Alpha radiolysis of nitric acid and sodium nitrate with 4He2+ beam of 13.5MeV energy

Pourbaix Diagram of Astatine Revisited: Experimental Investigations

The Pourbaix diagram of an element displays its stable chemical forms with respect to the redox potential and pH of the solution, whose knowledge is fundamental for understanding and anticipating the chemistry of the element in a specified solution. Unlike most halogens, the Pourbaix diagram in the aqueous phase for astatine (At, Z = 85) is still under construction. In particular, the predominant domains of two astatine species assumed to exist under alkaline conditions, At^- and AtO(OH)₂^-, need to be refined. Through high-performance ion-exchange chromatography, electromobility measurements, and competition experiments, the existence of At^- and AtO(OH)₂^- has been confirmed and the associated standard potential has been determined for the first time (0.86 ± 0.05 V vs the standard hydrogen electrode). On the basis of these results, a revised version of astatine's Pourbaix diagram is proposed, covering the three oxidation states of astatine that exist in the thermodynamic stability range of water: At(-I), At(I), and At(III) (as At^-, At⁺, AtO⁺, AtO(OH), and AtO(OH)₂^-).

Influence of plutonium oxidation state on the formation of molecular hydrogen, nitrous acid and nitrous oxide from alpha radiolysis of nitric acid solution

The study of the formation of radiolytic products, such as molecular hydrogen and nitrous acid, is of primary importance in the reprocessing of spent nuclear fuel and the storage of aqueous solutions containing radioactive materials. The radiolytic yields of molecular hydrogen, nitrous acid and nitrous oxide from alpha radiolysis of nitric acid solutions containing plutonium have been experimentally investigated. The results have shown that the yields of radiolytic products depends on the nitric acid concentration as well as the oxidation state of plutonium. However, the influence of plutonium oxidation state on radiolytic yields is less notable as the nitric acid concentration increases. Molecular hydrogen production decreases with increasing nitric acid concentration while nitrous acid and nitrous oxide productions increase. While radiolytic yields from plutonium(IV) nitric acid solutions have been previously investigated, this study provides radiolytic yields from alpha radiolysis of plutonium(III) and plutonium(VI) nitric acid solutions for molecular hydrogen, nitrous acid and nitrous oxide. These information provide insight into the role played by plutonium redox behaviour on the formation of radiolytic products.

New insight on the simultaneous H2 and HNO2 production in concentrated HNO3 aqueous solutions under alpha radiation

Knowledge of hydrogen and nitrous acid yields (G(H₂) and G(HNO₂)) from α radiolysis of nitric acid solutions is of critical importance for the technological aspects of reprocessing of spent nuclear fuel (SNF). This study provides critical information on the G values for external alpha irradiation of concentrated HNO₃ solutions. An investigation-specifically developed experimental setup allows performing this investigation without encountering issues related to extreme high local doses. In situ monitoring of the UV-visible induced absorption in irradiated HNO₃ solutions permitted quantification of HNO₂ production, and mass spectrometry was used to quantify H₂. The influence of the dose rate and HNO₃ concentration was investigated, and the primary yields of these two species were determined. It was found that dose rate increase leads to diminished production of HNO₂ and H₂, while HNO₃ concentration increase leads to increased HNO₂ formation and reduced H₂ production. The values of the primary yields of these two species were determined and compared to the literature reported values. While the determined values show similar trends as those reported, this study provides accurate radiolytic yields for H₂ and HNO₂ that are radioelement-independent compared to the α radiolysis using radioisotope/HNO₃ mixtures and provides the basis for perfecting numerical codes used for simulating the radiolytic processes associated with SNF reprocessing.

Speciation and reactivity of heptavalent technetium in strong acids

The speciation of Tc(vii) and its reactivity with H₂O₂, methanol and H₂S in H₂SO₄, HClO₄and HNO₃is reviewed.