solid surfaces or particles. Microbubbles oscillating in liquids also enable nucleation and precipitation of nanosized actinide compounds with specific morphology. This review focuses on the versatile sonochemical processes with actinide ions and particles in homogenous solutions and heterogenous systems. The redox reactions in aqueous solutions, dissolution or precipitation of refractory solids, synthesis of actinide nanoparticles, and ultrasonically driving decontamination are considered. The guideline for further research is also discussed.